Air does not move in straight lines over the Earth’s surface. Its motion is influenced by several forces that shape global wind patterns and weather systems. One of the most important of these forces is the Coriolis force, which results from the rotation of the Earth and plays a key role in atmospheric circulation.
In general atmospheric circulation theory, areas of low pressure exist over the equatorial regions and areas of high pressure exist over the polar regions due to differences in temperature. The resulting pressure gradient causes air to flow along the Earth’s surface from the poles toward the equator. While this pattern of air circulation is correct in theory, it is modified by several forces, the most important of which is the rotation of the Earth.
The force created by the rotation of the Earth is known as the Coriolis force. This force is not perceptible to humans as they walk around because humans move slowly and travel relatively short distances compared to the size and rotation rate of the Earth. However, the Coriolis force significantly affects motion over large distances, such as air masses and bodies of water.
The Coriolis force deflects moving air to the right in the Northern Hemisphere, causing it to follow a curved path instead of a straight line. The amount of deflection depends on latitude. It is greatest at the poles and decreases to zero at the equator. The magnitude of the Coriolis force also depends on the speed of the moving object—the greater the speed, the greater the deflection. In the Northern Hemisphere, Earth’s rotation deflects moving air to the right and modifies the general circulation pattern of the atmosphere.
The Coriolis force causes global circulation to break up into three distinct cells in each hemisphere. [Figure 1]
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| Figure 1. Three-cell circulation pattern due to the rotation of the Earth |
In the Northern Hemisphere, warm air at the equator rises, travels northward, and is deflected eastward by Earth’s rotation. By the time it has traveled about one-third of the distance from the equator to the North Pole, it is no longer moving northward but primarily eastward. This air cools and sinks in a belt-like region at about 30° latitude, creating an area of high pressure. It then flows southward along the surface back toward the equator.
The Coriolis force bends this flow to the right, creating the northeasterly trade winds that prevail from 30° latitude to the equator. Similar circulation cells form between 30° and 60° latitude and between 60° and the poles. This global circulation pattern also contributes to the prevailing westerly winds in the conterminous United States.
Circulation patterns are further complicated by seasonal changes, differences between land and ocean surfaces, and frictional forces caused by Earth’s surface topography, all of which modify atmospheric motion. For example, within 2,000 feet of the ground, friction between the surface and the atmosphere slows the wind. The wind is therefore deflected from its path due to this frictional force. As a result, wind direction at the surface often differs from wind direction just a few thousand feet above the ground.