Weather1

Weather

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Does anyone here watch the weather channel?

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When you see your local forecast do you really know what you’re seeing?

You Will!

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What types of reported weather do you see on the weather channel?

Weather Predictions 

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The farther into the future you try to predict the less accurate you will be. Weather is the daily account of temperature + moisture.

Weather 

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Most weather occurs in troposphere Due to our tilt, variations in insolation cause heat energy to be unevenly distributed The movement of heat energy results in changes in the atmosphere that are the cause of WEATHER

Much information is needed to interpret what is happening with the atmosphere.

Need to condense this information in a small space

Station Model Allows the data to be plotted in a condensed and usable format

Location of the station on the map.

The name of the location or airport abbreviation might be given nearby.

Temperature information in degrees Fahrenheit.

Temp

Dew Point Temperature in degrees Fahrenheit. Temp

dew point

Wind direction A staff is rotated around the station.

Temp dewpt Remember, winds are identified as the direction FROM which they come. This would be a west northwest wind or about 290º.

Wind Speed Attached to the staff are symbols for speed.

Temp dewpt

The coding for the wind speed symbols can be generalized as: half staff about 5 knots note, it is not on the end of the wind direction staff

full staff about 10 knots these are additive for the total amounts three full staffs=30 knots

Pressure – weight of air DIFFERENCES IN AIR PRESSURE ARE CAUSED BY UNEQUAL HEATING OF EARTH’S SURFACE

PRIMARILY

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Increase density, increase pressure

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Increase temperature, decrease pressure and density

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Increase altitude, decrease pressure

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Increase water vapor content, decrease pressure

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Measured by a barometer Isobars = lines of equal pressure

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Barometric (Air) Pressure 

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Pressure exerted by airs weight Heavy Air – high pressure Light Air – low pressure

Barometer 

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The instrument used to measure air pressure Measured in units called millibars (mb) or inches

ISOBARS ARE ISOLINES OF EQUAL PRESSURE

Air pressure and Temperature 

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What direction does warm air tend to move? Thing of a hot air balloon…….Why does it go up.

Warm Air 

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When the air warms up, it rises because the density of the air is decreasing. If the air rises it creates low pressure because the air is not pushing down on the earth.

Cold Air 

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When gasses get colder, they begin to condense, get smaller. Because of this their densities go up and the air begins to fall. Cold air causes high pressure because the air is falling to the earth.

Increase temperature – decrease pressure

PRESSURE

T

Pressure and Density Decrease with ALTITUDE

Air Pressure and Altitude 

The higher up you go, the less amount of air is sitting on top of you, meaning that there is less air pressure.

Air movement 

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What causes the wind to blow? Wind is caused by differences in   

Air temp Air Density Air Pressure

Wind from Temperature 

As Hot air rises, cooler air will “blow” in to replace the air that is rising.

Wind and Pressure 

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The wind will always blow from higher pressure to lower pressure Why does it do this? There are more air particles causing the higher pressure, so the sir moves to where there is more space.

Pressure Gradient 

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Def: rate of pressure change bewteen two locations When looking at a map with isobars. The strongest winds are where the lines are closest together because the gradient

What weighs more????

Pressure and Moisture

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Which is heavier, dry air or moist air???? Air is mainly made up of nitrogen (N2) which has a mass of 28g and oxygen (O2) which has a mass of 32g

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Water (H2O) has a mass of 18g.

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Because water is lighter than air, when the air has moisture in it, it has less mass which causes less pressure.

Water vapor replaces other air molecules that are heavier MOIST AIR WEIGHS LESS

WIND – Horizontal movement of air parallel to earth’s surface due to pressure differences 

Winds move from HIGH pressure to LOW pressure but Coriolis Effect kicks in and modifies direction

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Winds named for the direction from which they come Wind vane – measures direction Anemometer – measures wind speed

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Wind speed is DIRECTLY related to the pressure gradient Closer the isobars Greater the pressure gradient Greater the wind speed

Air Pressure (pg 14 ESRT) 

High Pressure  

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Lower Temp. Low Moisture in the air Lower Altitude

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Low Pressure   

High Temps. High Moisture in Air Higher Altitudes

Current Barometric Pressure

Temp dewpt

pressure

Barometric Pressure is VERY important! average barometric pressure at sea level is 1013.25 mb. current barometric pressure is reported to the nearest TENTH BUT, they don’t want to waste space

average barometric pressure at sea level is 1013.25 mb. barometric pressure varies around this value + about 30 = 1043 somewhat higher

1013.3 somewhat lower - about 30 = 983

average barometric pressure at sea level is 1013.25 mb. barometric pressure varies around this value they do not PRINT the beginning number or numbers

---1013.3

13.3 x

133

they, also, do not PRINT the decimal point, but report to the nearest TENTH

From the three numbers, you must INTERPRET whether the preceding value is 10 or 9 Which would make the value most realistic?

add the decimal point and then consider which, a 10 or a 9 places the value NEAREST to the average pressure of 1013.25 mb.

What is the pressure? 084

(1013.3)

08.4

1008.4

962

96.2

996.2

281

28.1

1028.1

875

87.5

987.5

322

32.2

1032.2

Not only is the pressure itself important, but so is the way the pressure has been changing.

So, more information may be given and must be coded.

How has the barometer been changing? Temp

pressure trend

dewpt + it is higher now than in the past - it is lower now than in the past

How much the pressure has changed

Temp

pressure trend

change

dewpt the amount of change in the past three hours again, reported in TENTHS without

What has the pressure been doing? Shown diagrammatically Temp

pressure trend

Dew point

change shown

Total amount of cloud

Temp

pressure trend

dewpt

changeshow n

graphic depiction of percentage of the sky covered by clouds

no clouds about 25% about 50%

about 75%

completely overcast

What is the present weather?

Temp

pressure trend

changeshow n

dewpt The current weather is very important.

A detailed set of symbols provides weather descriptions.

Samples of present weather symbols. fog haze

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thunderst orm drizzle lightni ng

rain

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snow

slight rain showers . ice or snow pellets cloud development not observed

The visibility is especially important for pilots.

Temp visibil ity dewpt

pressure trend

changeshow n

Visibility is given in miles and fractions

The past weather uses symbols.

Temp

trend

visibil ity dewpt

,

pressure

*

changeshow PAST n

The amount of precipitation

Temp visibil ity dewpt

pressure trend

changeshow n PAST ppt

amoun t Amount of precipitation is plotted in inches to the nearest hundredth of an inch.

Land/Sea Breeze 

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Because of the specific heat of water, we find that there are typical breezes during the day. Land Breeze occurs during the day when it is warmer on land then it is on the sea, so the wind blows from the sea to land Sea Breeze occurs at night when the sea is warmer then the land, causing the breeze to go from land to see

Factors Affecting Rates of Evaporation 

Increase Temperature – increase evaporation

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Increase Surface Area (the more spread out the water) – increase evaporation

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Increase wind speed – increase evaporation

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Increase saturation of the air with water vapor – decrease evaporation

How does moisture get into the atmosphere? Evaporation - liquid water changes state into water vapor Requires input of energy from sun – Heat energy is transferred from the atmosphere to the water vapor The water vapor absorbs energy - 540 calories. This added heat energy is being converted to a kind of potential energy – stored energy

How does moisture get returned to earth? 

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Condensation of water vapor– gas to liquid Clouds and precipitation Heat energy transferred from the water vapor to the atmosphere Releases energy gained from evaporation – 540 calories

RELATIVE HUMIDITY – given in % 

Does not tell you the amount of water vapor in the air – DOES TELL YOU HOW CLOSE IT IS TO BEING SATURATED

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SATURATION – filled to capacity with water vapor – the RH is 100%

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Relative Humidity DECREASES as Temperature INCREASES

Dew Point Temperature 

The temperature that the air is saturated – filled with water vapor

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Relative Humidity is 100%

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Air Temperature drops below dew point – CONDENSATION begins

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Dew Point Temperature is a measure of how much water vapor in the air Dew Point Temperature will rise as water vapor content of the air increases

Dew Point Temp 

Temperature at which condensation occurs (water vapor turns to liquid)

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When the dew point temperature = air temperature, this means that the air is saturated and it will rain. The dew point will always be less than or equal (