Environmental Engineering Reference
In-Depth Information
Figure 6.4
Force exerted on an air parcel produced by
density differences.
lost our atmosphere long ago. Most of the air movements that we observe are horizontal.
Where the atmosphere is denser, the lateral pressure on the parcel of air is great; where
the atmosphere has a lower density, the lateral pressure is less. Variations in the density
of the atmosphere from one part of the globe to another result in an imbalance of forces
and lateral movement of the air (Figure 6.4). The air is 'pushed' from areas of high
pressure to areas of low pressure.
This, in fact, is the basic force affecting atmospheric movement. It is called the
pressure gradient force
. Pressure decreases vertically because, as we move upward
through the atmosphere, the weight of overlying air diminishes. It varies laterally because
of differences in the intensity of solar heating of the atmosphere. Where solar radiation is
intense the air warms up, expands and its density declines; air pressure falls. Where
cooling occurs, the air contracts, its density increases and air pressure becomes greater.
A corollary of this principle is that the pattern of air pressure close to the surface is
reversed in the upper atmosphere. Because cold air contracts, the upward decline in
pressure is rapid and at any constant height above a zone of cool air the pressure is
relatively low. Conversely, warm air expands and rises, so that the vertical pressure
gradient is less steep. Above areas of warm air, therefore, the pressure tends to be
relatively high (Figure 6.5). The effect upon atmospheric motion is clear. At the surface
the air will move from cold to warm zones; at higher altitudes the flow will be from
warm to cold.
Differences in air pressure may be mapped by defining lines of equal pressure. These
are known as isobars. Air movement occurs at right-angles to the isobars, down the
pressure gradient; that is, from areas of high pressure to
