Circulation of the Wind Upon Earth, Continued
The tradewinds are found just north and south of the doldrums.
Whenever the doldrums are absent in some part of the equatorial region,
the tradewinds of the Northern and Southern Hemispheres converge,
causing heavy rain squalls. A feature of the trade belt is the regularity
of the systems, especially over the oceans. The wind blowing above and
counter to the trade is called the ANTITRADE.
The areas of the subtropical high-pressure cells, where the winds are
light and variable are about 30°N to 40°N and 30°S to 40°S. They are
called the horse latitudes. Fair weather is characteristic of this region,
due to the descending air. The pressure decreases outward from this
area, and the prevailing westerlies are on the poleward side, with the
tradewinds on the equatorial side.
The prevailing westerlies, which are on the poleward side of the
tradewinds are persistent through the midlatitudes. In the Northern
Hemisphere their direction at the surface is from the southwest, and in
the Southern Hemisphere from the northwest. This is a result of the
deflection caused by the Coriolis force as air moves poleward. The
Coriolis effect is the apparent force exerted by the rotation of Earth.
The front zone lies poleward of the prevailing westerlies.
In the polar cells, polewards of the polar front zone, the surface winds
are known as the polar easterlies (polar northeasterlies at the North Pole
and southeasterlies at the South Pole). They move the northeast in the
Northern Hemisphere and from the southeast in the Southern
Hemisphere. They are very shallow due to the low temperatures and are
overlain by the westerlies. This circulation pattern is temporarily
disrupted by the migratory pressure systems in all areas but returns to
the original pattern.