1. Air Pressure

2. State Variables Describe the “state” of the gas “Variable” means they can change Physicists use P, V, T, N and constant k Chemists use P, V, T, n and constant R Can also use P, D, T and constant R/M M is average mass of specific number of air molecules – different for humid and dry air Dry: M = 28.97 g/molHumid: M = 27.87 g/mol

3. Ideal Gas Law An enclosed gas in rigid container – V and N fixed  density is constant – P changes in direct proportion to T An air parcel – Only N is fixed – If T is constant, P changes in direct proportion to D – If P is constant, D is inversely proportional to T In atmosphere all 3 may change  more complex

4. Thermodynamics Internal Energy is U – Includes Kinetic Energy of motion – Includes Potential Energy from forces between atoms and molecules Heat transfer is Q – Entering system it’s positive – Leaving system it’s negative Work is W (W = −P  V) – Done on the system it’s positive – Done by the system it’s negative

5. Adiabatic Processes No heat exchanged between system and its surroundings For atmospheric air parcels assume – No radiative heating/cooling – No sensible heating/cooling – No latent heating/cooling

6. Air Parcels Rising/Sinking Use 1 st Law of Thermodynamics with Q = 0 U = Q + W = −P  V Rising air parcels expand (  V > 0) – U decreases so T decreases Sinking air parcels compress (  V < 0) – U increases so T increases Dry adiabatic lapse rate is change in T with height

7. Moist Adiabatic Lapse Rate (for Saturated Air) Atmosphere contains water vapor As water vapor cools it releases latent heat Expansional cooling is partially offset Warmer air can hold more water vapor than cooler air More or less water gives more or less offset

9. Dry Adiabat Wet Adiabat