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Wind

Wind is simply air in motion relative to the earth's surface. We normally think of the wind as the horizontal motion of the air, although air actually moves in three dimensions. The vertical component of the wind is generally quite small, except in thunderstorm updrafts. The vertical motion of air, however, is very important in determining our weather. Air that is rising cools, which may cause it to reach saturation and form clouds and precipitation. Conversely, air that is sinking warms, which causes clouds to evaporate and produce clear weather. (See clouds section.)


Surface maps usually have H's and L's at various locations. The H's and L's represent high and low pressure systems. On weather maps highs and lows are surrounded by lines called isobars. Isobars are lines of constant pressure; they connect every location that has the same value of pressure. When isobars are packed close together, the pressure is changing rapidly over a small distance. The closer the isobars are packed together, the stronger the pressure gradient (the rate of pressure change over a given distance.) Also, notice that (in the Northern Hemisphere) the wind blows clockwise around a high pressure system and also slightly outward from its center. Around a low pressure system, the wind blows counterclockwise and slightly in towards its center.



Why does the wind blow? There are three forces that cause the wind to blow in the direction that it does:

1. Pressure Gradient Force
2. Coriolis Force
3. Friction

Pressure Gradient Force


The Pressure Gradient Force (PGF) arises due to differences in pressure within the atmosphere. In a physical sense, this force is trying to move air to eliminate pressure differences. The PGF causes air to flow from high pressure to low pressure. In the absence of any other forces, wind would blow directly from high to low pressure. The PGF also affects the speed of the wind. As the PGF becomes stronger (i.e. pressure changing rapidly with distance), the wind speed increases. When looking on a surface map, strong winds would occur in locations where the isobars are packed close together (strong PGF).



Coriolis Force


A complicated force that affects the wind is the Coriolis Force. The Coriolis Force is due to the earth's rotation. This force causes moving objects (i.e. air, planes, birds, etc) to deflect to the right of their motion in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis Force is strongest near the poles and zero at the equator. In most of the atmosphere, it is nearly equal and opposite the PGF. If the PGF and the Coriolis Force are exactly equal and opposite, the wind would blow parallel to isobars, with high pressure on the right.



Friction


  The third force acting on the wind is friction. Friction becomes very important near the earth's surface because the surface of the earth is rough. Friction is the force that causes air to slow down and spiral into lows and out of highs. When air spirals into the low, it is converging into the low. When air converges near the surface, it is forced to rise. As air rises, it may condense and form clouds and precipitation. This is why low pressure systems are often associated with adverse weather conditions. Conversely, high pressure systems are generally associated with fair weather. When air spirals out of the high, it is actually diverging. As air diverges from the high, the air above the surface must sink in order to replace the air that is moving away from the high. Sinking air warms and tends to evaporate any clouds that may be present.

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