required to increase the pressure of a gas and work is
done by the gas when the pressure diminishes. It
follows that if internal energy (heat) is added to a
simple gas, this energy must show up as an increase in
either temperature or pressure, or both. Also, if work is
performed on the gas, the work energy must show up as
an increase in either pressure or temperature, or both.
An example of the thermodynamic process is a
manual tire pump. The pump is a cylinder enclosed by a
thermodynamics, any increase in the pressure exerted
by the piston as you push down on the handle results in
work being done on the air. As a consequence, either
the temperature and pressure must be increased or the
heat equivalent of this work must be transmitted to the
surrounding bodies. In the case of a tire pump, the work
done by the force on the piston is changed into an
increase in the temperature and the pressure in the air. It
also results in some increase in the temperature of the
surrounding body by conduction.
If the surrounding body is considered to be
insulated so it is not heated, there is no heat transferred.
Therefore, the air must utilize this additional energy as
an increase in temperature and pressure. This occurs in
the adiabatic process.
THE ADIABATIC PROCESS
The adiabatic process is the process by which a gas,
such as air, is heated or cooled, without heat being
added to or taken away from the gas, but rather by
expansion and compression.
In the atmosphere,
adiabatic and nonadiabatic processes are taking place
continuously. The air near the ground is receiving heat
from or giving heat to the ground. These are
atmosphere somewhat removed from Earths surface,
the short-period processes are adiabatic. When a parcel
of air is lifted in the free atmosphere, pressure
decreases. To equalize this pressure, the parcel must
expand. In expanding, it is doing work. In doing work,
it uses heat. This results in a lowering of temperature as
well as a decrease in the pressure and density. When a
parcel of air descends in the free atmosphere, pressure
increases. To equalize the pressure, the parcel must
contract. In doing this, work is done on the parcel. This
work energy, which is being added to the parcel, shows
up as an increase in temperature. The pressure and
density increase in this case also.
In discussing the adiabatic process several terms
are used that you should understand.
LAPSE RATE.In general, lapse rate is the rate
of decrease in the value of any meteorological element
with elevation. However, it is usually restricted to the
rate of decrease of temperature with elevation; thus, the
lapse rate of the temperature is synonymous with the
vertical temperature gradient. The temperature lapse
rate is usually positive, which means that the
temperature decreases with elevation.
atmospheric conditions when the temperature increases
with altitude, rather than decreases as it usually does.
Inversions result from the selective absorption of
Earths radiation by the water vapor in the air, and also
from the sinking, or subsidence, of air, which results in
its compression and, therefore, heating. Either effect
alone may cause an inversion; combined, the inversion
When air is subsiding (sinking), the compressed air
heats. This frequently produces a subsidence inversion.
When subsidence occurs above a surface inversion, the
surface inversion is intensified. Such occurrences are
common in wintertime high-pressure systems. The air
in the inversion layer is very stable, and the cold air
above the inversion acts as a lid trapping fog, smoke,
and haze beneath it. Poor visibility in the lower levels of
the atmosphere results, especially near industrial areas.
Such conditions frequently persist for days, notably in
the Great Basin region of the western United States. An
inversion is a frequent occurrence (especially at night)
in the Tropics and in the Polar regions. For night
conditions all over the world, polar and tropical regions
included, it may be said that low- level inversions are
the rule rather than the exception.
ISOTHERMAL.In the isothermal lapse rate, no
cooling or warming is noted and the rate is neutral with
heightno change in temperature with height.
Adiabatic Heating and Cooling
Air is made up of a mixture of gases that is subject
to adiabatic heating when it is compressed and
adiabatic cooling when it is expanded. As a result, air
rises seeking a level where the pressure of the body of
air is equal to the pressure of the air that surrounds it.
There are other ways air can be lifted, such as through
the thermodynamic processes of a thunderstorm or
mechanically, such as having colder, denser air move