Introduction - The Global Climate System: Patterns, Processes, and Teleconnections - page 7

LOW LOW

Figure 1.5 Idealized
diagrams showing how
interacting forces result in
surface friction layer winds
(left) and upper air,
geostrophic winds (right).
PGF, pressure gradient force;
FR, friction; CE, coriolis
effect.

PGF PGF

WIND

Balanced

WIND Balanced

FR

PGF PGF

WIND CE

WIND CE

Intermediate

Intermediate CE

FR CE

PGF PGF

Initial

Initial

HIGH HIGH

energy originally absorbed and retained as latent heat. The same is true when
water freezes and water vapor sublimates to ice.
The significance of the release of latent heat shows in many ways. For
example, it plays a critical role in the redistribution of heat energy over the
Earth's surface. Because of the high evaporation in low latitudes, air transported
to higher latitudes carries latent heat with it. The vapor in this air condenses and
releases energy to warm the atmosphere in higher latitudes.

Air in motion
Newton's first law of motion deals with inertia. It states that a body will change
its velocity of motion only if acted upon by an unbalanced force. In effect, if
something is in motion, it will keep going until a force modifies its motion. On
Earth, a parcel of air seldom moves continuously and in a straight line. This is
because, as Newton's second law states, the acceleration of any body, in this case
the parcel of air, is directly proportional to the magnitude of the net forces acting
upon it and inversely proportional to its mass. Note that these laws concern
acceleration, which is change of velocity with time.
By identifying the forces that act upon a parcel of air, it becomes possible to
understand more fully the processes that lead to the acceleration (or deceleration) of
air. If we consider a unit parcel of air (m ¼ 1), then Newton's second law becomes

Acceleration ¼ Sum of forces
or F a ¼ X F
The P F is made up of the atmospheric forces so that:

Acceleration ¼ Pressure gradient force þ Coriolis force
þ Frictional forces þ Rotational forces

The understanding and evaluation of each of these forces (or accelerations)
provide the key to winds that blow, at both the surface and aloft, over the globe.
This is schematically illustrated in Figure 1.5 where the interacting forces are
shown to produce friction winds at lower levels of the atmosphere, and geo-
strophic winds aloft.