CHAPTER 3
ATMOSPHERIC CIRCULATION
To understand large-scale motions of the
atmosphere, it is essential that the Aerographer’s Mate
study the general circulation of the atmosphere. The
sun’s radiation is the energy that sets the atmosphere in
motion, both horizontally and vertically. The rising and
expanding of the air when it is warmed, or the
descending and contracting of the air when it is cooled
causes the vertical motion. The horizontal motion is
caused by differences of atmospheric pressure; air
moves from areas of high pressure toward areas of low
pressure. Differences of temperature, the cause of the
pressure differences, are due to the unequal absorption
of the Sun’s radiation by Earth’s surface. The
differences in the type of surface; the differential
heating; the unequal distribution of land and water; the
relative position of oceans to land, forests to mountains,
lakes to surrounding land, and the like, cause different
types of circulation of the air. Due to the relative
position of Earth with respect to the Sun, much more
radiation is absorbed near the equator than at other
areas, with the least radiation being absorbed at or near
the poles. Consequently, the principal factor affecting
the atmosphere is incoming solar radiation, and its
distribution depends on the latitude and the season.
GENERAL CIRCULATION
LEARNING OBJECTIVE: Recognize how
temperature, pressure, winds, and the 3-cell
theory affect the general circulation of Earth’s
atmosphere.
The general circulation theory attempts to explain
the global circulation of the atmosphere with some
minor exceptions. Since Earth heats unequally, the heat
is carried away from the hot area to a cooler one as a
result of the operation of physical laws. This global
movement of air, which restores a balance of heat on
Earth, is the general circulation.
WORLD TEMPERATURE GRADIENT
Temperature gradient is the rate of change of
temperature with distance in any given direction at any
point. World temperature gradient refers to the change
in temperature that exists in the atmosphere from the
equator to the poles. The change in temperature or
temperature differential, which causes atmospheric
circulation can be compared to the temperature
differences produced in a pan of water placed over a gas
burner. As the water is heated, it expands and its density
is lowered. This reduction in density causes the
warmer, less dense water to rise to the top of the pan. As
it rises, it cools and is forced to the edges of the pan.
Here it cools further and then sinks to the bottom,
eventually working its way back to the center of the pan
where it started. This process sets up a simple
circulation pattern due to successive heating and
cooling.
Ideally, the air within the troposphere may be
compared to the water in the pan. The most direct rays
of the Sun hit Earth near the equator and cause a net
gain of heat. The air at the equator heats, rises, and
flows in the upper atmosphere toward both poles. Upon
reaching the poles, it cools sufficiently and sinks back
toward Earth, where it tends to flow along the surface of
Earth back to the equator. (See fig. 3-1).
Simple circulation of the atmosphere would occur
as described above if it were not for the following
factors:
3-1
