Geoscience Reference
In-Depth Information
6
CHAPTER SIX
Atmospheric motion:
principles
LEARNING OBJECTIVES
When you have read this chapter you will:
n
know the basic laws of horizontal motion in the atmosphere
n
know how the Coriolis force arises and its effects
n
be able to define the geostrophic wind
n
know how friction modifies wind velocity in the boundary layer
n
understand the principles of divergence/convergence and vorticity and their roles in atmospheric
processes
n
understand the thermodynamic, dynamic and topographic factors that lead to distinctive local
wind regimes.
The atmosphere is in constant motion on scales
ranging from short-lived, local wind gusts to
storm systems spanning several thousand kilo-
meters and lasting about a week, and to the more
or less constant global-scale wind belts circling
the earth. Before considering the global aspects,
however, it is important to look at the immediate
controls on air motion. The downward-acting
gravitational field of the earth sets up the observed
decrease of pressure away from the earth's surface
that is represented in the vertical distribution
of atmospheric mass (see
Figure 2.13
). This
mutual balance between the force of gravity and
the vertical pressure gradient is referred to as
hydrostatic equilibrium
(p. 31). This state of
balance, together with the general stability of the
atmosphere and its shallow depth, greatly limits
vertical air motion. Average horizontal wind
speeds are of the order of 100 times greater than
average vertical movements, although individual
exceptions occur - particularly in convective
storms.
A LAWS OF HORIZONTAL
MOTION
There are four controls on the horizontal move-
ment of air near the earth's surface: the pressure-
gradient force, the Coriolis force, centripetal
acceleration, and frictional forces. The primary
cause of air movement is the development of
a horizontal pressure gradient through spatial
differences in surface heating and consequent
changes in air density and pressure. The fact that
