|WHAT IS ADIABATIC THEORY?
METEOROLOGIST JEFF HABY
Adiabatic theory is used to understand how the temperature and dewpoint change as air rises or sinks within the atmosphere. This theory
is important in that it is used to aid in the forecasting of clouds, cloud base, precipitation, the temperature profile, thunderstorms
and severe thunderstorms.
The theory answers questions to how the thermodynamic characteristics of a parcel of air should change as air rises or sinks. This is
generally broken up into two cases: one in which the air has a relative humidity of less than 100% (unsaturated air) and one is which
the air has a relative humidity of 100% (saturated air). These are then analyzed for rising air or sinking air. The following are the
general idealizations that result from using this theory as it relates to how the temperature and dewpoint of air change as air rises
or sinks in the atmosphere.
1) Unsaturated air cools at 9.8 C/km (~10C/km) as it rises. This is called the dry adiabatic lapse rate. For example, if air is 20 C at
the 1 km level then it will cool to 10 C once that air rises to 2 km. This occurs because air expands as it rises (since the parcel is
surrounded by lower pressure as it rises). Expanding air cools.
2) Dewpoint decreases by 2 C/km as unsaturated air rises. This is not much of a change but it is important in determining how much air
needs to be lifted in order to be saturated. Once the air is saturated then the dewpoint cools the same amount the temperature cools
and this rate of cooling is explained in the next paragraph.
3) Saturated air cools at the wet adiabatic lapse rate. This rate is not constant but depends on how much moisture is in the air. Condensing
air releases latent heat and the more moisture that is present then the more latent heat that is released. Cold saturated air that will
have very little moisture will cool at near the dry adiabatic lapse rate. Air that is very warm and saturated will cool at about half
the dry adiabatic lapse rate (~5 C/km). The rate will vary between 5 and 10 C/km depending on how much moisture content is in the
saturated air. An implication from this is that saturated air does not cool as quickly as unsaturated air when it rises. Also, air
with a high moisture content will not cool as much as air with a lower moisture content when air rises.
4) Sinking air follows the same rules as rising unsaturated air but the lapse rate is reversed. Instead of air cooling, air warms at
the dry adiabatic lapse rate as it sinks. There is no wet adiabatic lapse rate for sinking air since the sinking process results in
air becoming unsaturated. The dewpoint increases by 2 C/km as air sinks.