Wind

The term wind refers to any flow of air relative to the Earth's surface in a roughly horizontal direction. Breezes that blow back and forth from a body of water to adjacent land areas—on-shore and off-shore breezes—are examples of wind.

The ultimate cause of Earth's winds is solar energy . When sunlight strikes Earth's surface, it heats that surface differently. Newly turned soil , for example, absorbs more heat than does snow.

Uneven heating of Earth's surface, in turn, causes differences in air pressure at various locations. On a weather map, these pressure differences can be found by locating isobars , lines that connect points of equal pressure. The pressure at two points on two different isobars will be different. A pressure gradient is said to exist between these two points. It is this pressure gradient that provides the force that drives air from one point to the other, causing wind to blow from one point to the other. The magnitude of the winds blowing between any two points is determined by the pressure gradient between those two points.

The Coriolis effect and wind direction

In an ideal situation, one could draw the direction of winds blowing over an area simply by looking at the isobars on a weather map. But the earth is not an ideal situation. At least two important factors affect the direction in which winds actually blow: the Coriolis effect and friction . The Coriolis effect is a pseudoforce that appears to be operating on any moving object situated on a rotating body, such as a stream of air traveling on the surface of the rotating planet . The effect of the Coriolis force is to deflect winds from the straight-forward direction that we might expect them to take simply from an examination of isobars. In the Northern Hemisphere, the Coriolis effect tends to deflect winds to the right and in the Southern Hemisphere, it tends to drive winds to the left.

Imagine how the Coriolis effect will determine the movement of winds in the Northern Hemisphere. Suppose that air initially begins to move from west to east as a result of pressure gradient forces. At once, the Coriolis effect will begin to drive the stream of air to the right, that is, to the south. The actual path followed by the wind, then, is a compromise between the pressure gradient force and the Coriolis force. Since each of these forces can range widely in value, the precise movement of wind in any one case is also variable.

At some point, the two forces driving the wind are likely to come into balance. At that point, the wind begins to move in a straight line that is perpendicular to the direction of the two forces. Such a wind is known as a geostrophic wind.

Friction and wind movement

The picture described above applies to winds that blow in the upper atmosphere. At distances of more than a kilometer or so above the ground, pressure gradient and Coriolis forces are the only factors affecting the movement of winds. Thus, air movements eventually reach an equilibrium point between pressure gradient forces and the Coriolis force, and geostrophic winds blow parallel to the isobars on a weather map.

Such is not the case near ground level, however. An additional factor affecting air movements near the earth's surface is friction. As winds pass over the earth's surface, they encounter surface irregularities and slow down. The decrease in wind speed means that the Coriolis effect acting on the winds also decreases. Since the pressure gradient force remains constant, the wind direction is driven more strongly toward the lower air pressure. Instead of developing into geostrophic winds, as is the case in the upper atmosphere, the winds tend to curve inward towards the center of a low pressure area or to spiral outward away from the center of a high pressure area.

Friction effects vary significantly with the nature of the terrain over which the wind is blowing. On very hilly land, winds may be deflected by 30 degrees or more, while on flat lands, the effects may be nearly negligible.

Local winds

In many locations, wind patterns exist that are not easily explained by the general principles outlined above. In most cases, unusual topographic or geographic features are responsible for such winds, known as local winds. Land and sea breezes are typical of such winds. Because water heats up and cools down more slowly than does dry land, the air along a shoreline is alternately warmer over the water and cooler over the land, and vice versa. These differences account for the fact that winds tend to blow offshore during the evening and on-shore during the day.

The presence of mountains and valleys also produces specialized types of local winds. For example, Southern Californians are familiar with the warm, dry Santa Ana winds that regularly sweep down out of the San Gabriel and San Bernadino Mountains, through the San Fernando Valley, and into the Los Angeles Basin , often bringing with them widespread and devastating wildfires.

See also Atmospheric circulation; Atmospheric pressure.

Resources

books

Ahrens, C. Donald. Meteorology Today. 2nd ed. St. Paul: West Publishing Company, 1985.

Battan, Louis J. Fundamentals of Meteorology. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1979.

Holton, James R. An Introduction to Dynamic Meteorology. 2nd ed. New York: Academic Press, 1979.

Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere: An Introduction to Meteorology. 4th ed. Englewood Cliffs, NJ: Prentice Hall, 1989.

David E. Newton

KEY TERMS

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Coriolis effect

—An apparent force experienced by any object that is moving across the face of a rotating body.

Geostrophic wind

—A movement of air that results from the balance of pressure gradient and Coriolis forces and that travels across isobars.

Isobar

—A line on a weather map connecting points of equal atmospheric pressure.