My Book 1

Meteorology Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries. After the development of the computer in the latter half of the 20th century, breakthroughs in weather forecasting were achieved. Meteorological phenomena are observable weather events which illuminate, and are explained by the science of meteorology. Those events are bound by the variables that exist in Earth's atmosphere; temperature, air pressure, water vapor, and the gradients and interactions of each variable, and how they change in time. Different spatial scales are studied to determine how systems on local, region, and global levels impact weather and climatology. Meteorology, climatology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology compose the interdisciplinary field of hydrometeorology. Interactions between Earth's atmosphere and the oceans are part of coupled ocean-atmosphere studies. Meteorology has application in many diverse fields such as the military, energy production, transport, agriculture and construction. The word "meteorology" is from Greek μετέωρος metéōros "lofty; high (in the sky)" (from μετα- meta"above" and ἐωρ eōr "to lift up") and -λογία -logia "-(o)logy".

History of Meteorology The beginnings of meteorology can be traced back to ancient India, as the Upanishads contain serious discussion about the processes of cloud formation and rain and the seasonal cycles caused by the movement of Earth around the sun. Varāhamihira’s classical work Brihatsamhita, written about 500 A.D. provides a clear evidence that a deep knowledge of atmospheric processes existed even in those times. In 350 BC, Aristotle wrote Meteorology. Aristotle is considered the founder of meteorology. One of the most impressive achievements described in the Meteorology is the description of what is now known as the hydrologic cycle. The Greek scientist Theophrastus compiled a book on weather forecasting, called the Book of Signs. The work of Theophrastus remained a dominant influence in the study of weather and in weather forecasting for nearly 2,000 years In 25 AD, Pomponius Mela, a geographer for the Roman Empire, formalized the climatic zone system. Around the 9th century, Al-Dinawari, an Iranian Kurdish naturalist, writes the Kitab al-Nabat (Book of Plants), in which he deals with the application of meteorology to agriculture during the Muslim Agricultural Revolution. He describes the meteorological character of the sky, the planets and constellations, the sun and moon, the lunar phases indicating seasons and rain, the anwa (heavenly bodies of rain), and atmospheric phenomena such as winds, thunder, lightning, snow, floods, valleys, rivers, lakes, wells and other sources of water. In 1021, Ibn al-Haytham (Alhazen) wrote on the atmospheric refraction of light. He showed that the twilight is due to atmospheric refraction and only begins when the Sun is 19 degrees below the horizon, and uses a complex geometric demonstration to measure the height of the Earth's atmosphere as 52,000 passuum (49 miles (79 km)), which is very close to the modern measurement of 50 miles (80 km). He also realized that the atmosphere also reflects light, from his observations of the sky brightening even before the Sun rises.

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St. Albert the Great was the first to propose that each drop of falling rain had the form of a small sphere, and that this form meant that the rainbow was produced by light interacting with each raindrop. Roger Bacon was the first to calculate the angular size of the rainbow. He stated that the rainbow summit can not appear higher than 42 degrees above the horizon. In the late 13th century and early 14th century, Theodoric of Freiberg and Kamāl al-Dīn al-Fārisī continued the work of Ibn al-Haytham, and they were the first to give the correct explanations for the primary rainbow phenomenon. Theoderic went further and also explained the secondary rainbow In 1716, Edmund Halley suggests that aurorae are caused by "magnetic effluvia" moving along the Earth's magnetic field lines. In 1441, King Sejongs son, Prince Munjong, invented the first standardized rain gauge.These were sent throughout the Joseon Dynasty of Korea as an official tool to assess land taxes based upon a farmer's potential harvest. In 1450, Leone Battista Alberti developed a swinging-plate anemometer, and is known as the first anemometer. In 1607, Galileo Galilei constructs a thermoscope. In 1611, Johannes Kepler writes the first scientific treatise on snow crystals: "Strena Seu de Nive Sexangula (A New Year's Gift of Hexagonal Snow)". In 1643, Evangelista Torricelli invents the mercury barometer. In 1662, Sir Christopher Wren invented the mechanical, self-emptying, tipping bucket rain gauge. In 1714, Gabriel Fahrenheit creates a reliable scale for measuring temperature with a mercury-type thermometer. In 1742, Anders Celsius, a Swedish astronomer, proposed the 'centigrade' temperature scale, the predecessor of the current Celsius scale. In 1783, the first hair hygrometer is demonstrated by Horace-Bénédict de Saussure. In 1802-1803, Luke Howard writes On the Modification of Clouds in which he assigns cloud types Latin names. In 1806, Francis Beaufort introduced his system for classifying wind speeds. Near the end of the 19th century the first cloud atlases were published, including the International Cloud Atlas, which has remained in print ever since. The April 1960 launch of the first successful weather satellite, TIROS-1, marked the beginning of the age where weather information became available globally. In 1648, Blaise Pascal rediscovers that atmospheric pressure decreases with height, and deduces that there is a vacuum above the atmosphere. In 1738, Daniel Bernoulli publishes Hydrodynamics, initiating the kinetic theory of gases and established the basic laws for the theory of gases In 1761, Joseph Black discovers that ice absorbs heat without changing its temperature when melting. In 1772, Black's student Daniel Rutherford discovers nitrogen, which he calls phlogisticated air, and together they developed the phlogiston theory. In 1777, Antoine Lavoisier discovers oxygen and develops an explanation for combustion.In 1783, in Lavoisier's book Reflexions sur le phlogistique,[25] he deprecates the phlogiston theory and proposes a caloric theory.[26][27] In 1804, Sir John Leslie observes that a matte black surface radiates heat more effectively than a polished surface, suggesting the importance of black body radiation. In 1808, John Dalton defends caloric theory in A New System of Chemistry and describes how it combines with matter, especially gases; he proposes that the heat capacity of gases varies inversely with atomic weight. In 1824, Sadi Carnot analyzes the efficiency of steam engines using caloric theory; he develops the notion of a reversible process and, in postulating that no such thing exists in nature, lays the foundation for the second law of thermodynamics. In 1494, Christopher Columbus experiences a tropical cyclone, leads to the first written European account of a hurricane.[28] In 1686, Edmund Halley presents a systematic study of the trade winds and monsoons and identifies solar heating as the cause of atmospheric motions.[29] In 1735, an ideal explanation of global circulation through study of the Trade winds was written by George Hadley.[30] In 1743, when Benjamin Franklin is prevented from seeing a lunar eclipse by a hurricane, he decides that cyclones move in a contrary manner to the winds at their periphery.[31] Understanding the kinematics of how exactly the rotation of the Earth affects airflow was partial at first. Gaspard-Gustave Coriolis published a paper in 1835 on the energy yield of machines with rotating parts, such as waterwheels.[32] In 1856, William Ferrel proposed the existence of a circulation cell in the mid-latitudes with air being deflected by the Coriolis force to create the prevailing westerly winds.[33] Late in the 19th century the full extent of the large scale interaction of pressure gradient force and deflecting force that in the end causes air masses to move along isobars was understood. By 1912, this deflecting force was named the Coriolis effect.[34] Just after World War I, a group of meteorologists in Norway led by Vilhelm Bjerknes developed the Norwegian cyclone model that explains the generation, intensification and ultimate decay (the life cycle) of mid-latitude cyclones, introducing the idea of fronts, that is, sharply defined boundaries between air masses.[35] The group included Carl-Gustaf Rossby (who was the first to explain

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the large scale atmospheric flow in terms of fluid dynamics), Tor Bergeron (who first determined the mechanism by which rain forms) and Jacob Bjerknes. In 1654, Ferdinando II de Medici establishes the first weather observing network, that consisted of meteorological stations in Florence, Cutigliano, Vallombrosa, Bologna, Parma, Milan, Innsbruck, Osnabrück, Paris and Warsaw. Collected data was centrally sent to Florence at regular time intervals. [36] In 1832, an electromagnetic telegraph was created by Baron Schilling.[37] The arrival of the electrical telegraph in 1837 afforded, for the first time, a practical method for quickly gathering surface weather observations from a wide area.[38] This data could be used to produce maps of the state of the atmosphere for a region near the Earth's surface and to study how these states evolved through time. To make frequent weather forecasts based on these data required a reliable network of observations, but it was not until 1849 that the Smithsonian Institution began to establish an observation network across the United States under the leadership of Joseph Henry.[39] Similar observation networks were established in Europe at this time. In 1854, the United Kingdom government appointed Robert FitzRoy to the new office of Meteorological Statist to the Board of Trade with the role of gathering weather observations at sea. FitzRoy's office became the United Kingdom Meteorological Office in 1854, the first national meteorological service in the world. The first daily weather forecasts made by FitzRoy's Office were published in The Times newspaper in 1860. The following year a system was introduced of hoisting storm warning cones at principal ports when a gale was expected. Over the next 50 years many countries established national meteorological services. The India Meteorological Department (1875) was established following tropical cyclone and monsoon related famines in the previous decades. The Finnish Meteorological Central Office (1881) was formed from part of Magnetic Observatory of Helsinki University. Japan's Tokyo Meteorological Observatory, the forerunner of the Japan Meteorological Agency, began constructing surface weather maps in 1883. The United States Weather Bureau (1890) was established under the United States Department of Agriculture. The Australian Bureau of Meteorology (1906) was established by a Meteorology Act to unify existing state meteorological services. In 1904, Norwegian scientist Vilhelm Bjerknes first argued in his paper Weather Forecasting as a Problem in Mechanics and Physics that it should be possible to forecast weather from calculations based upon natural laws. It was not until later in the 20th century that advances in the understanding of atmospheric physics led to the foundation of modern numerical weather prediction. In 1922, Lewis Fry Richardson published "Weather Prediction By Numerical Process", after finding notes and derivations he worked on as an ambulance driver in World War I. He described therein how small terms in the prognostic fluid dynamics equations governing atmospheric flow could be neglected, and a finite differencing scheme in time and space could be devised, to allow numerical prediction solutions to be found. Richardson envisioned a large auditorium of thousands of people performing the calculations and passing them to others. However, the sheer number of calculations required was too large to be completed without the use of computers, and the size of the grid and time steps led to unrealistic results in deepening systems. It was later found, through numerical analysis, that this was due to numerical instability. Starting in the 1950s, numerical forecasts with computers became feasible.[46] The first weather forecasts derived this way used barotropic (that means, single-vertical-level) models, and could successfully predict the large-scale movement of midlatitude Rossby waves, that is, the pattern of atmospheric lows and highs..[47] In the 1960s, the chaotic nature of the atmosphere was first observed and mathematically described by Edward Lorenz, founding the field of chaos theory.[48] These advances have led to the current use of ensemble forecasting in most major forecasting centers, to take into account uncertainty arising from the chaotic nature of the atmosphere.[49] Climate models have been developed that feature a resolution comparable to older weather prediction models. These climate models are used to investigate long-term climate shifts, such as what effects might be caused by human emission of greenhouse gases. Essence of meteorology •

Climate – the average and variations of weather in a region over long periods of time.

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Meteorology – the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting (in contrast with climatology).

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Weather – the set of all the phenomena in a given atmosphere at a given time.

Branches of meteorology •

Microscale meteorology – the study of atmospheric phenomena about 1 km or less, smaller than mesoscale, including small and generally fleeting cloud "puffs" and other small cloud features

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Mesoscale meteorology – the study of weather systems about 5 kilometers to several hundred kilometers, smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems, such as sea breezes, squall lines, and mesoscale convective complexes

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Synoptic scale meteorology – is a horizontal length scale of the order of 1000 kilometres (about 620 miles) or more

Meteorologists Meteorologists are scientists who study meteorology.[50] Meteorologists work in government agencies, private consulting and research services, industrial enterprises, utilities, radio and television stations, and in education. In the United States, meteorologists held about 9,400 jobs in 2009.[51] Meteorologists are best known for forecasting the weather. Many radio and television weather forecasters are professional meteorologists, while others are merely reporters (weather specialist, weatherman, etc.) with no formal meteorological training. The American Meteorological Society and National Weather Association issue "Seals of Approval" to weather broadcasters who meet certain requirements. Each science has its own unique sets of laboratory equipment. In the atmosphere, there are many things or qualities of the atmosphere that can be measured. Rain, which can be observed, or seen anywhere and anytime was one of the first ones to be measured historically. Also, two other accurately measured qualities are wind and humidity. Neither of these can be seen but can be felt. The devices to measure these three sprang up in the mid-15th century and were respectively the rain gauge, the anemometer, and the hygrometer.[52] Sets of surface measurements are important data to meteorologists. They give a snapshot of a variety of weather conditions at one single location and are usually at a weather station, a ship or a weather buoy. The measurements taken at a weather station can include any number of atmospheric observables. Usually, temperature, pressure, wind measurements, and humidity are the variables that are measured by a thermometer, barometer, anemometer, and hygrometer, respectively.[53] Upper air data are of crucial importance for weather forecasting. The most widely used technique is launches of radiosondes. Supplementing the radiosondes a network of aircraft collection is organized by the World Meteorological Organization. Remote sensing, as used in meteorology, is the concept of collecting data from remote weather events and subsequently producing weather information. The common types of remote sensing are Radar, Lidar, and satellites (or photogrammetry). Each collects data about the atmosphere from a remote location and, usually, stores the data where the instrument is located. RADAR and LIDAR are not passive because both use EM radiation to illuminate a specific portion of the atmosphere.[54] Weather satellites along with more generalpurpose Earth-observing satellites circling the earth at various altitudes have become an indispensable tool for studying a wide range of phenomena from forest fires to El Niño.

Meteorological Applications

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Weather forecasting

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather informally for millennia, and formally since at least the nineteenth century. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere and using scientific understanding of atmospheric processes to project how the atmosphere will evolve. Once an all-human endeavor based mainly upon changes in barometric pressure, current weather conditions, and sky condition, forecast models are now used to determine future conditions. Human input is still required to pick the best possible forecast model to base the forecast upon, which involves pattern recognition skills, teleconnections, knowledge of model performance, and knowledge of model biases. The chaotic nature of the atmosphere, the massive computational power required to solve the equations that describe the atmosphere, error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes mean that forecasts become less accurate as the difference in current time and the time for which the forecast is being made (the range of the forecast) increases. The use of ensembles and model consensus help narrow the error and pick the most likely outcome. There are a variety of end uses to weather forecasts. Weather warnings are important forecasts because they are used to protect life and property. Forecasts based on temperature and precipitation are important to agriculture, and therefore to commodity traders within stock markets. Temperature forecasts are used by utility companies to estimate demand over coming days. On an everyday basis, people use weather forecasts to determine what to wear on a given day. Since outdoor activities are severely curtailed by heavy rain, snow and the wind chill, forecasts can be used to plan activities around these events, and to plan ahead and survive them.

Aviation meteorology ~ Penalosa R.N.

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Aviation meteorology deals with the impact of weather on air traffic management. It is important for air crews to understand the implications of weather on their flight plan as well as their aircraft, as noted by the Aeronautical Information Manual

Agricultural meteorology Meteorologists, soil scientists, agricultural hydrologists, and agronomists are persons concerned with studying the effects of weather and climate on plant distribution, crop yield, water-use efficiency, phenology of plant and animal development, and the energy balance of managed and natural ecosystems. Conversely, they are interested in the role of vegetation on climate and weather.

Hydrometeorology Hydrometeorology is the branch of meteorology that deals with the hydrologic cycle, the water budget, and the rainfall statistics of storms.[80] A hydrometeorologist prepares and issues forecasts of accumulating (quantitative) precipitation, heavy rain, heavy snow, and highlights areas with the potential for flash flooding. Typically the range of knowledge that is required overlaps with climatology, mesoscale and synoptic meteorology, and other geosciences.[81]

Maritime meteorology Maritime meteorology deals with air and wave forecasts for ships operating at sea. Organizations such as the Ocean Prediction Center, Honolulu National Weather Service forecast office, United Kingdom Met Office, and JMA prepare high seas forecasts for the world's oceans.

Instruments and equipment of meteorology •

Anemometer – a device for measuring wind speed; used in weather stations

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Barograph – an aneroid barometer that records the barometric pressure over time and produces a paper or foil chart called a barogram

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Barometer – an instrument used to measure atmospheric pressure using either water, air, or mercury; useful for forecasting short term changes in the weather

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Ceiling balloon – a balloon, with a known ascent rate, used to measure the height of the base of clouds during daylight

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Ceiling projector – a device that is used, in conjunction with an alidade, to measure the height of the base of clouds

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Ceilometer – a device that uses a laser or other light source to measure the height of the base of clouds.

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Dark adaptor goggles – clear, red-tinted plastic goggles used either for adapting the eyes to dark prior to night observation or to help identify clouds during bright sunshine or glare from snow

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Disdrometer – an instrument used to measure the drop size, distribution, and velocity of falling hydrometeors

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Field mill – an instrument used to measure the strength of electric fields in the atmosphere near thunderstorm clouds

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Hygrometer – an instrument used to measure humidity

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Ice Accretion Indicator – an L-shaped piece of aluminum 15 inches (38 cm) long by 2 inches (5 cm) wide used to indicate the formation of ice, frost, or the presence of freezing rain or freezing drizzle

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LIDAR (LIght Detection And Ranging) – an optical remote sensing technology used in atmospheric physics (among other fields) that measures the properties of scattered light to find information about a distant target

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Lightning detector – a device, either ground-based, mobile, or space-based, that detects lightning produced by thunderstorms

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Nephelometer – an instrument used to measure suspended particulates in a liquid or gas colloid. Gas-phase nephelometers are used to provide information on atmospheric visibility and albedo

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Nephoscope – an instrument for measuring the altitude, direction, and velocity of clouds

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Pyranometer – A type of actinometer found in many meteorological stations used to measure broadband solar irradiance

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Radar – see Weather radar

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Radiosonde – an instrument used in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver

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Rain gauge – an instrument that gathers and measures the amount of liquid precipitation over a set period of time

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Snow gauge – an instrument that gathers and measures the amount of solid precipitation over a set period of time

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SODAR (SOnic Detection And Ranging) – an instrument that measures the scattering of sound waves by atmospheric turbulence

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Solarimeter – a pyranometer, an instrument used to measure combined direct and diffuse solar radiation

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Sounding rocket – an instrument-carrying sub-orbital rocket designed to take measurements and perform scientific experiments

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Stevenson screen – part of a standard weather station, it shields instruments from precipitation and direct heat radiation while still allowing air to circulate freely

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Sunshine recorders – devices used to indicate the amount of sunshine at a given location

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Thermograph – a chart recorder that measures and records both temperature and humidity

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Thermometer – a device that measures temperature or temperature gradient

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Weather balloon – a high-altitude balloon that carries instruments aloft and uses a radiosonde to send back information on atmospheric pressure, temperature, and humidity

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Weather radar – a type of radar used to locate precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.) and forecast its future position and intensity

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Weather vane – a movable device attached to an elevated object such as a roof that shows the direction of the wind

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Windsock – a conical textile tube designed to indicate wind direction and relative wind speed

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Wind profiler – equipment that uses radar or SODAR to detect wind speed and direction at various elevations

Meteorological Instruments and Equipment

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Radar

Wind vane

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Aneroid Barometer

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Atmosphere of Earth ~ Penalosa R.N.

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The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation). Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10 18 kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during atmospheric reentry of spacecraft. The Kármán line, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space. Air is the name given to atmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. While air content and atmospheric pressure varies at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres.

Composition of Earth's atmosphere. The lower pie represents the trace gases which together compose 0.039% of the atmosphere. Values normalized for illustration. The numbers are from a variety of years (mainly 1987, with CO2 and methane from 2009) and do not represent any single source.

Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases are often referred to as trace gases,[1] among which are the greenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many natural substances may be present in tiny amounts in an unfiltered air sample, including dust, pollen and spores, sea spray, and volcanic ash. Various industrial pollutants also may be present, such as chlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulfur compounds such as sulfur dioxide [SO2].

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Composition of dry atmosphere, by volume ppmv: parts per million by volume (note: volume fraction is equal to mole fraction for ideal gas only, see volume (thermodynamics))

Gas

Volume

Nitrogen (N2)

780,840 ppmv (78.084%)

Oxygen (O2)

209,460 ppmv (20.946%)

Argon (Ar)

9,340 ppmv (0.9340%)

Carbon dioxide (CO2)

390 ppmv (0.039%)

Neon (Ne)

18.18 ppmv (0.001818%)

Helium (He)

5.24 ppmv (0.000524%)

Methane (CH4)

1.79 ppmv (0.000179%)

Krypton (Kr)

1.14 ppmv (0.000114%)

Hydrogen (H2)

0.55 ppmv (0.000055%)

Nitrous oxide (N2O)

0.3 ppmv (0.00003%)

Carbon monoxide (CO)

0.1 ppmv (0.00001%)

Xenon (Xe)

0.09 ppmv (9×10−6%) (0.000009%)

Ozone (O3)

0.0 to 0.07 ppmv (0 to 7×10−6%)

Nitrogen dioxide (NO2)

0.02 ppmv (2×10−6%) (0.000002%)

Iodine (I2)

0.01 ppmv (1×10−6%) (0.000001%)

Ammonia (NH3)

trace

Not included in above dry atmosphere:

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Water vapor (H2O)

~0.40% over full atmosphere, typically 1%-4% at surface

Structure of the atmosphere Exosphere The outermost layer of Earth's atmosphere extends from the exobase upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These freemoving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind. Thermosphere Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. Unlike in the stratosphere, where the inversion is caused by absorption of radiation by ozone, in the thermosphere the inversion is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The air is so rarefied, that an individual molecule (of oxygen, for example) travels an average of 1 kilometer between collisions with other molecules.[3] The International Space Station orbits in this layer, between 320 and 380 km (200 and 240 mi). Because of the relative infrequency of molecular collisions, air above the mesopause is poorly mixed compared to air below. While the composition from the troposphere to the mesosphere is fairly constant, above a certain point, air is poorly mixed and becomes compositionally stratified. The point dividing these two regions is known as the turbopause. The region below is the homosphere, and the region above is the heterosphere. The top of the thermosphere is the bottom of the exosphere, called the exobase. Its height varies with solar activity and ranges from about 350–800 km (220–500 mi; 1,100,000–2,600,000 ft). Mesosphere The mesosphere extends from the stratopause to 80–85 km (50–53 mi; 260,000–280,000 ft). It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The mesopause, the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −85 °C (−120 °F; 190 K).[4] At the mesopause, temperatures may drop to −100 °C (−150 °F; 170 K).[5] Due to the cold temperature of the mesosphere, water vapor is frozen, forming ice clouds (or Noctilucent clouds). A type of lightning referred to as either sprites or ELVES, form many miles above thunderclouds in the troposphere. Stratosphere The stratosphere extends from the tropopause to about 51 km (32 mi; 170,000 ft). Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. While the temperature may be −60 °C (−76 °F; 210 K) at the tropopause, the top of the stratosphere is much warmer, and may be near freezing[citation needed]. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km (31 to 34 mi; 160,000 to 180,000 ft). The pressure here is 1/1000 sea level. Troposphere The troposphere begins at the surface and extends to between 9 km (30,000 ft) at the poles and 17 km (56,000 ft) at the equator,[6] with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή", trope, meaning turn or overturn). The troposphere contains roughly 80% of the mass of the atmosphere.[7] The tropopause is the boundary between the troposphere and stratosphere.

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Other layers Within the five principal layers determined by temperature are several layers determined by other properties. •

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The ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about 15–35 km (9.3–22 mi; 49,000–110,000 ft), though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.

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The ionosphere, the part of the atmosphere that is ionized by solar radiation, stretches from 50 to 1,000 km (31 to 620 mi; 160,000 to 3,300,000 ft) and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is responsible for auroras.

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The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.[8] The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.

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The planetary boundary layer is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through turbulent diffusion. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, calm nights to 3000 m or more during the afternoon in dry regions.

Pressure and thickness The average atmospheric pressure at sea level is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.92 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×1018 kg (1.135×1019 lb),[15] about 2.5% less than would be inferred from the average sea level pressure and the Earth's area of 51007.2 megahectares, this portion being displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather.

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If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at 8.50 km (27,900 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every 5.6 km (18,000 ft) and by a factor of e = 2.718… approximately every 7.64 km (25,100 ft), the latter being the average scale height of Earth's atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of atmospheric drag on satellites. In summary, the equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference:[16] •

50% of the atmosphere by mass is below an altitude of 5.6 km (18,000 ft).

•

90% of the atmosphere by mass is below an altitude of 16 km (52,000 ft). The common altitude of commercial airliners is about 10 km (33,000 ft) and Mt. Everest's summit is 8,848 m (29,029 ft) above sea level.

•

99.99997% of the atmosphere by mass is below 100 km (62 mi; 330,000 ft), although in the rarefied region above this there are auroras and other atmospheric effects. The highest X-15 plane flight in 1963 reached an altitude of 108.0 km (354,300 ft).

Atmospheric Circulation

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Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller ocean circulation) by which thermal energy is distributed on the surface of the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains fairly constant. However, individual weather systems - midlatitude depressions, or tropical convective cells occur "randomly"[citation needed], and it is accepted that weather cannot be predicted beyond a fairly short limit: perhaps a month in theory, or (currently) about ten days in practice (see Chaos theory and Butterfly effect). Nonetheless, as the climate is the average of these systems and patterns - where and when they tend to occur again and again -, it is stable over longer periods of time. As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials compared to glacials), but remain largely constant even due to continental drift. Tectonic uplift can significantly alter major elements of it, however - for example the jet stream -, and plate tectonics shift ocean currents. In the extremely hot climates of the Mesozoic, indications of a third desert belt at the Equator has been found; it was perhaps caused by convection. But even then, the overall latitudinal pattern of Earth's climate was not much different from the one today.

Latitudinal circulation features The wind belts girdling the planet are organised into three cells: the Hadley cell, the Ferrel cell, and the Polar cell. Contrary to the impression given in the simplified diagram, the vast bulk of the vertical motion occurs in the Hadley cell; the explanations of the other two cells are complex. Note that there is one discrete Hadley cell that may split, shift and merge in a complicated process over time. Low and high pressures on earth's surface are balanced by opposite relative pressures in the upper troposphere.

1. Hadley cell The Hadley cell mechanism is well understood. The atmospheric circulation pattern that George Hadley described to provide an explanation for the trade winds matches observations very well. It is a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in equatorial low pressure areas (the Intertropical Convergence Zone, ITCZ) to the tropopause and carried poleward. At about 30°N/S latitude, it descends in a high pressure area. Some of the descending air travels equatorially along the surface, closing the loop of the Hadley cell and creating the Trade Winds. Though the Hadley cell is described as lying on the equator, it is more accurate to describe it as following the sun’s zenith point, or what is termed the "thermal equator," which undergoes a semiannual north-south migration.

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2. Polar cell The Polar cell is likewise a simple system. Though cool and dry relative to equatorial air, air masses at the 60th parallel are still sufficiently warm and moist to undergo convection and drive a thermal loop. Air circulates within the troposphere, limited vertically by the tropopause at about 8 km. Warm air rises at lower latitudes and moves poleward through the upper troposphere at both the north and south poles. When the air reaches the polar areas, it has cooled considerably, and descends as a cold, dry high pressure area, moving away from the pole along the surface but twisting westward as a result of the Coriolis effect to produce the Polar easterlies. The outflow from the cell creates harmonic waves in the atmosphere known as Rossby waves. These ultralong waves play an important role in determining the path of the jet stream, which travels within the transitional zone between the tropopause and the Ferrel cell. By acting as a heat sink, the Polar cell also balances the Hadley cell in the Earth’s energy equation. It can be argued that the Polar cell is the primary weathermaker for regions above the middle northern latitudes. While Canadians and Europeans may have to deal with occasional heavy summer storms, there is nothing like a winter visit from a Siberian high to give one a true appreciation of real cold. In fact, it is the polar high which is responsible for generating the coldest temperature recorded on Earth: -89.2°C at Vostok Station in 1983 in Antarctica. The Hadley cell and the Polar cell are similar in that they are thermally direct; in other words, they exist as a direct consequence of surface temperatures; their thermal characteristics override the effects of weather in their domain. The sheer volume of energy the Hadley cell transports, and the depth of the heat sink that is the Polar cell, ensures that the effects of transient weather phenomena are not only not felt by the system as a whole, but — except under unusual circumstances — are not even permitted to form. The endless chain of passing highs and lows which is part of everyday life for mid-latitude dwellers is unknown above the 60th and below the 30th parallels. There are some notable exceptions to this rule. In Europe, unstable weather extends to at least 70° north. These atmospheric features are also stable, so even though they may strengthen or weaken regionally or over time, they do not vanish entirely.

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3. Ferrel cell The Ferrel cell, theorized by William Ferrel (1817-1891), is a secondary circulation feature, dependent for its existence upon the Hadley cell and the Polar cell. It behaves much as an atmospheric ball bearing between the Hadley cell and the Polar cell, and comes about as a result of the eddy circulations (the high and low pressure areas) of the mid-latitudes. For this reason it is sometimes known as the "zone of mixing." At its southern extent (in the Northern hemisphere), it overrides the Hadley cell, and at its northern extent, it overrides the Polar cell. Just as the Trade Winds can be found below the Hadley cell, the Westerlies can be found beneath the Ferrel cell. Thus, strong high pressure areas which divert the prevailing westerlies, such as a Siberian high (which could be considered an extension of the Arctic high), could be said to override the Ferrel cell, making it discontinuous. While the Hadley and Polar cells are truly closed loops, the Ferrel cell is not, and the telling point is in the Westerlies, which are more formally known as "the Prevailing Westerlies." While the Trade Winds and the Polar Easterlies have nothing over which to prevail, their parent circulation cells having taken care of any competition they might have to face, the Westerlies are at the mercy of passing weather systems. While upper-level winds are essentially westerly, surface winds can vary sharply and abruptly in direction. A low moving polewards or a high moving equator wards maintains or even accelerates a westerly flow; the local passage of a cold front may change that in a matter of minutes, and frequently does. A strong high moving polewards may bring easterly winds for days. The base of the Ferrel cell is characterized by the movement of air masses, and the location of these air masses is influenced in part by the location of the jet stream, which acts as a collector for the air carried

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aloft by surface lows (a look at a weather map will show that surface lows follow the jet stream). The overall movement of surface air is from the 30th latitude to the 60th. However, the upper flow of the Ferrel cell is not well defined. This is in part because it is intermediary between the Hadley and Polar cells, with neither a strong heat source nor a strong cold sink to drive convection and, in part, because of the effects on the upper atmosphere of surface eddies, which act as destabilizing influences.

Walker circulation The Pacific cell is of such importance that it has been named the Walker circulation after Sir Gilbert Walker, an early-20th-century director of British observatories in India, who sought a means of predicting when the monsoon winds would fail. While he was never successful in doing so, his work led him to the discovery of an indisputable link between periodic pressure variations in the Indian Ocean and the Pacific, which he termed the "Southern Oscillation". The movement of air in the Walker circulation affects the loops on either side. Under "normal" circumstances, the weather behaves as expected. But every few years, the winters become unusually warm or unusually cold, or the frequency of hurricanes increases or decreases, and the pattern sets in for an indeterminate period.The behavior of the Walker cell is the key to the riddle, and leads to an understanding of the El Niño (more accurately, ENSO or El Niño - Southern Oscillation) phenomenon. If convective activity slows in the Western Pacific for some reason (this reason is not currently known), the climate dominoes next to it begin to topple. First, the upper-level westerly winds fail. This cuts off the source of cool subsiding air, and therefore the surface Easterlies cease. The consequence of this is twofold. In the eastern Pacific, warm water surges in from the west since there is no longer a surface wind to constrain it. This and the corresponding effects of the Southern Oscillation result in long-term unseasonable temperatures and precipitation patterns in North and South America, Australia, and Southeast Africa, and disruption of ocean currents. Meanwhile in the Atlantic, high-level, fast-blowing Westerlies which would ordinarily be blocked by the Walker circulation and unable to reach such intensities, form. These winds tear apart the tops of nascent hurricanes and greatly diminish the number which are able to reach full strength. El Niño - Southern Oscillation El Niño and La Niña are two opposite surface temperature anomalies in the Southern Pacific, which heavily influence the weather on a large scale. In the case of El Niño warm water approaches the coasts of South America which results in blocking the upwelling of nutrient-rich deep water. This has serious impacts on the fish populations. In the La Niña case, the convective cell over the western Pacific strengthens inordinately, resulting in colder than normal winters in North America, and a more robust cyclone season in South-East Asia and Eastern Australia. There is increased upwelling of deep cold ocean waters and more intense uprise of surface air near South America, resulting in increasing numbers of drought occurrence, although it is often argued that fishermen reap benefits from the more nutrient-filled eastern Pacific waters. The neutral part of the cycle - the "normal" component - has been referred to humorously by some as "La Nada", which means "the nothing" in Spanish.

Weather Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy.[1] Most weather phenomena occur in the troposphere,[2][3] just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time.[4] When used without qualification, "weather" is understood to be the weather of Earth.

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Weather is driven by density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in Earth's orbit affect the amount and distribution of solar energy received by the Earth and influence long term climate and global climate change. Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes due to differences in compression heating. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout human history, and there is evidence that human activity such as agriculture and industry has inadvertently modified weather patterns.

Cause On Earth, common weather phenomena include wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes, typhoons and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere).[3] Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.[5] Weather occurs primarily due to density (temperature and moisture) differences between one place to another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. In other words, the farther from the tropics you lie, the lower the sun angle is, which causes those locations to be cooler due to the indirect sunlight.[6] The strong temperature contrast between polar and tropical air gives rise to the jet stream.[7] Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow (see baroclinity).[8] Weather systems in the tropics, such as monsoons or organized thunderstorm systems, are caused by different processes.

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Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate).[9] This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence longterm climate. (see Milankovitch cycles).[10] The uneven solar heating (the formation of zones of temperature and moisture gradients, or frontogenesis) can also be due to the weather itself in the form of cloudiness and precipitation. [11] Higher altitudes are cooler than lower altitudes, which is explained by the lapse rate.[12][13] On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or man-made objects) have differing physical characteristics such as reflectivity, roughness, or moisture content. Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands, lowering the air pressure and its density.[14] The resulting horizontal pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation then causes curvature of the flow via the Coriolis effect.[15] The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell while a smaller scale example would be coastal breezes. The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole.[16] This makes it difficult to accurately predict weather more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, meteorology. It is theoretically impossible to make useful day-to-day predictions more than about two weeks ahead, imposing an upper limit to potential for improved prediction skill.[17]

Climate Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long

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periods. Climate can be contrasted to weather, which is the present condition of these elements and their variations over shorter periods. A region's climate is generated by the climate system, which has five components: Atmosphere, hydrosphere, cryosphere, land surface, and biosphere.[1] The climate of a location is affected by its latitude, terrain, and altitude, as well as nearby water bodies and their currents. Climates can be classified according to the average and the typical ranges of different variables, most commonly temperature and precipitation. The most commonly used classification scheme was originally developed by Wladimir Köppen. The Thornthwaite system,[2] in use since 1948, incorporates evapotranspiration along with temperature and precipitation information and is used in studying animal species diversity and potential effects of climate changes. The Bergeron and Spatial Synoptic Classification systems focus on the origin of air masses that define the climate of a region. Paleoclimatology is the study of ancient climates. Since direct observations of climate are not available before the 19th century, paleoclimates are inferred from proxy variables that include non-biotic evidence such as sediments found in lake beds and ice cores, and biotic evidence such as tree rings and coral. Climate models are mathematical models of past, present and future climates. Climate change may occur over long and short timescales from a variety of factors; recent warming is discussed in global warming.

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Monthly average surface temperatures from 1961–1990. This is an example of how climate varies with location and season

Monthly global images from NASA Earth Observatory

The Köppen classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E. These primary types are A, tropical; B, dry; C, mild mid-latitude; D, cold mid-latitude; and E, polar. The five primary classifications can be further divided into secondary classifications such as rain forest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic climate, Mediterranean climate, steppe, subarctic climate, tundra, polar ice cap, and desert. •

Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 millimetres (69 in) and 2,000 millimetres (79 in). Mean monthly temperatures exceed 18 °C (64 °F) during all months of the year.[15]

•

A monsoon is a seasonal prevailing wind which lasts for several months, ushering in a region's rainy season.[16] Regions within North America, South America, Sub-Saharan Africa, Australia and East Asia are monsoon regimes.[17]

•

A tropical savanna is a grassland biome located in semiarid to semi-humid climate regions of subtropical and tropical latitudes, with average temperatures remain at or above 18 °C (64 °F) year round and rainfall between 750 millimetres (30 in) and 1,270 millimetres (50 in) a year. They are widespread on Africa, and are found in India, the northern parts of South America, Malaysia, and Australia.[18]

•

The humid subtropical climate zone where winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during

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thunderstorms and from occasional tropical cyclones.[19] Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator.[20]

Humid continental climate, worldwide

•

A humid continental climate is marked by variable weather patterns and a large seasonal temperature variance. Places with more than three months of average daily temperatures above 10 °C (50 °F) and a coldest month temperature below −3 °C (27 °F) and which do not meet the criteria for an arid or semiarid climate, are classified as continental.[21]

•

An oceanic climate is typically found along the west coasts at the middle latitudes of all the world's continents, and in southeastern Australia, and is accompanied by plentiful precipitation year round.[22]

•

The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of western North America, parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters.[23]

•

A steppe is a dry grassland with an annual temperature range in the summer of up to 40 °C (104 °F) and during the winter down to −40 °C (−40 °F).[24]

•

A subarctic climate has little precipitation,[25] and monthly temperatures which are above 10 °C (50 °F) for one to three months of the year, with permafrost in large parts of the area due to the cold winters. Winters within subarctic climates usually include up to six months of temperatures averaging below 0 °C (32 °F).[26]

Map of arctic tundra

•

Tundra occurs in the far Northern Hemisphere, north of the taiga belt, including vast areas of northern Russia and Canada.[27]

•

A polar ice cap, or polar ice sheet, is a high-latitude region of a planet or moon that is covered in ice. Ice caps form because high-latitude regions receive less energy as solar radiation from the sun than equatorial regions, resulting in lower surface temperatures.[28]

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•

A desert is a landscape form or region that receives very little precipitation. Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures (in summer up to 45 °C or 113 °F), and low nighttime temperatures (in winter down to 0 °C; 32 °F) due to extremely low humidity. Many deserts are formed by rain shadows, as mountains block the path of moisture and precipitation to the desert.[29]

Season A season is a division of the year, marked by changes in weather, ecology, and hours of daylight. Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution.[1] In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant. During May, June and July, the northern hemisphere is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere in November, December and January. It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. However, due to seasonal lag, June, July and August are the hottest months in the northern hemisphere and December, January and February are the hottest months in the southern hemisphere. In temperate and subpolar regions, generally four calendar-based seasons (with their adjectives) are recognized: spring (vernal), summer (estival), autumn (autumnal) and winter (hibernal). However, ecologists are increasingly using a six-season model for temperate climate regions that includes pre-spring (prevernal) and late summer (serotinal) as distinct seasons along with the traditional four (See Ecological Seasons below). In some tropical and subtropical regions it is more common to speak of the rainy (or wet, or monsoon) season versus the dry season, because the amount of precipitation may vary more dramatically than the average temperature. For example, in Nicaragua, the dry season (November to April) is called 'summer' and the rainy season (May to October) is called 'winter', even though it is located in the northern hemisphere. In other tropical areas a three-way division into hot, rainy, and cool season is used. In some parts of the world, special "seasons" are loosely defined based on important events such as a hurricane season, tornado season or a wildfire season.

Astronomical The precise timing of the seasons[9] as viewed by astronomers is determined by the exact times of transit of the sun over the tropics of Cancer and Capricorn for the solstices and the times of the sun's transit over the equator for the equinoxes. [10] For 2011 these times are: Equinoxes

Solstices

Mar 20 @2321 UTC June 21 @1716 UTC Sept 23 @0905 UTC Dec 22 @0530 UTC

The following diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December.

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In astronomical reckoning, the solstices and equinoxes ought to be the middle of the respective seasons, but, because of thermal lag, regions with a continental climate often consider these four dates to be the start of the seasons as in the diagram, with the cross-quarter days considered seasonal midpoints. The length of these seasons is not uniform because of the elliptical orbit of the earth and its different speeds along that orbit.[11] From the March equinox it takes 92.75 days until the June solstice, then 93.65 days until the September equinox, 89.85 days until the December solstice and finally 88.99 days until the March equinox. In North America and most of Europe the educational systems and media consider the astronomical seasons "official" over all other reckonings. Because of the differences in the Northern and Southern Hemispheres, it is no longer considered appropriate to use the northern-seasonal designations for the astronomical quarter days. The modern convention for them is: March Equinox, June Solstice, September Equinox and December Solstice. The oceanic climate of the Southern Hemisphere produces a shorter temperature lag, so the start of each season is usually considered to be several weeks before the respective solstice or equinox in this hemisphere, in other countries with oceanic climates, and in cultures with Celtic roots.

Ecological seasons Ecologically speaking, a season is a period of the year in which only certain types of floral and animal events happen (e.g.: flowers bloom—spring; hedgehogs hibernate—winter). So, if we can observe a change in daily floral/animal events, the season is changing. Hot regions

Hot regions have two seasons: •

Rainy season

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•

Dry season

Temperate areas

Six seasons can be distinguished. Mild temperate regions tend to experience the beginning of the hibernal season up to a month later than cool temperate areas, while the prevernal and vernal seasons begin up to a month earlier. For example, prevernal crocus blooms typically appear as early as February in mild coastal areas of British Columbia, the British Isles, and western and southern Europe. The actual dates for each season vary by climate region and can shift from one year to the next. Average dates listed here are for cool temperate climate zones in the Northern Hemisphere: •

Prevernal (ca.1 March–1 May)

•

Vernal (ca.1 May–15 June)

•

Estival (ca.15 June–15 August)

•

Serotinal (ca.15 August–15 September)

•

Autumnal (ca.15 September–1 November)

•

Hibernal (ca.1 November–1 March)

Cold regions

There are again only two seasons: •

Polar Day (spring and summer)

•

Polar Night (autumn and winter)

Traditional season divisions Traditional temperate seasonal changes on a city road in Manchester, UK:

Spring

Summer

Autumn

Winter

Traditional seasons are reckoned by insolation, with summer being the quarter of the year with the greatest insolation and winter the quarter with the least. These seasons begin about four weeks earlier than the meteorological seasons and 7 weeks earlier than the astronomical seasons. In traditional reckoning, the seasons begin at the cross-quarter days. The solstices and equinoxes are the midpoints of these seasons. For example, the days of greatest and least insolation are considered the "midsummer" and "midwinter" respectively. This reckoning is used by various traditional cultures in the Northern Hemisphere, including East Asian and Irish cultures.[citation needed] In Iran, Afghanistan and some other parts of Middle East the beginning of the astronomical spring is the beginning of the new year which is called Nowruz.

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So, according to traditional reckoning, winter begins between 5 November and 10 November, Samhain, 立冬 (lìdōng or rittou); spring between 2 February and 7 February, Imbolc, 立春 (lìchūn or risshun); summer between 4 May and 10 May, Beltane, 立 夏 (lìxià or rikka); and autumn between 3 August and 10 August, Lughnasadh, 立秋 (lìqiū or risshū). The middle of each season is considered Mid-winter, between 20 December and 23 December, 冬至 (dōngzhì or touji); Mid-spring, between 19 March and 22 March, 春分 (chūnfēn or shunbun); Mid-summer, between 19 June and 23 June, 夏至 (xiàzhì or geshi); and Mid-autumn, between 21 September and 24 September, 秋分 (qiūfēn or shūbun).

Earth's Water Cycle Water is always on the move. Rain falling where you live may have been water in the ocean just days before. And the water you see in a river or stream may have been snow on a high mountaintop. Water can be in the atmosphere, on the land, in the ocean, and even underground. It is recycled over and over through the water cycle. In the cycle, water changes state between liquid, solid (ice), and gas (water vapor).Most water vapor gets into the atmosphere by a process called evaporation. This process turns the water that is at the top of the ocean, rivers, and lakes into water vapor in the atmosphere using energy from the Sun.

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Water vapor can also form from snow and ice through the process of sublimation and can evaporate from plants by a process called transpiration. The water vapor rises in the atmosphere and cools, forming tiny water droplets by a process called condensation. Those water droplets make up clouds. If those tiny water droplets combine with each other they grow larger and eventually become too heavy to stay in the air. Then they fall to the ground as rain, snow, and other types of precipitation.

Most of the precipitation that falls becomes a part of the ocean or part of rivers, lakes, and streams that eventually lead to the ocean. Some of the snow and ice that falls as precipitation stays at the Earth surface in glaciers and other types of ice. Some of the precipitation seeps into the ground and becomes a part of the groundwater.Water stays in certain places longer than others. A drop of water may spend over 3,000 years in the ocean before moving on to another part of the water cycle while a drop of water spends an average of just eight days in the atmosphere before falling back to Earth.

Cloud and Cloud formation A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology. Two processes, possibly acting together, can lead to air's becoming saturated: cooling the air or adding water vapor to the air. In general, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface. Clouds are formed in Earth's atmosphere when water evaporates into vapor from oceans, lakes, and ponds or by evapotranspiration over moist areas of Earth's land surface. The vapor rises up into colder areas of the atmosphere due to convective, orographic, or frontal lifting. The water vapor attaches itself to condensation nuclei which could be anything from dust to microscopic particles of salt and debris. Once the vapor has been

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cooled to saturation, the cloud becomes visible. All weather producing clouds form in the troposphere, the lowest major layer of the atmosphere. However very small amounts of water vapor can be found higher up in the stratosphere and mesosphere and may condense into very thin clouds if the air temperatures are sufficiently cold. One branch of meteorology is focused on the study of nephology or cloud physics. Tropospheric clouds can be divided into three main categories with names based on Latin root words that indicate physical structure and process of formation. Clouds of the cirriform category are generally thin and occur mostly in the form of filaments. The other two categories are stratiform with clouds that are mostly sheetlike in structure, and cumuliform that appear heaped, rolled, and/or rippled.[1] In the troposphere, nine of the ten genus types are derived by cross-classifying the three categories into four families defined by altitude range; high, middle, low, and moderate vertical. Each of these families includes one stratiform and one cumuliform genus. Cirriform clouds differ in that they are only found in the high altitude family as a third member, and therefore only constitute a single genus cirrus. High stratiform and cumuliform clouds carry the prefix cirro which yield the genera cirrostratus and cirrocumulus. Middle cloud genera have the prefix alto (altostratus and altocumulus) to distinguish them from the high clouds, while low altitude stratiform and cumuliform genera (stratus and stratocumulus) carry no height-related prefixes. The fourth family comprises stratiform and cumuliform genera of moderate vertical extent (nimbostratus and cumulus) that form in the low or middle altitude range. This group also has no height-related prefixes, but its stratiform genus carries the prefix nimbo to denote its ability to produce widespread precipitation.

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A fifth family or sub-family of towering vertical clouds comprises only cumuliform types. One is cumulonimbus, the tenth genus type, and the other is cumulus congestus, a towering species of the genus cumulus whose other species belong to the family of moderate vertical clouds. All cloud genera except nimbostratus are divided into species and/or varieties based on specific physical characteristics of the clouds, but the cumulus genus is the only one that has species in two different altitude families.

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The essentials of the modern nomenclature system for tropospheric clouds were proposed by Luke Howard, a British manufacturing chemist and an amateur meteorologist with broad interests in science, in an 1802 presentation to the Askesian Society. Since 1890, clouds are classified and illustrated in cloud atlases. Clouds that form above the troposphere have a generally cirriform structure, but are not given Latin names based on that characteristic. Polar stratospheric clouds form at very high altitudes in polar regions of the stratosphere. They are given the name Nacreous due to the mother-of-pearl colors that are typically seen, and are sub-classified alpha-numerically according to their chemical makeup. Polar mesospheric clouds are the highest in the atmosphere and are given the Latin name noctilucent which refers to their illumination during deep twilight. They are sub-classified alphanumerically according to specific details of their cirriform physical structure.

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Following the official classification of the World Meteorological Organization (WMO) there are 10 different cloud types. These can be divided further into subspecies. Here you find a description of the 10 cloud types.

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Cirrus Form:thin fibers or threads, rarely also bundles; edges usually frayed by the high winds.

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Description:consists of ice crystals. Interpretation:Fair weather cloud; when compressed it can be a sign for a warm front (precipitation).

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Altitude: 8-12 km Precipitation: none

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Cirrocumulus Form:Heap cloud; occurs mostly in more or less expanded fields, which consist of small granular cloud parts, rarely also in small ripped to pieced bundles. Description:consists almost exclusively of ice crystals; strongly

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undercooled water drops will mostly freeze inside the cloud. Interpretation: indicate strong vertical movement in the altitude in which they form.

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Altocumulus Form: Layer cloud; occurs either as a fibrous veil in which thin stripe can form, or as a veil-like fog; it can never completely cover the sun. Under certain conditions, these clouds produce a "halo" around the sun, caused by the refraction of the sunlight.

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Description:consists primarily of small ice particles. Interpretation: Indicate the arrival of a warm front (with precipitation) within 1 to 2 days. Altitude: 8-12 km Precipitation: none

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Altocommulus lenticularis ~

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Form: Thread medium high layer cloud without contours. Description: Composed of to ice-crystals as well as water droplet.s Interpretation: Indicator for precipitation within next few hours.

Altostratus ~

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Altitude: 2-8 km Precipitation: rain or snow

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Stratocumulus Form: Layer cloud; appears in spots, fields or layers which aggregate into steadily arranged clods, bales or rolls. Description: Consists primarily of water droplets; these are the most

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frequent clouds; often have grey colouring, because the water droplets absorb a lot of light. Altitude: 0,6-2 km Precipitation: sometimes rain or snow

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Form: Misty layer cloud; absolutely without structure. Description: Consists of small water droplets; it can generate halos;

Stratus ~

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often originates with high pressure and low air movement. Interpretation: Generally indicates a rather quiet weather condition. Altitude: 0-2 km Precipitation: often sprinkling of rain

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Form: Cumulus are thick heap clouds sharply separated from each other; the edges sometimes look tattered and change constantly. Description: Consists almost exclusively of water droplets; only at low temperatures ice-crystals can appear; originate with locally restricted upward wind; for gliders and pilots, cumuli are an indicator for upward winds.

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Interpretation: Nice weather cloud. If the cloud reaches the medium levels of the atmosphere and is turning into a Cumulonimbus cloud, light showers may arrive. Altitude: 0,6-2 km Precipitation: seldom

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Nimbostratus

Form: Very vast, dark grey layer; strong vertical expansion Description: Consists of water droplets and/or ice-crystals; originates

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from the upward movement of moist air moving within a warm front. Interpretation: Long-term rain / snow about several hours or days.

Altitude: 0,6-12 Km Precipitation: rain or snow

Cumulonimbus

however, primarily in the upper parts; originates from a big Cumuluswolke which if it owns enough humidity and elevation impulse, spreads out upwards, later the upper one spreads distribute to the cloud horizontally further, Sodas the so-called "anvil" originates Interpretation: From Cumulonimbi precipitation falls in the form of

Form: very big heap cloud with the massive vertical expansion which originates from a Cumulus cloud Description: exists of water droplet and ice-crystals which seem,

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rain, hail or snow, often are present also thunderstorm; a full-grown cloud can take up up to 100 million tonnes of water, hence, violent showers and hail can fall, moreover, are to be calculated with Cumulonimbi on violent hoists which can reach a speed from up to

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120 km/h. Also within the cloud there is strong turbulence, so that they themselves can become dangerous for big airplanes and should be also therefore flew around

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Meteorological Phenomena Ice Formation

Waterspout

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Supercell

Cyclone

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Dust Storm

Fog

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Wind Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in our solar system occur on Neptune and Saturn. In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termed squalls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, hurricane, and typhoon. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Within the tropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds. Wind is caused by differences in pressure. When a difference in pressure exists, the air is accelerated from higher to lower pressure. On a rotating planet, the air will be deflected by the Coriolis effect, except exactly on the equator. Globally, the two major driving factors of large-scale winds (the atmospheric circulation) are the differential heating between the equator and the poles (difference in absorption of solar energy leading to buoyancy forces) and the rotation of the planet. Outside the tropics and aloft from frictional effects of the surface, the large-scale winds tend to approach geostrophic balance. Near the Earth's surface, friction causes the wind to be slower than it would be otherwise. Surface friction also causes winds to blow more inward into low pressure areas.[1]

Wind force scale Historically, the Beaufort wind force scale provides an empirical description of wind speed based on observed sea conditions. Originally it was a 13-level scale, but during the 1940s, the scale was expanded to 17 levels.[17] There are general terms that differentiate winds of different average speeds such as a breeze, a gale, a storm, tornado, or a hurricane. Within the Beaufort scale, gale-force winds lie between 28 knots (52 km/h) and 55 knots (102 km/h) with preceding adjectives such as moderate, fresh, strong, and whole used to differentiate the wind's strength within the gale category.[18] A storm has winds of 56 knots (104 km/h) to 63 knots (117 km/h). [19] The terminology for tropical cyclones differs from one region to another globally. Most ocean basins use the average wind speed to determine the tropical cyclone's category. Below is a summary of the classifications used by Regional Specialized Meteorological Centers worldwide:

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General wind classifications

Tropical cyclone classifications (all winds are 10-minute averages)

10Beaufo minute N Indian rt sustain General Ocean scale[1 ed term[20] IMD 7] winds (knots)

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Super cyclonic storm

intense tropical cyclone

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typhoon Major hurricane (5) (5)

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Tropics The trade winds (also called trades) are the prevailing pattern of easterly surface winds found in the tropics towards the Earth's equator.[26] The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. [27] The trade winds act the steering flow for tropical cyclones that form over world's oceans.[28] Trade winds also steer African dust westward across the Atlantic Ocean into the Caribbean Sea, as well portions of southeast North America.[29]

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A monsoon is a seasonal prevailing wind that lasts for several months within tropical regions. The term was first used in English in India, Bangladesh, Pakistan, and neighboring countries to refer to the big seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest bringing heavy rainfall to the area.[30] Its poleward progression is accelerated by the development off a heat low over the Asian, African, and North American continents during May through July, and over Australia in December.[31][32][33] The Westerlies or the Prevailing Westerlies are the prevailing winds in the middle latitudes between 35 and 65 degrees latitude. These prevailing winds blow from the west to the east to the north of the subtropical ridge, [34][35] and steer extratropical cyclones in this general manner. The winds are predominantly from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere.[27] They are strongest in the winter when the pressure is lower over the poles, and weakest during the summer and when pressures are higher over the poles.[36] Together with the trade winds, the westerlies enabled a round-trip trade route for sailing ships crossing the Atlantic and Pacific Oceans, as the westerlies lead to the development of strong ocean currents on the western sides of oceans in both hemispheres through the process of western intensification.[37] These western ocean currents transport warm, sub tropical water polewards toward the polar regions. The westerlies can be particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, which slows the winds down. The strongest westerly winds in the middle latitudes are within a band known as the Roaring Forties, between 40 and 50 degrees latitude south of the equator.[38] The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents,[39][40] especially in the southern hemisphere because of its vast oceanic expanse.

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Fronts Air masses create weather as they are moved by winds around the globe. Fronts develop at the boundary where two air masses with different temperatures—and, usually, different humidities—come into contact with each other. The term front was suggested by the Bjerkneses because the collision of two air masses reminded them of battlefront during a military operation. Cold fronts. A cold front develops when a cold air mass moves into an area occupied by a warm air mass. Because cold air is heavier or more dense than warm air, the cold air mass moves under the warm air mass. Cold fronts are usually accompanied by a decrease in air pressure and the development of large cumulus and cumulonimbus clouds that bring rain showers and thunderstorms. Rainfall and winds

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are most severe along the boundary between the two air masses. Cold fronts are represented on weather maps by solid lines with solid triangles. The direction in which the triangles point shows the direction in which the cold front is moving. Warm fronts. A warm front develops when a warm air mass approaches and then slides up and over a cold air mass. As the warm air mass comes into contact with the cold air mass, it is cooled and some of the moisture held within it condenses to form clouds. In most cases, the first clouds to appear are high cirrus clouds. Some time later, lower-level stratus and nimbostratus clouds form, usually bringing widespread rainfall. Warm fronts are designated on weather maps by solid lines with solid half circles. The direction in which the half circles point shows the direction in which the warm front is moving. Occluded fronts. A more complex type of front is one in which a cold front overtakes a slower-moving warm front. When that happens, the cold air mass behind the cold front eventually catches up and comes into contact with the cold air mass underneath the warm front. The boundary between these two cold air masses is an occluded front. Clouds form along this boundary, usually resulting in steady and moderate rainfall. An occluded front is represented on a weather map by means of a solid line that contains alternating triangles and half circles on the same side of the line. Stationary fronts. In some instances, the collision of two air masses results in a stand-off. Neither mass is strong enough to displace the other, and essentially no movement occurs. The boundary between the air masses in this case is known as a stationary front and is designated on a weather map by a solid line with triangles and half circles on opposite sides of the line. Stationary fronts are often accompanied by fair, clear weather, although some light precipitation may occur.

Cyclone In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth.[1][2] This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. A cyclone is a synonym for hurricane . Most large-scale cyclonic circulations are centered on areas of low atmospheric pressure.[3][4] The largest low-pressure systems are cold-core polar cyclones and extratropical

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cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size.[5][6] Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the Tropical Upper Tropospheric Trough during the summer months in the Northern Hemisphere. Cyclones have also been seen on extraterrestrial planets, such as Mars and Neptune.[7][8]

Structure There are a number of structural characteristics common to all cyclones.The cyclones have high pressure outside and low pressure inside. A cyclone is a low pressure area.[13] A cyclone's center (often known in a mature tropical cyclone as the eye), is the area of lowest atmospheric pressure in the region.[13] Near the center, the pressure gradient force (from the pressure in the center of the cyclone compared to the pressure outside the cyclone) and the force from the Coriolis effect must be in an approximate balance, the cyclone would collapse on itself as a result of the difference in pressure.[14]

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Because of the Coriolis effect, the wind flow around a large cyclone is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.[15] In the Northern Hemisphere, the fastest winds relative to the surface of the Earth therefore occur on the eastern side of a northward-moving cyclone and on the northern side of a westward-moving one; the opposite occurs in the Southern Hemisphere.[16] (The wind flow around an anticyclone, on the other hand, is clockwise in the northern hemisphere, and counterclockwise in the southern hemisphere.)

Six Main Types of Cyclones • Polar cyclone A polar, sub-polar, or Arctic cyclone (also known as a polar vortex)[26] is a vast area of low pressure which strengthens in the winter and weakens in the summer.[27] A polar cyclone is a low pressure weather system, usually spanning 1,000 kilometres (620 mi) to 2,000 kilometres (1,200 mi), in which the air circulates in a counterclockwise direction in the northern hemisphere, and a clockwise direction in the southern hemisphere. In the Northern Hemisphere, the polar cyclone has two centers on average. One center lies near Baffin Island and the other over northeast Siberia.[26] In the southern hemisphere, it tends to be located near the edge of the Ross ice shelf near 160 west longitude.[28] When the polar vortex is strong, westerly flow descends to the Earth's surface. When the polar cyclone is weak, significant cold outbreaks occur.[29]

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• Polar low A polar low is a small-scale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach −45 °C (−49 °F) move over open waters, deep convection forms which allows polar low development to become possible.[30] The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s.[31]

• Extratropical An extratropical cyclone is a synoptic scale low pressure weather system that has neither tropical nor polar characteristics, being connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".[32] The descriptor "extratropical" refers to the fact that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where extratropical transition has occurred,[32][33] and are often described as "depressions" or "lows" by weather forecasters and the general public. These are the everyday phenomena which along with anti-cyclones, drive the weather over much of the Earth. Although extratropical cyclones are almost always classified as baroclinic since they form along zones of temperature and dewpoint gradient within the westerlies, they can sometimes become barotropic late in their life cycle when the temperature distribution around the cyclone becomes fairly uniform with radius.[34] An extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone, if it dwells over warm waters and develops central convection, which warms its core.[10]

• Subtropical A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form between the equator and the 50th parallel.[35] As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical cyclones or extratropical cyclones, and used terms such as quasi-tropical and semi-tropical to describe the cyclone hybrids. [36] By 1972, the National Hurricane Center officially recognized this cyclone category.[37] Subtropical cyclones began to receive names off the official tropical cyclone list in the Atlantic Basin in 2002.[35] They

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have broad wind patterns with maximum sustained winds located farther from the center than typical tropical cyclones, and exist in areas of weak to moderate temperature gradient.[35]

Since they form from initially extratropical cyclones which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required for their formation are lower than the tropical cyclone threshold by three degrees Celsius, or five degrees Fahrenheit, lying around 23 degrees Celsius.[38] This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season.

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Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.[39]

• Tropical A tropical cyclone is a storm system characterized by a low pressure center and numerous thunderstorms that produce strong winds and flooding rain. A tropical cyclone feeds on heat released when moist air rises, resulting in condensation of water vapour contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows, leading to their classification as "warm core" storm systems.[10] The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in Maritime Tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on their location and strength, tropical cyclones are referred to by other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply as a cyclone. A tropical cyclone is generally referred to as a hurricane (from the name of the ancient Central American deity of wind, Huracan) in the Atlantic basin, and a cyclone in the Indian Ocean and parts of the Pacific.

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge.[40] They develop over large bodies of warm water,[41] and lose their strength if they move over land.[42] This is the reason coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions.[43] They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere. Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system

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deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses;[10] From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.[44]

• Mesocyclone A mesocyclone is a vortex of air, 2.0 kilometres (1.2 mi) to 10 kilometres (6.2 mi) in diameter (the mesoscale of meteorology), within a convective storm.[45] Air rises and rotates around a vertical axis, usually in the same direction as low pressure systems in both northern and southern hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a supercell.[46] Such storms can feature strong surface winds and severe hail. Mesocyclones often occur together with updrafts in supercells, where tornadoes may form. About 1700 mesocyclones form annually across the United States, but only half produce tornadoes.[11]

Typhoons in the Philippines In the Philippines, tropical cyclones (typhoons) are called bagyo.[1] Tropical cyclones entering the Philippine area of responsibility are given a local name by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), which also raises public storm signal warnings as deemed necessary.[2][3] Around 19 tropical cyclones or storms enter the Philippine Area Of Responsibility in a typical year and of these usually 6 to 9 make landfall.[4][5]

Public Storm Warning Signals Signal #1

winds of 30–60 km/h (20-35 mph) are expected to occur within 36 hours

Signal #2

winds of 60–100 km/h (40-65 mph) are expected to occur within 24 hours

Signal #3

winds of 100–185 km/h, (65-115 mph) are expected to occur within 18 hours.

Signal #4

winds of at least 185 km/h, (115 mph) are expected to occur within 12 hours.

The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) releases tropical cyclone warnings in the form of Public Storm Warning Signals.[3] An area having a storm signal may be under: • PSWS #1 - Tropical cyclone winds of 30 km/h (19 mph) to 60 km/h (37 mph) are expected within the next 36 hours. (Note: If a tropical cyclone forms very close to the area, then a shorter lead time is seen on the warning bulletin.) • PSWS #2 - Tropical cyclone winds of 60 km/h (37 mph) to 100 km/h (62 mph) are expected within the next 24 hours. • PSWS #3 - Tropical cyclone winds of 100 km/h (62 mph) to 185 km/h (115 mph) are expected within the next 18 hours. • PSWS #4 - Tropical cyclone winds of greater than 185 km/h (115 mph) are expected within 12 hours. These storm signals are usually raised when an area (in the Philippines only) is about to be hit by a tropical cyclone. As a tropical cyclone gains strength and/or gets nearer to an area having a storm signal, the warning may be upgraded to a higher one in that particular area (e.g. a signal #1 warning for an area may be increased to signal #3). Conversely, as a tropical cyclone weakens and/or gets farther to an area, it may be downgraded to a lower signal or may be lifted (that is, an area will have no storm signal). Classes for preschool are canceled when Signal #1 is in effect. High school classes and below are canceled under Signal #2 and classes for colleges and universities and below are canceled under Signal #3.

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The following terms are in common use when reference is made to a T.R.S.: • PATH: The direction in which the storm is moving. • TRACK: The area, which the storm centre has traversed. • STORM FIELD: The horizontal area covered by the cyclone conditions of the storm. • SOURCE REGION: The region where the storm first forms. • VERTEX: The furthest westerly point reached by the storm centre. • EYE OF THE STORM: The storm centre. • BAR OF THE STORM: The advancing edge of the storm field. • ANGLE OF INDRAUGHT: The angle, which the wind makes with the isobars. • VORTEX: The central calm of the storm. • DANGEROUS SEMI-CIRCLE: The half of the storm, which lies to the right of the path in the Northern Hemisphere and to the left of the path in the Southern Hemisphere. • DANGEROUS QUADRANT: The leading portion of the dangerous semicircle where • NAVIGABLE SEMI-CIRCLE: The half of the storm, which lies to the left of the path in the Northern Hemisphere and to the right of the path in the Southern Hemisphere. • TROUGH LINE: A line through the centre of the storm at right angles to the path. The dividing line between falling and rising pressure.

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Sources http://www.eoearth.org/article/Wind?vm=r http://en.wikipedia.org/wiki/Prevailing_wind?vm=r http://en.wikipedia.org/wiki/Cyclone?vm=r http://en.wikipedia.org/wiki/Weather?vm=r http://en.wikipedia.org/wiki/Climate?vm=r http://en.wikipedia.org/wiki/Season?vm=r http://thatscienceguy.files.wordpress.com/2011/02/cyclone.jpg http://thatscienceguy.files.wordpress.com/2011/02/storm-surge_21.jpg http://thatscienceguy.files.wordpress.com/2011/02/cyclone-map.gif http://en.wikipedia.org/wiki/Monsoon?vm=r http://en.wikipedia.org/wiki/Meteorology?vm=r http://en.wikipedia.org/wiki/Earth%27s_atmosphere http://en.wikipedia.org/wiki/List_of_cloud_types?vm=r http://www.superteacherworksheets.com/weather/cloud-types-article.pdf?vm=r http://www.crh.noaa.gov/lmk/soo/docu/cloudchart.pdf?vm=r http://www.aopa.org/asf/online_courses/skyspotter/clouds.pdf?vm=r http://nenes.eas.gatech.edu/Cloud/Clouds.pdf?vm=r http://www.lessonsnips.com/docs/pdf/meteorology.pdf?vm=r http://www.srh.noaa.gov/jetstream/ http://www.geog.ucsb.edu/~joel/g110_w08/lecture_notes/midlat_upper/midlat_upper.html http://schoolworkhelper.net/2011/05/atmospheric-circulation-models/ http://www.britannica.com/EBchecked/topic/41463/atmospheric-circulation http://www.scienceclarified.com/A-Al/Air-Masses-and-Fronts.html http://ffden-2.phys.uaf.edu/645fall2007_web.dir/Tapas_project/index-6.html http://www.takdangaralin.com/science/earth-science/the-water-cycle/ http://ga.water.usgs.gov/edu/watercycle.html http://www.metoffice.gov.uk/media/pdf/r/i/Cloud_types_for_observers.pdf http://www.metlink.org/pdf/articles/weather_tropics_7.pdf

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