variable. These areas are referred to as the horse
latitudes. The prevailing westerlies, which are on the
poleward side of the subtropical high-pressure belt, are
persistent throughout the mid-latitudes. In the Northern
Hemisphere, the direction of the westerlies at the
surface is from the southwest. In the Southern
Hemisphere, westerlies are from the northwest. This is
due to the deflection area resulting from the Coriolis
effect as the air moves poleward.
Poleward of the prevailing westerlies, near 60°N
and 60°S latitudes, lies the belt of low-pressure basic
pressure known as the polar front zone. Here,
converging winds result in ascending air currents and
consequent poor weather.
WIND THEORY
Newton’s first two laws of motion indicate that
motion tends to be in straight lines and only deviates
from such lines when acted upon by another force or by
a combination of forces. Air tends to move in a straight
line from a high-pressure area to a low-pressure area.
However, there are forces that prevent the air from
moving in a straight line.
Wind Forces
There are four basic forces that affect the
directional movement of air in our atmosphere:
pressure gradient force (PGF), the Coriolis effect,
centrifugal force, and frictional force. These forces,
working together, affect air movement. The forces that
are affecting it at that particular time determine the
direction that the air moves. Also, the different names
given to the movement of the air (geostrophic wind,
gradient wind, etc.) depends on what forces are
affecting it.
Pressure Gradient
The rate of change in pressure in a direction
perpendicular to the isobars is called pressure gradient.
Pressure applied to a fluid is exerted equally in all
3-7
POLAR FRONT
POLAR FRONT
WESTERLY
WESTERLY
WIND
WIND
BELT
BELT
POLAR EASTERLIES
POLAR EASTERLIES
60O
NORTHEAST TRADEWINDS
SOUTHEAST TRADEWINDS
INTERTROPICAL
CONVERGENCE
ZONE
DOLDRUM BELT
DRUM
30O
60O
30O
UB
UB
DOL-
POLAR HIGH
CUMULONIMBUS
TOPS AS HIGH AS
60,000 FT CEILING
BELOW 1,000 FT
WINDS EASTERLY AT SURFACE
WESTERLY ALOFT
WINDS WESTERLY
AT ALL ELEVATIONS
WINDS EASTERLY TO 5,000 FT
THEN WESTERLY
WINDS VARIABLE
WINDS EASTERLY
TO 25,000 FT,
THEN WESTERLY
60,000 FT.
25,000 FT.
AG5f0306
Figure 3-6.—Idealized pattern of the general circulation. (The 3-cell theory).
