Weather

1. Weather

2. Weather • Weather is the local, short term atmospheric condition resulting from the interaction of certain variables. • All weather occurs in the troposphere • The weather in the United States generally moves from the southwest to the northeast.

3. Atmospheric Variables • There are five (5) constantly changing factors that cause weather: • Temperature • Air Pressure • Wind • Moisture • Atmospheric Transparency

4. 1. Temperature • The Earth’s surface heats up at different rates due to duration and intensity of insolation, land and water differences and altitude. • Temperature is a measure of how cold or hot something is.

5. 2. Air Pressure • Caused by the weight of the air, which varies due to the temperature, altitude and moisture content. • Warm air is less dense, therefore, it’s lighter. • The higher the altitude, the less air above, therefore, less pressure. • Water vapor is lighter than dry air. • Air pressure is measured with a barometer. • At sea level, air pressure is 14.7 lbs/in2 (one atmosphere) or 29.92 inches of mercury, or 1013.2 millibars.

6. 3. Wind • Movement of air horizontal to the Earth’s surface caused by differences in air pressure. • Wind blows from high pressure areas to low pressure areas. • The greater the difference between the air pressures of two areas, the higher the wind speed.

7. 4. Moisture • Water vapor enters the atmosphere by a process called evapotranspiration. • Evapo ~ comes from evaporation (liquid into gas) • Transpiration ~ when plants release water vapor into the atmosphere.

8. 5. Atmospheric Transparency • If the atmosphere was made out of clear, colorless, gases, it would be transparent. However, there are aerosols in the air. • The more aerosols, the less transparent; resulting in an increase in insolation absorbed or reflected by the atmosphere and the more chance of clouds and rain.

9. Atmospheric Temperature • Isotherms ~ Temperature data shown on a map by the use of isolines.

10. 6 Factors determine temperature: • Amount of insolation • Generally, the more insolation at a location, the warmer the temperature. • Amount of radiation • Terrestrial radiation is reradiated back into the atmosphere and heats it. • Condensation • Change of water vapor into liquid water. Releases a large amount of latent heat.

11. Sublimation • Change of water vapor into ice. Releases a large amount of latent heat. 5. Frictional Drag: • Due to the Coriolis Effect, the interface between the atmosphere and the Earth’s surface produces frictional heat. • Wind • Heat is moved around by the wind.

12. Adiabatic Heating and Cooling • Due to molecular movement: • When gas expands, it’s temperature decreases. • When gas is compressed, the temperature increases. • This automatic change in temperature of a gas due to expansion or compression is Adiabatic temperature change.

13. Atmospheric Moisture/Energy of Evaporation • It takes 540 calories/gram to change liquid water to water vapor (esrt) • When evapotranspiration occurs, the more energetic water molecules leave first. • The molecules that are left behind are not as energetic. • The result is a low temperature of an evaporating liquid.

14. Factors that effect evaporation rate: • The higher the temperature, the faster the rate of evaporation. • The greater the surface area of the water, the faster the rate of evaporation. • The stronger the wind speed, the faster the rate of evaporation. • If the air has a high moisture content (saturated), it cannot absorb any more.

15. Saturation Vapor Pressure • Saturation: When air is holding the maximum amount of water vapor it can. • Vapor Pressure: The pressure that water vapor exerts. • Saturation Vapor Pressure: The pressure that is exerted when the air is saturated. • Dynamic equilibrium is reached when saturated vapor pressure is reached (condensation equals evaporation)

16. Measuring Relative Humidity and Dew Point Temperature • Sling Psychrometer: Contains an ordinary thermometer (dry-bulb) and a thermometer with a cloth around the bulb (wet-bulb). • When the cloth is moistened and the psychrometer is whirled around, the wet-bulb temp drops due to evaporation. • The amount of cooling depends on the rate of evaporation.

17. Humidity • Humidity: The water vapor content in the atmosphere. • Absolute Humidity: The weight of water vapor in a given volume of air. • Directly related to vapor pressure. • Air can hold more water vapor at higher temperatures. • Warmer temps = more humidity

18. Relative Humidity • Relative Humidity: The ratio of the amount of water in the air to the maximum amount that could be present at that temperature. • High temperatures can hold more water. • RH is expressed as a percent. • When RH reaches 100%, the air is saturated.

19. Dew Point • Dew Point Temperature: The temperatured at which the air is saturated with moisture (relative humidity is 100%) • A further drop in temperature would result in condensation (rain) or sublimation (snow). • The reason is that colder air can not hold as much water as warmer air. • Dew Point does not depend on relative humidity; only on the amount of water vapor in the air.

20. Air Masses and Fronts • Weather is caused by the interaction of Air Masses. • Air Mass ~ Large body of air with similar characteristics (pressure, moisture, and temperature) • Air masses form when a large air mass sits over a part of the Earth’s surface for a long time. • The part of the Earth that an air mass forms over is called a source region.

21. Temperature: Arctic (A) Polar (P) Tropical (T) Moisture: Continental (c) – dry Maritime (m) – wet Air masses combine two of five characteristics:

22. An air mass is described by its source region . . . Continental: Dry (c) Maritime: Wet (m) Cold: • Arctic (A) • Polar (P) Warm: • Tropical (T)

23. Air will either: ·Rise, expand, cool and condense in a low-pressure, convergent, counterclockwise system. Or . . . Sink, compress, heat and evaporate in a high-pressure, divergent, clockwise system Everything comes down to . . .The interaction of weather variables: temperature, air pressure, moisture content (relative humidity and dew point), precipitation (rain, snow, hail, sleet, etc.) wind speed/direction and cloud cover.