Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 127

Chapter 6

Effects of Meteorology on Air Pollution

The concentrations of gases and aerosol particles in
the air are affected by winds, temperatures, vertical
temperature profiles, clouds, and the relative humid-
ity. These meteorological parameters are influenced
by large- and small-scale weather systems. Large-scale
weather systems are controlled by vast regions of high
and low pressure, whereas small-scale weather systems
are controlled by ground temperatures, soil moisture,
and small-scale variations in pressure. The first section
of the chapter examines the forces acting on air, and the
second section examines how forces combine to form
winds. In the third section, the way in which radia-
tion coupled with forces and the rotation of the Earth
generates the global circulation of the atmosphere is
discussed. Sections four and five discuss characteristics
of the two major types of large-scale pressure systems.
In the sixth section, the effects of such pressure systems
on air pollution are addressed. The last section focuses
on the effects of local meteorology on air pollution.

(PGF), which is proportional to the difference in pres-
sure divided by the distance between the two locations
and always acts from high to low pressure.

6.1.2. Apparent Coriolis Force
When air is in motion over a rotating Earth, it appears
to an observer fixed in space to accelerate to the right in
the Northern Hemisphere and to the left in the Southern
Hemisphere by the apparent Coriolis force (ACoF;
Figure 6.1). The ACoF is not a real force; rather, it is
an acceleration that arises when the Earth rotates under
a body (air in this case) in motion. The ACoF is zero at
the Equator, maximum at the poles, zero for bodies at
rest, proportional to the speed of the air, and always acts
90 degrees to the right (left) of the moving body in the
Northern (Southern) Hemisphere.
Figure 6.1 illustrates the ACoF. If the Earth did not
rotate, an object thrown from point A directly north
would be received at point B, along the same longitude
as point A. Because the Earth rotates, objects thrown to
the north have a west-to-east velocity equal to that of
the Earth's rotation rate at the latitude from which they
originate. The Earth's rotation rate near the Equator
(low latitude) is greater than that near the poles (high
latitudes); thus, objects thrown from low latitudes have
agreater west-to-east velocity than does the Earth below
them when they reach a high latitude. For example, an
object thrown from point A to point B in the north will
end up at point C, instead of at point B by the time
the person at point A reaches point A , and the object

6.1. Forces
Winds arise due to forces acting on the air. The four
major forces - pressure gradient force, apparent Corio-
lis force, friction force, and apparent centrifugal force -
are described in this section.

6.1.1. Pressure Gradient Force
When air pressure is high in one location and low
nearby, air moves from high to low pressure. The force
causing this motion is the pressure gradient force