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The Wind & How It Forms
Updated: Wednesday, August 14 2013, 10:12 PM EDT
When you see the leaves shake in a tree or a plastic bag roll across the grass, you are seeing the result of pressure differences in the atmosphere.
The wind is simply the horizontal motion of air; this motion is fueled by the positioning and strength of high and low pressure centers near and above the ground. The force that sets the air is motion is called the pressure gradient force, which is a force that acts from high to low pressure in an effort to eliminate the pressure difference between the two. In essence, the pressure gradient force is constantly working to restore balance in the atmosphere.
|A diagram of how the Corilois Force deflects objects over time in the Northern and Southern Hemispheres (via UIUC).|
The pressure gradient force isn't the only force working to make the wind. The closer you get to the ground, the more friction must be considered. The rough surface of Earth is an example of friction, a force that works against the wind. One more force will have a say in where the wind blows: the Coriolis Force. This is an apparent force that deflects objects in the atmosphere from their intended path (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). Because we live on a rotating, spherical planet, points at different latitudes rotate at different speeds. The Coriolis Force corrects for this difference in rotational speeds at different latitudes.
The wind is what becomes of these forces acting together. Just like people around a table fighting for an object at the center of the table, sometimes one or more of these forces oppose each other or balance out with another force.
While this balance of forces may seem complex, it's often easy to know how the wind will behave around areas of high and low pressure. Just based on the pressure gradient force and the Coriolis Force alone, wind tends to flow clockwise around high pressure and counterclockwise around low pressure near the ground in the Northern Hemisphere. For example, the wind directly to the west of high pressure and to the east of low pressure tends to be out of the south.
|A surface map from the morning of March 5, 2013 shows isobars (lines of equal pressure) tightly packed west of a strong area of low pressure. Tightly packed iisobars are often a sign of strong winds (via NOAA).|
The strength and positioning of high and low pressure also an impact on how strong the wind will be. If a strong low or high pressure center is approaching a city, the wind speed there is likely to increase. If a strong low or high center is moving away from a city, the wind speed there is likely to decrease. When areas of low and high pressure come close together, the pressure gradient between them increases, and therefore the wind speeds will increase.
The wind may be also influenced by forces in the atmosphere, such as sea breezes, land breezes, mountains, or outflow boundaries from nearby, dying thunderstorms. These forces may affect the speed and direction of the wind.
While it is important to forecast the wind at the ground, the direction and speed of winds above the ground are also important. Differences in wind speeds and directions above the ground help to create disturbances, which can tend help to create clouds, showers, and storms.