T/G Layout 1

Air that is forced into, or over, a topographic barrier will also rise and cool to form
clouds (figure 31b). This occurs near mountain ranges. For example, warm and moist
air from the Gulf of Mexico can be pushed northwestward and up the eastern slope of
the Rockies to form extensive cloud decks.

F i n a l l y, lifting occurs where there is large scale convergence of air (figure 27c, page 40).
Cold fronts are a location of strong convergence as cold, dense southward moving air
displaces warmer air. Convergence can also occur on smaller scales along the leading
edge of the sea or bay breeze boundaries.

The formation of clouds is an application of the First Law of Thermodynamics. Accord i n g
to the First Law, a change in the internal energy of a system can be due to the addition
(or loss) of heat or to the work done on (or by) the system. In the atmosphere system, the
change of internal energy is measured as a change in temperature and the work done is
manifested as a change in pressure. Because air is a relatively poor conductor of heat
energy, the assumption is made that the parcel of air upon which work is being done
is insulated from the surrounding environment. This is the adiabaticassumption. For a
r i sing air parcel, the change in internal energy is there f o re due entirely to pre s s u re work
with no addition or loss of heat to the surrounding environment. A simple relationship
for temperature change for a rising parcel of air can then be determined. This change
of temperature with height is the dry adiabatic lapse rate of -9.8°C per kilometer.

adiabatic

The process without transfer of heat, compression results in warm i n g ,

expansion results in cooling.

Air is, of course, not entirely dry and always contains some water vapor which can
condense as the air parcel rises and cools. Condensation creates clouds and affects the
temperature and vertical motion of the parcel. During condensation, heat is released

figure 32.

5 10 15 20

1

moist adiabatic

dry laps e rate

adiabatic

lapse rate

Z (km) Lapse Rates

T (C°)