Page 301 - Fundamentals of Air Pollution

III. Laws of Motion 257

sphere, the top portion of the descending layer will be further warmed
due to its greater descent than will the bottom portion of the layer (Fig.
17-9). Occasionally a subsidence inversion descends all the way to the
surface, but usually its base is well above the ground.
Inversions are of considerable interest in relation to air pollution because
of their stabilizing influence on the atmosphere, which suppresses the
vertical motion that causes the vertical spreading of pollutants.

III. LAWS OF MOTION

The atmosphere is nearly always in motion. The scales and magnitude
of these motions extend over a wide range. Although vertical motions
certainly occur in the atmosphere and are important to both weather pro-
cesses and the movement of pollutants, it is convenient to consider wind
as only the horizontal component of velocity.
On the regional scale (hundreds to thousands of kilometers), the winds
are most easily understood by considering the balance of various forces in
the atmosphere. The applicable physical law is Newton's second law of
motion, F = ma; if a force F is exerted on a mass m, the resulting acceleration
a equals the force divided by the mass. This can also be stated as the rate
of change of momentum of a body, which is equal to the sum of the forces
that act on the body. It should be noted that all the forces to be discussed
are vectors; that is, they have both magnitude and direction. Although
Newton's second law applies to absolute motion, it is most convenient to
consider wind relative to the earth's surface. These create some slight
difficulties, but they can be rather easily managed.

Fig. 17-9. Formation of a subsidence inversion in subsiding (sinking) air. Note the vertical
compression of the sinking layer which is usually accompanied by horizontal divergence.