Coriolis force or Coriolis effect stems from the fact that Earth rotates under the
wind resulting in the wind being bent (to the right in the Northern Hemisphere and
the left in the Southern Hemisphere) of the pressure gradient direction. Coriolis
effect is negligible in the tropics and at maximum in the polar regions. Addition-
ally, the magnitude of Coriolis effect corresponds to the wind speed. Coriolis
effect is crucial in making large atmospheric disturbances rotate.
Friction is the third force. Friction slows down the wind and alters its direction.
Friction is imparted to the wind by contact with Earth’s surface and is quite varia-
ble over the planet. The least friction is encountered over bodies of water and the
greatest over mountains. The rougher the landscape, the greater the friction so that
forests and cities are places with slower winds. Over a kilometer or so above
Earth’s surface air is virtually frictionless as distance from the friction-causing
surface increases.
The result is the wind that actually blows near Earth’s surface. Wind moves
from the high to the low pressure side of the isobars. Winds aloft, over a kilometer
from the surface, are frictionless and blow faster than the near-surface winds and
parallel to the isobars. In this case, only the pressure gradient and Coriolis effect
act on the air, and the wind is termed “geostrophic.”
If near-surface isobars are arranged around a center of low or high pressure, air
will rotate about the center. In the case of low pressure, the motion will be a counter-
clockwise spiral into the center in the Northern Hemisphere and a clockwise spiral
into the center in the Southern Hemisphere; such is the case withmiddle latitude
cyclones, hurricanes, and tornadoes. In the case of high pressure, the motion will
be a clockwise spiral away from the center in the Northern Hemisphere and a
counterclockwise spiral away from the center in the Southern Hemisphere.
In the vertical dimension, there is not as much flow in the atmosphere as in
the horizontal. In fact, worldwide there is a vertical balance between pressure
gradient force and gravity. Although air is transported upward and downward from
imbalances in these two forces, horizontal imbalances are so much greater that
horizontal motion transports thousands oftimes more air than vertical motion.
Vertical imbalances, though, are crucial when considering the rise of air causing
precipitationor the sinking of air causing cloud-free conditions.
Latitudinal imbalances of energy are key to understanding the world’s wind and
pressure belts sometimes called the global circulation. In the equatorial latitudes
where there is an energy surplus, there is a broad zone of low pressure into which
winds converge. This is known as the Intertropical Convergence or the doldrums.
Winds are light and variable. The winds converging into the Intertropical Conver-
gence are the trade winds that, on average, extend 5–20°in latitude away from the
equator. These are the most dependable flows of air on the planet, and in the
Northern Hemisphere blow from the northeast and in the Southern Hemisphere368 Winds and Pressure Systems