Lecture 11: Adiabatic Processes

How Adiabatic air occurs

** A parcel of air that contains warm air is less dense (lighter) and thus may rise. As this parcel of air rises in the atmosphere, it will encounter lower pressure and thus expand. As it expands the molecules in it will start to slow down and thus kinetic energy levels will decrease. As this occurs, the T° will also decrease. ** Conversely, a parcel of cold air is more dense (heavier) and thus sinks. As this parcel of air sinks in the atmosphere, it will encounter higher pressure and thus be compressed. As it is compressed the molecules in it will start to speed up and thus kinetic energy levels will increase. As this occurs, the T° will also increase. So, Expanding air = Tº decrease Compressing air = Tº increases ** BUT, the Tº changes WITHOUT heat energy being added or subtracted. ** Rising air in the atmosphere expands & cools. ** Sinking air in the atmosphere is compressed & warms.

Adiabatic Temperature Change

Changing the Temperature of the air without adding or subtracting heat energy. Simply the result of compressing the air or allowing it to expand.

Adiabatic

a parcel of air changing temperature without heat energy being added or removed

Saturated Adiabatic Rate (Saturated Adiabatic Lapse Rate): (SAR or SALR) (May also see this as WAR, Wet Adiabatic Rate; or MAR, Moist Adiabatic Rate)

air parcel is saturated its air Tº = d.p.Tº and thus its RH = 100%. * The SAR is not a constant rate but a variable rate of .5º to .9ºC / 100 m or 5º to 9ºC / 1000m * Need to know, but the variable rate will be given. * It is dependent on the moisture content of the air. The more water vapor condensing to liquid there is in the air, the slower the rate of decline. (closer to 5°C / 1000m). * This is because condensation releases latent heat energy, thus slowing the rate of cooling (or warming).

Dry Adiabatic Rate (Dry Adiabatic Lapse Rate): (DAR or DALR)

air parcel is unsaturated. Its air Tº > dew point Tº and thus its RH < 100%. * The DAR of unsaturated air is a constant rate of ** 1ºC / 100 m or 10ºC / 1000m. Thus: rising air cools at 1ºC/100 m (10ºC / 1000m) sinking air warms at 1ºC/100 m (10ºC / 1000m).

Condensation level

is the height in the atmosphere at which condensation occurs, where cloud formation begins (usually seen as the bottom of a cloud mass. RH = 100%; air T° = d.p.T°

Rate of Adiabatic Temperature Change

varies with the humidity condition (water vapor content) of the air, is it Dry (unsaturated) or Wet (saturated).