The atmosphere in action - Fundamentals of the Physical Environment - page 115

Polar front: a zone of
slantwise convection

Figure 6.12
Main features of atmospheric
circulations at the surface and
in the atmosphere. PFJpolar
front jet; STJsubtropical jet.
The surface brown lines
represent frontal zones
separating warm subtropical
air from cool polar air.

Sinking

Arctic
front

8km

Pole

PFJ

60°

STJ 30°

Hadley
cell

Equator

Again we can understand the cause of these eddies with
the help of a simple experiment. A pan of water is heated
at the rim and cooled at the centre. If the pan is slowly
rotated it is seen that a simple thermal circulation is
produced. If the rate of rotation is increased, however, the
flow suddenly becomes unstable. New patterns form like
those we see in the atmosphere of the temperate latitudes
- eddies and waves. It seems that rapid rotation, like that
of Earth, sets up forces which disturb the simple
circulation of the atmosphere, particularly near the axis
of rotation (i.e. in higher latitudes). These forces destroy
the simple pattern and produce more complex circulation
( Figure 6.13 ).

Effect of surface configuation

Even now our picture of atmospheric circulation is far
from complete. Earth's surface is not uniform, and the
variations in its surface form cause ever more disruption
of the pattern of circulation. Friction affects the winds,
reducing the effect of the Coriolis force, and, locally, it
deflects the surface flow of air to produce highly compli-
cated systems of movement. Temperature differences
produced by different types of surface, such as land, sea
and ice, also have an impact.
It is difficult to model the effects of surface configura-
tion, but a general indication of its influence can be

( , ) symmetric
(w = 0.341 rad s -1 )

( ,, ) steady waves
(w = 1.19 rad s -1 )

( ,,, ) irregular
(w = 5.02 rad s -1 )

Figure 6.13 Streaks indicating surface flow patterns in a rotating fluid subject to heating at the outer side wall and cooling at
the inner side wall. At low rates of rotation (left) the flow is symmetrical about the axis of rotation. As the rotation rate increases
(centre) the flow develops jet streams and waves. At higher rates (right) the flow is highly irregular, with resemblance to the
cyclonic and anticyclonic eddies found in the westerly circulation. Original by courtesy of Dr R. Hide