|ASSUMPTIONS OF ADIABATIC PROCESS
METEOROLOGIST JEFF HABY
An adiabatic process is one is which when air rises or sinks there is no exchange of mass, moisture and
momentum between the parcel and the environment. What is meant by this? It means the air does not mix. Think
of a balloon. The air inside the balloon is trapped and can not mix with the air outside the balloon. The
adiabatic process gives us important thermodynamic information such as: a. unsaturated air cools at the
dry adiabatic lapse rate when rising and warms at the same rate when sinking (10 C/km), b. saturated air
cools at the wet adiabatic lapse rate when rising, c. the dewpoint lapse
rate is a decrease of 2 C/km in rising unsaturated air and increases at same rate
when sinking and d. the dewpoint falls with the temperature as saturated air rises.
There are several important points of adiabatic theory I want to go over. The first is time. The longer
a parcel of air is exposed to the environment then the more mixing that will occur. Thus an adiabatic
process works best in shorter time scales (such as a
convective updraft). Once a parcel is
exposed to the surrounding environment beyond several hours there will be considerable mixing between
the parcel and the environment. This contaminates an adiabatic process. Even in short time scales the
adiabatic process will be somewhat contaminated. Unlike a balloon, a parcel of air does not have
a membrane separating the parcel air from the environmental air. Think of a helium balloon.
When you first buy the balloon it is the most buoyancy but over time it eventually will not
rise since air has escaped from the balloon and there has been mixing with the air outside the
balloon. If drier, more moist, warmer or cooler air is mixed into a parcel it will change the
thermodynamic characteristics of that parcel. The parcel has been contaminated so that adiabatic
theory will have some error.
The second point I want to go over is the Earth's surface. While air motions are more smooth and less mixed above
the planetary boundary layer, air motions near the surface are influenced by
friction and this causes
a mixing of air (wind gusts, convective eddies, turbulence). These turbulent processes erode the
adiabatic process over time. Thus a parcel of air near the surface will mix out with the environment
quicker than a parcel of air above the planetary boundary layer. Also, surface heat fluxes and surface
evaporation contaminate an adiabatic process. It is easier for moisture to be added or subtracted or
heat added or subtracted to a parcel of air if it is exposed to the Earth's surface.
The third point is that an adiabatic process is a theory that only works perfectly under ideal
conditions. The atmosphere is not ideal for adiabatic theory. The term adiabatic approximation
is sometimes used to make you aware that this process is a theory that does not work
perfectly in the real atmosphere. However, it works well enough so that meteorologists are
able to use it to gain insight into how thermodynamic characteristics of the air will change as
the air rises or sinks. The theory can be used to help forecast thunderstorms, temperature
changes in the troposphere and
(in)stability of the atmosphere.