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INTRODUCTION The weatherman just like any professional requires certain instruments to assist him in the conduct of his calling. He uses sophisticated instruments developed through the years. Little by little, due to advances in the science of meteorology and the advent of more sophisticated instruments, a forecaster is approaching the threshold where he can forecasts with confidence the weather for the following day and optimistically, a year later. The Philippines is not far behind developed countries in instrumentations, specially when one speaks of basic weather instruments. They are all the same the world over with slight differences in construction and gradation as dictated by geographical requirements. BASIC WEATHER PARAMETERS AND THE INSTRUMENTS USED The following weather parameters are the minimum requirements to effectively forecast weather. A brief description of the instruments that PAGASA uses accompanies the discussion of these weather parameters. Simplified illustrations given are practicable. TEMPERATURE The temperature is the degree of hotness or coldness of a certain body. In the Philippines, it is measured in degrees Celsius (0C). In weather forecasting, temperature (actual, surface and temperature ranges) are important as they give indications, to a certain extent, of the development and changes of weather conditions. Temperature change is one of the principal causes in changes of other basic weather elements. Temperature variations over lands and ocean result to a range of weather conditions from the gentlest breeze to the most violent storms. Temperature also affects the development and formation of clouds, the source of our precious water, when these clouds eventually fall as rain. It is, then, imperative that variations in temperature be considered in weather forecasting as they play an important part in the improvement or deterioration of weather conditions. Through modern instruments, actual temperature in the atmosphere and surface temperatures are obtained. Surface temperature is the temperature of free air at a height between 1.25 and 2.00 meters above the ground.
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The following instruments that measure temperature are commonly used: a.) Thermometer A thermometer (Fig. 1a) measures the degree of hotness or coldness of a given substance. It operates on the principle of thermal expansion of the material used, e.g. liquids like mercury and alcohol, metallic materials etc. Mercury is one of the liquids very sensitive to changes of temperature. When the substance to be measures is warm, mercury expands and rises in the capillary tube. When it cools, mercury contracts. b.) Maximum Minimum thermometer In order to measure the temperature range, a set of maximum and minimum thermometer (Fig. 1b) are used. A maximum thermometer has a constriction above the bulb that permits the mercury to rise in the capillary tube but does not allow it to descend the capillary tube unless the thermometer is reset. The highest point that the mercury reaches indicates the maximum temperature for the period. The minimum thermometer, on the other hand, gives the lowest temperature. It uses colored alcohol (because of its low freezing point). It is placed at an angle of about 200. The black float B called index (Fig. 1c) is pulled down slope to the lowest temperature of the day by two forces; a) the surface tension at the top of the alcohol column and b) the force of gravity. c.) Thermograph A thermograph (Fig 2) is an instrument that records air temperature continuously on graphing paper. It usually consists of a cylinder made to revolve once each week by means of clockworks inside. A sheet of graph paper is fastened on the outside. A pen point that rests on the paper traces the temperature curve, according to the expansion and contraction
of a sensitive metallic coil or strip corresponding to the reading of a thermometer. These instruments are housed in a thermometer shelter (Fig. 3) which has double-louvered sides and double-top roofing designed to permit air to circulated freely through the shelter.
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TEMPERATURE MEASURING DEVICES
Fig. 1c Fig 1c. Diagram showing the distinctive characteristics of the maximum-minim maximum-minimum thermometer
Fig. 1b. Maximum-Minimum thermometer mounted on a Townsend Support which clamps these in the proper position and allows for their setting
Fig. 1a. Ordinary Thermometer
Fig. 2. Thermograph
Fig. 3. Thermometer Shelter or Screen. In the Northern Hemisphere its door faces north to prevent the sun’s rays from directly affecting the intrument readings whenever it is opened
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ATMOSPHERIC PRESSURE Gas molecules exert forces on each other and their environmental they collide. The magnitude of these forces depends upon the temperature of the gas and the number of molecules involved. These collision forces are expressed in terms of quantity called pressure. Pressure difference is principally related to temperature differences and to the number of molecules exerting pressure forces. Atmospheric motion results from pressure variations. The atmospheric pressure on a given surface is the force exerted by an overlying column of air extending to the outer limit of the atmosphere per unit area. To measure atmospheric pressure, a barometer is used, which is commonly of two types. These are: a.) Mercurial Barometer Mercurial barometer A mercury barometer (Fig.4a) is a simple barometer made by filling a glass tube 32 inches long with mercury and inverting it so that the open end of the tube is below the surface of mercury in a cistern. The height of the mercury column is measured by sliding a vernier attached on a scale. To obtain accurate measurements, corrections are made for temperature expansion of the instruments, gravity and latitude. Values are read in millibars, millimeter or inches of mercury. b.) Aneroid Barometer An aneroid barometer (Fig. 4b) is made by exhausting the air from a thin, circular, metallic box, with practically no air on the inside and an air pressure of 14.6 pounds per square inch on the outside, the box would collapse except for a strong spring inside. If one side of the box is fixed, the other side will move due to changes in atmospheric pressure. The surface of the metallic box is corrugated to increase the area exposed to the air. The movement of the spring causes a pointer to move over a scale of figures corresponding to the readings of a mercury barometer. Since air pressure decreases with increase in altitude, the aneroid is used to make altimeters (Fig. 5) On the altimeter, the scales is marked off in hundred and thousands of feet or meters above sea level. The altimeter is a basic instrument in aeronautical stations and on board an aircraft. c.) Barograph A barometer (Fig. 5a), on the other hand, is a recording barometer. The pen point that traces the pressure curve on the paper is made to move up
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or down by means of a series of levers attached to the aneroid cells in tandem. The aneroid cells in tandem provide a more pronounced response to changes in atmospheric pressure than would be indicated by a single aneroid of the same size. WIND Wind is measured in terms of its velocity. Wind velocity has a vectorial notation and (usually) refers to both the speed and direction. Speed is the distance to which an object travels at a certain instant. Wind speed is usually expressed in meters per second (mps) and the more popular kilometers per hour (kph). On the other hand, wind direction refers to the direction of the compass point from where the wind is coming. Thus, when we say southwest winds, the wind is coming from the southwest and blowing towards the northwest.
Fig. 4b. Aneroid Barometer
Fig. 4a. Mercurial Barometer
ATMOSPHERIC PRESSURE MEASURING DEVICES
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Fig. 5. Altimeter
Fig. 5a. Barograph
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SURFACE WIND VELOCITY AND DIRECTION To accurately measure the wind speed and direction PAGASA uses several instruments: These are: a. Wind Vane A wind vane (Fig.6) is used to indicate wind direction. It consists basically of an asymmetrically shaped object with its center of gravity about a vertical axis. The front end of this object (in most cases as arrow) which officers the greater resistance to the motion of the air points to the direction from where the winds comes. The direction of the wind is determined by reference to an attached oriented compass rose. b. Anemometer An anemometer (fig. 6a) measures the wind speed and is made of propeller cups which are rotated by the motion of the wind. The essential parts of the cup anemometer are the cup wheel, a vertical shaft, the necessary mechanism for counting the revolution of the shaft or indicating its instantaneous speed of rotation. c. Aerovane An aerovane (Fig. 6b) indicates both the wind direction and wind speed or simply the wind velocity. It is shaped like an airplane. The nose of the plane ports to the direction from which the wind comes and the two-bladed propeller measures the wind speed. The propeller shaft is coupled to a small dynamo which generates current. The amount of current generated depends on the rate of rotation of the propeller which depends on the speed of the wind. The generated current activates a dial which gives the direct reading of the wind speed. d. Anemographs Anemograph gives direct record of the variations of wind velocity.
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Fig. 6a. Rotating Cup Anemograph
Fig. 6. Windvane
Fig.6b. An aerovane with its component, a wind indicator.
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ATMOSPHERIC HUMIDITY Humidity is the amount of water vapor or moisture content of the air. The amount of water vapor in the air affects human comfort. When the air is very moist or has high humidity, evaporation is very slow so much so that perspiration remains on the surface of the skin. This makes a person feel warm and uncomfortable. Humidity measurement is a useful parameter for weather forecasting in determining whether or not it will rain. To measure humidity, the following instruments are used: a. Sling Psychrometer The sling psychrometer (Fig. 7a) consists of a dry and wet-bulb thermometer. The term bulb refers to that portion of the glass tube where the mercury is stored. The dry and wet bulbs are exactly alike in construction. The only difference is that the wet-bulb has a piece of muslin cloth or wick wrapped around its bulb and which is dipped in water shortly before the psychrometer is read. This is how it is done. The weather observer first wets the cloth cladding the wet-bulb, whirls the psychrometer a few times, then reads the wet bulb. He reads the dry-bulb last. Normally, the wet-bulb reading will be lower than the dry-bulb’s. The dry-bulb reading is the air temperature. The difference between the dry and wet-bulb reading will give, with the aid of psychrometric table, the dew point temperature and the relative humidity. (Dew point temperature at which the water will condense while relative humidity is the ratios of the amount of water vapor actually present in the air to the maximum amount of water vapor the air can hold at a given temperature. b. Hygrometer The other instrument used to measure humidity is the hygrometer (Fig. 7b). The hygrometer is less accurate than the psychrometer. It uses human air from which the oil has been removed by using ether. The hair becomes longer as the relative humidity of the air increases. This change can be made to move an indicator needle which moves over a scale, the graduations of which reads from 0% to 100%.
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HUMIDITY MEASURING DEVICES
Fig. 7a. Sling Psychometer
Fig. 7b. Hygrometer
Fig. 7b. Hygrothermograph
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c. Hygrothermograph The hygrothermograph (Fig. 7c) measures and records relative humidity and temperature on graph paper in the same manner as the thermograph and barograph do. PRECIPITATION When the water vapor in the air aloft cools, it is transformed into water droplets that form the cloud we see in the sky. When these water droplets become large and heavy enough that the air could no longer support them, the water droplets eventually fall as rain, snow, sleet or hail. Rainfall is one such results of precipitation process.
a.) measuring stick
Fig. 8. An 8-inch raingauge and its parts. a.) measuring stick; b.) tube; c.) receiver and d.) overflow can.
PRECIPITATION MEASURING DEVICES
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To measure the amount of rainfall, raingauge is used. There are two types of raingauge used by PAGASA. The 8-inch raingauge and the tipping bucket raingauge. a. 8-inch Raingauge An 8-inch raingauge (Fig. 8), so called because the inside diameter of the receiver is exactly 8 inches, is provided with a funnel that conducts rain into a cylindrical measuring tube. The volume of the receiver is 10 times the volume of the measuring tube. Therefore the actual depth of rainfall is increased ten times on being collected in the smaller measuring tube. To measure the amount of rainfall accumulated in the measuring tube, a thin measuring stick with the magnified scale printed on its face is used. The precisely dimensioned measuring tube has a capacity of 2 inches (50.8 millimeters). Rainfall exceeding this amount spills into the overflow but can be easily measured by pouring it into the measuring tube for total rainfall. Used this way, the gauge has a total capacity of 20 inches. b. Tipping Bucket Raingauge Another type of rainfall recording instrument is the tipping-bucket raingauge (Fig. 9). It is an upright cylindrical that has a funnel-shaped receiver. The precipitation collected by the receiver empties into one side of a “tipping bucket”, an inverted triangular contraption partitioned transversely at its center, and is pivoted about a horizontal axis. Once it is filled with rain, it tips, spilling out water and placing the other half of the bucket under the funnel. The tipping activates a mercury switch causing an electrical current to move the pen in the recorder. Each tipping is equal to one millimeter of rainfall. CLOUDS Clouds are either composed of water-droplets or ice-crystals dependent upon their altitude and temperature conditions. In observing clouds, an accurate description of both type and size plays an important part in the analysis and forecasting of weather.
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Parts of an 80-inch raingauge
Fig. 9 Tipping Bucket Raingauge (and parts inside)
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Thus, for this purpose an International Classification of clouds was prepared and adopted by most countries. In observing cloudiness (the extent where clouds cover the sky), the observer uses his eyes to determine the presence of cloud layers and the lateral extent of cloud coverage. He must also be familiar with the genus and species of each cloud present. On the basis of knowledge and experience, he estimates the height of each layer or measure it with the aid of instruments. To determine the height of the cloud base, PAGASA uses a ceiling light projectorr (Fig. 10) and a ceiling balloon. a.) Ceiling Light Projector A ceiling light projector is vertically a narrow beam of light into a cloud base. The height of the cloud base is determined by using a clinometer located at a known distance from the projector to measure the angle included by the illuminated spot on the cloud, the observer, and the projector. From trigonometry, the height of the cloud base is equal to the distance of the observer from the ceiling light projector multiplied by the tangent of the elevation angle.
b.) Ceiling Balloon Another away of determining the height of the cloud base is by using a ceiling balloon. A ceiling balloon is a meteorological balloon whose rate of ascent has been predetermined. It is filled with gas lighter than air, usually hydrogen, and released. The time of release and the time the balloon disappears into the cloud are recorded. The time difference multiplied by the rate of ascent will give the height of the base cloud. SPECIAL INSTRUMENTS The instruments described earlier are tools for measuring weather elements prevailing at the “surface” or near the surface of the earth at a height not exceeding 10 meters from wherever the observers stands.
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Fig. 10. Ceiling Light Projector F
Fig. 10a. Ceiling Balloon
The art of weather forecasting however is never completed if the conditions of the air above us are not known. The weather forecaster needs to know the humidity, temperature, pressure, and speed direction at different levels of the atmosphere so that he could obtain a better picture of what the prevailing weather conditions are from the surface upwards.
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These data are called Upper-Air Data. Most of the marked weather changes and the resulting effects on our daily lives occur at levels higher that what we observe or feel near the surface of the earth. Some of the widely used instruments to obtain upper air data are as follows: a. PIBAL/Theodolite = Pilot balloon Theodolite b. Radiosonde; c. Rawinsonde; d. Rawin; e. Wind-Finding Radar; and f. Weather Surveillance Radar a. Pilot Balloon/Theodolite A pilot balloon (Fig. 11a) is a meteorological balloon that is filled with gas lighter than air. When the pilot balloon is used in conjunction with a theodolite it is used to determine the speed and direction of winds at different levels of the atmosphere. The theodolite (Fig. 10b) is similar to The elevation and angles of azimuth of the balloon are recorded and these data at the end of the flight which may last for more than an hour are transferred to a plotting board. The wind speed and direction at selected levels are calculated by trigonometric methods. Night observation is accomplished by attaching a lit paper lantern to the balloon. b.) Radiosonde An airborne instrument used for measuring pressure, temperature and relative humidity in the upper air is the radiosonde (Fig. 11). The instrument is carried aloft by a meteorological balloon inflated with hydrogen. The radiosonde has a built-in high frequency transmitter that transmits data from the radiosonde meter and recorded on the ground by a specially designed radiosonde receiver.
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Fig. 10a. Pilot Balloon
Fig. 10b. Theodolite
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Fig. 12. Radiosonde attached to a Meteorological balloon
c.) Rawinsonde A more sophisticated version of this instrument is the rawinsonde. The rawinsonde (Fig. 12) is an electronic device used for measuring wind velocity, pressure, temperature and humidity aloft. It is also attached to a balloon and as it rises through the atmosphere, it makes the required measurements. The data gathered are then converted to radio signals which are received by a receiving set on the ground where they are decoded and evaluated. d.) Rawin Another special instrument is the Rawin which is short for Radar and Wind. It is an electronic device that measures pressure, temperature and humidity. e.) Wind Finding Radar Another instrument is the Wind Finding Radar (Fig. 13). It determines the speed and direction of winds aloft by means of radar echoes. A radar target is attached to a balloon and it is this target that is tracked by ground radar.
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Fig. 12. Rawinsonde Antenna
Fig. 13. Wind Finding Radar Antenna
Fig. 14. Weather Surveillance Radar
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The bearing and time of interval of the echoes is evaluated by a receiver. f.) Weather Surveillance Radar A weather surveillance radar (Fig. 14) is of the long range type which detects and tracks typhoons and clouds masses at distance of 400 kilometers or less. This radar has a rotating antenna disk preferably mounted on top of a building free from any physical obstruction. Radio energy emitted by the transmitter and focused by the antenna shoots outward through the atmosphere in a narrow beam. The cloud mass, whenever it is part of a typhoon or not, reflects a small fraction of the energy back to the antenna. This reflected energy is amplified and displayed visually on a radar scope. The distance or slant range of the target from the radar is determined through the elapsed time to signal is transmitted and then received as an echo. Its direction is determined by the direction at which the focused beam is pointing at an instant the echo is received. WEATHER SATELLITE Modern Tool for Weather Analysis Polar-Orbiting Satellites The National Oceanic and Atmospheric Administration (NOAA) satellite system consists of satellites in polar orbit at 833 and 870 km. above the earth’s surface. Each satellite transmits data from a circular area of the earth’s surface with diameter of 2,800 kms. Inboth satellites, one of the sensors is the Advance Very High Resolution Radiometer (AVHRR) which is sensitive to visible near infrared and infrared radiation. This instrument is used for measuring cloud distribution and for determining temperature of radiating surface (clouds or surface). Another sensor is the TIROS Operational Vertical Sounder (TOVS) system which is used to calculate the temperature profiles from the surface to 10 mb, water vapor content at three levels of the atmosphere and total ozone content. Geo-stationary Meteorological Satellite The most valuable feature of Geostationary Meteorological Satellites (GMS) is that they can globally observe atmospheric phenomena uniformly, including overlying areas in sea, desert and mountain regions where weather observation is difficult.
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The GMS of Japan is a spin stabilized satellite that is placed in geosynchronous orbit about the equator and 140 degree longitude. The GMS provides a real time digital cloud image (Stretched-VISSR) broadcast to the users, which are the Medium Scale Data Utilization Station (MSDUS). The S-VISSR data can be processed not only by a high grade computer system but also by an ordinary personal computer system. PAGASA has both the GMS AVHRR, the NOAA polar orbiting satellite ground receiving facility. Both are located in Diliman, Quezon City. Satellite data coming from both the orbital and geo-stationary satellites are used for monitoring the development of severe weather systems, locating tropical cyclones centers, determining the cyclone’s present intensity and future movement and weather forecasting.
Fig. 15a. Geostationary Meteorological Satelite Antenna
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MODIS MODIS (Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra’s orbit around the Earth is timed so that it passes from north to south over the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS are viewing the entire Earth’s surface every 1 to 2 days, acquiring data in 36 spectral bands or groups of wavelengths. These data will improve our understanding of global dynamics and processes occurring on the land, in the oceans and in the lower atmosphere. MODIS is playing a vital role in the development of validated, global, interactive Earth system models able to predict global change accurately enough to assist policy makers in making sound decisions concerning the protection of our environment. The qualitative and quantitative estimates and display of atmospheric parameters and a few oceanographic elements from newly acquired NOAA HRPT Receiving Systems of PAGASA enables the agency to monitor, forecast and predict weather and climate and issue early warning of associated hazards. MODIS also provides finer horizontal-scale atmospheric vapor gradient estimates which is a valuable input in weather forecasting. Data derived from the system used to monitor flood inundation areas. Acquisition of this new technology strengthens PAGASA farm weather forecasting using data from the multi-spectral band imaging instrument.
Fig. 16. MODIS Satellite Reciever
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Multi-functional Transport Satellite (MTSAT) To improve meteorological services over a wide field of activity (such as weather forecasts, natural-disaster countermeasures and securing safe transportation), the MTSAT series replaced the GMS series that had been in operation since 1977. It has taken over the role of the GMS series, covering East Asia and the Western Pacific region from 140 degrees east above the equator. It also provides information to 27 countries and territories in the region, including imagery for monitoring the distribution/motion of clouds, sea surface temperatures, and distribution of water vapor. The MTSAT series carries a new imager with a new infrared channel (IR4) in addition to the four channels (VIS, IR1, IR2 and IR3) of the GMS-5. Its imagery is more effective than GMS-5 imagery in detecting low-level cloud/ fog and estimating sea surface temperatures at night and has enhanced brightness levels, enabling a whole new level of image imagery. By further computation of cloud imagery, data obtained by MTSAT’s observations can be used to calculate wind data for numerical weather prediction; make nephanalysis charts and analyze the distribution of cloud amounts according to area. The Imager scans the earth by moving an internal scan mirror in an east-west and north-south direction. The light reflected by the mirror is converted into a beam and channeled through a system of lenses and filters and is separated into one visible and four infrared channels. The beam intensities are converted to electric signals by visible and infrared detectors and these signals are transmitted to the Meteorological Satellite Center’s Command and Data Acquisition Station (CDAS). PAGASA’s weather forecasting has significantly improved with the availability of high resolution satellite imageries both from the MTSAT and MODIS installed at the Weather and Flood Forecasting Center (WFFC) Building in Quezon City. A redundant Meteorological Satellite High Resolution Imaging (MTSAT-HRIT) was also installed at Cebu PAGASA Complex Station.
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Fig. 17. MTSAT Satellite Receiver Facility
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“tracking the sky... helping the country”
Department of Science and Technology
PHILIPPINE ATMOSPHERIC, GEOPHYSICAL AND ASTRONOMICAL SERVICES ADMINISTRATION Science Garden, Agham Road, Diliman, Q.C. www.pagasa.dost.gov.ph email:
[email protected] Telefax: 434-2696 / 927-9308
PAGASA Synoptic Station
Department ment of Science and Te Technology
PHILIPPINE ATMOSPHERIC, GEOPHYSICAL AND ASTRONOMICAL SERVICES ADMINISTRATION (PAGASA) Public Information and International Affairs Staff Revised 2007
