under it or by lifting as it flows up over a mountain
slope.
As the air rises, the pressure decreases which
allows the parcel of air to expand. This continues until it
reaches an altitude where the pressure and density are
equal to its own. As it expands, it cools through a
thermodynamic process in which there is no transfer of
heat or mass across the boundaries of the system in
which it operates (adiabatic process). As air rises, it
cools because it expands by moving to an altitude
where pressure and density is less. This is called
adiabatic cooling. When the process is reversed and air
is forced downward, it is compressed, causing it to heat.
This is called adiabatic heating, (See fig. 2-6.)
Remember, in an adiabatic process an increase in
temperature is due only to COMPRESSION when the
air sinks or subsides. A decrease in temperature is due
only to EXPANSION when air rises, as with convective
currents or air going over mountains. There is no
addition or subtraction of heat involved. The changes in
temperature are due to the conversion of energy from
one form to another.
STABILITY AND INSTABILITY
The atmosphere has a tendency to resist vertical
motion. This is known as stability. The normal flow of
air tends to be horizontal. If this flow is disturbed, a
stable atmosphere resists any upward or downward
displacement and tends to return quickly to normal
horizontal flow. An unstable atmosphere, on the other
hand, allows these upward and downward disturbances
to grow, resulting in rough (turbulent) air. An example
is the towering thunderstorm that grows as a result of a
large intense vertical air current.
2-13
AG5f0206
MOIST AIR BEING LIFTED
BY COLD FRONT.
AS THE LIFTED AIR EXPANDS,
IT COOLS ADIABATICALLY.
COLD FRONT
WHEN AIR RISES IN ALTITUDE,
IT EXPANDS AND COOLS AT ITS
ADIABATIC LAPSE RATE.
ADIABATIC
COOLING
WHEN AIR DESCENDS IN ALTITUDE,
IT COMPRESSES AND HEATS AT ITS
ADIABATIC LAPSE RATE.
ADIABATIC
HEATING
MOIST ADIABATIC LAPSE RATE
2 TO 3 F/1000FT.
MOIST ADIABATIC LAPSE RATE
5.5 F/1000FT.
Figure 2-6.—Adiabatic cooling and heating process.
