Air Pressure

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/mol Humid: M = 27.87 g/mol

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

𝑃=constant ×𝐷×𝑇

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

𝑈=𝑄+𝑊

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

0Q

Air Parcels Rising/Sinking• Use 1st 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

C/km75.9 z

T

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

C/km6ave

z

T

Dry Adiabat

Wet Adiabat