Wind
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, or land and sea breezes—are examples of local wind. Winds, driven by large pressure systems also exist in great wind belts that comprise the earth's atmospheric circulation.
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.
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. The earth, however, 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 an apparent force 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 Coriolis effect deflects winds from the straightforward direction across isobars. In the Northern Hemisphere, the Coriolis effect tends to deflect winds right of path and in the Southern Hemisphere, it tends to drive winds left of path.
For example, wind in the Northern Hemisphere initially begins to move from west to east as a result of pressure gradient forces. The Coriolis effect results in a deflection of the wind right of path. This results in air moving out of a high-pressure system (an area of divergence) to spin clockwise. Conversely, air moving into a low pressure area (an area of convergence) also deflected right of path, is spun counterclockwise.
The actual path followed by the wind 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.
The Coriolis effect is most pronounced on winds farther from the surface of the earth. At distances of more than a half a mile 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.
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. Annual changes in weather patterns produce seasonal winds such as the dry Santa Ana winds in Southern California.
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