Geoscience Reference
In-Depth Information
994
Force 1
996
Vertical
pressure
gradient
force
Resultant
Air
parcel
998
Gravitational
force
1000 hPa
Force 2
Figure 6.3 Pressure forces acting upon a parcel of air.
W - 2
Figure 6.2 The resultant of two forces acting in different
directions. The length of line is proportional to the strength of
the force.
Decreasing
density
W - 1
W
W + 1
Air
parcel
FORCES ACTING UPON THE AIR
Pressure gradient force
Let us imagine that we have a small parcel of air some
distance above the ground. What forces will act upon it?
The most obvious is the force of gravity , which tends to
attract all mass towards Earth's centre. In addition we have
the pressure exerted by the air surrounding the parcel
( Figure 6.3 ). If this pressure were the same on all sides of
the parcel its effects would cancel out. But it is not.
Pressure decreases upwards in our atmosphere, as we saw
in Chapter 3. The force pushing the parcel of air upwards
is greater than the downward force from the overlying
atmosphere; there is a potential upward acceleration of the
parcel. Luckily this vertical force is almost exactly balanced
by the force of gravity, otherwise we would have 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
Figure 6.4 Force exerted on a parcel of air produced by
density differences.
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
areas of low pressure ( Figure 6.6 ). The magnitude of the
force causes movement (the pressure gradient force), and
 
 
 
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