Parcels moving up and down in the atmosphere experience changes in pressure outside, which must be matched inside according to our definition of an air parcel (p_in = p_out). Since no air can go in or out of the parcel, the parcel must expand to reduce its interior pressure or compress in order to increase its interior pressure. Thus, rising parcels expand, and sinking parcels compress.

The act of expanding or compressing a parcel changes the speed of the molecules of air inside the parcel, so the temperature of the air inside changes while a parcel rises or sinks. Since no heat is going in or out of the parcel, the temperature changes occur adiabatically (if heat was actually added or taken away to make the temperature go up or down, respectively, we would call this a diabatic temperature change). Air parcels rising or sinking in the atmosphere are generally assumed to be losing or gaining temperature, respectively, by an adiabatic process.

[Higher-level physics for those who are interested: We use the terms "energy" and "heat" to mean the same thing, the "dQ" term in the First Law of Thermodynamics (below). dQ represents the amount of heat added/taken away from the system.

dU = dQ + dW

In an adiabatic process, dQ=0, so compressing or expanding the gases adiabatically (which is the "dW" or "work" term) causes a temperature change (via the "dU" or "internal energy" term, where U is a direct function of temperature T in an ideal gas).]

You have probably experienced this phenomenon if you have pumped up a bicycle tire or a rubber ball---it gets hot as you compress the air. If you have sprayed an aerosol can, you may have noticed the can and the spray become cold, due to the adiabatic expansion when you release the pressure of the can's contents.

Play the QuickTime movie of an adiabatic compression and expansion.

Putting this all together, since a rising parcel expands adiabatically, a rising air parcel loses temperature; an adiabatically sinking parcel gains temperature since it is compressing. For air that is dry (not enough water vapor to saturate the air), it turns out that in Earth's troposphere, all air parcels are losing or gaining temperature at the rate of 10°C per km change in altitude. This rate of change of temperature is referred to as the Adiabatic Lapse Rate*.

*In general meteorology, there is a case where the air is saturated with water vapor and is rising/sinking pseudoadiabatically. Because the condensation or evaporation of water while this is going on is adding or removing (latent) heat, the lapse rate in these saturated air parcels is altered. This Saturated Adiabatic Lapse Rate (4 to 10°C/km) is smaller than the Dry Adiabatic Lapse Rate (10°C/km) we will be using for all of our air pollution cases.