1. Thermal Properties of Troposphere September 10, 2007

2. Clouds: The Basics

3. Wind Direction Wind barbs point in the direction "from" which the wind is blowing. In the case of the diagram to the right, the orientation of the wind barb indicates winds from the Northeast.

4. Wind Direction

6. Buoyancy The buoyancy of an air parcel depends on its density Remember cooler air is denser than warmer air So if a cool air parcel is surrounded by warmer air, it will sink If a warm air parcel is surrounded by cold air, it will rise

7. Adiabatic Lapse Rates Lapse rate refers to the change in temperature with any change in altitude Adiabatic means occurring without a gain or loss of heat energy to the environment Sinking air parcels heat by compression Rising air parcels cool by expansion

8. Outside of the air parcel Normal Lapse Rate- The average decrease in temperature with increasing altitude in the troposphere –6.4ºC/1000m or 3.5ºF/1000ft Environmental Lapse Rate (ELR) – actual temperature decrease with increasing altitude in any one location under specific conditions

9. Inside the air parcel Two different lapse rates, depending on the physical state of water molecules in the air parcel Dry Adiabatic Lapse Rate (DAR) – if the air parcel is NOT saturated (the relative humidity is less than 100%) it will heat and cool at the DAR as it changes altitude –10ºC/1000m or 5.5ºF/1000ft

10. Examples Assume an unsaturated air parcel with an air temperature of 20ºC rises from the surface to 3000m, what is its new temperature? –DAR: lose 10ºC for every 1000m you rise –So, 3000m rise means loss of 30ºC –Answer: -10ºC

11. Inside the air parcel Moist adiabatic lapse rate (MAR) - if the air parcel is saturated (the relative humidity if 100%) it will heat and cool at the MAR as it changes altitude –6ºC/1000m or 3.3ºF/1000ft The MAR is lower than the DAR because of latent heat. Heat is lost when traveling from the DAR to MAR because water is changing phase

12. Example Assume a saturated air parcel with an air temperature of 20ºC rises from the surface to 3000m, what is its new temperature? –MAR = 6ºC/1000m –If you rise 3000m, you lose 6ºC for every 1000m you rise –So, you would lose 18ºC –Answer: 2ºC

13. Example Assume a saturated air parcel with an air temperature of -30ºC descends from 3000m to the surface. What is its new temperature? –MAR = 6ºC/1000m –If you descend 3000m, you gain 6ºC for every 1000m you sink –So, you would gain 18ºC –Answer: -12ºC

14. LCL What happens when we consider the dewpt temperature in an equation? Lapse rate changes when you hit the dewpt. The point in the troposphere where the air temperature tries to decrease below the dewpt causes condensation = clouds This point is called the Lifting Condensation Level (LCL)

15. LCL So when an air parcel rising in the troposphere hits the LCL, it changes from being influenced by the DAR to the MAR because it is now saturated

16. Example If an air parcel at the surface has an internal temperature of 25ºC, and the dewpt is 11ºC, at what altitude will the air parcel reach 11ºC? –25ºC - 11ºC = 14ºC, so we travel 14ºC –10ºC/1000m = 1ºC/100m * 14ºC = 14ºC/1400m –Answer: 1400m

17. Example DAR = 10ºC/1000m MAR = 6ºC/1000m LCL = 11ºC 25ºC Surface 14ºC 1400ft Now the air parcel travels from 1400m to 2400m. What is its final temperature? 5º C 2400m 1400m

18. Example An unsaturated air parcel at the surface has a temperature of 30ºC and the dewpt is 10ºC. What is the parcel’s temperature at 3000m? –30ºC - 10ºC = 20ºC, distance traveled –10ºC/1000m * x = 20ºC X = 2000m, height of dewpt –Must travel another 1000m at MAR 6ºC/1000m, so 10ºC (dewpt temp) - 6ºC = 4ºC

19. Who cares? Lapse rates are used in meteorology to determine if a parcel of air can rise high enough to produce clouds, what type of clouds, and what type of precipitation (if any)