Geoscience Reference
In-Depth Information
Atmospheric motion: principles
Learning objectives
When you have read this chapter you will:
Know the basic laws of horizontal motion in the atmosphere,
Know how the Coriolis force arises and its effects,
Be able to define the geostrophic wind,
Know how friction modifies wind velocity in the boundary layer,
Understand the principles of divergence/convergence and vorticity and their roles in atmospheric
processes,
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 kilometres and lasting for
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 equilib-
rium
(p. 23). This state of balance, together with the
general stability of the atmosphere and its shallow
depth, greatly limits vertical air motion. Average hori-
zontal wind speeds are of the order of one hundred 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 movement 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 devel-
opment of a horizontal pressure gradient through spatial
differences in surface heating and consequent changes
in air density and pressure. The fact that such a gradient
can persist (rather than being destroyed by air motion
towards the low pressure) results from the effect of the
earth's rotation in giving rise to the Coriolis force.
