Environmental Engineering Reference
In-Depth Information
Figure 6.2
The resultant of two forces acting in different
directions. The length of line is proportional to the strength of
the force.
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
Figure 6.3
Pressure forces acting upon a parcel of air.
