Air can move along a vertical or horizontal axis. When large quantities of air move up along a vertical axis, the end result is cloud formation. By contrast, horizontally moving air results in wind. Several factors play an important role in the initial creation of wind as well as the relative speed at which the wind blows, including the pressure-gradient force, the coriolis effect and friction.
The pressure-gradient force is the means by which wind is initially generated. It forces still air to start moving. Variations in the amount of horizontal pressure being applied to pockets of air by the atmospheric PGF can make still air start moving if a greater amount of pressure is applied from a certain direction. The intensity of the PGF determines to a large degree how strong the wind blows in a given area.
The coriolis effect, or coriolis force, does not wield a great deal of influence over the velocity of wind, but it does influence its direction. Winds directed in a certain way can become stronger, depending on local weather conditions. A natural result of the rotation of the Earth, the coriolis effect deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; this determines wind direction and will also influence its velocity if the wind is directed into a storm or other pattern of unsettled weather.
Friction has a mitigating effect on the PGF and actually acts to slow down the movement of horizontally flowing air. However, friction only affects wind speed within a mile or two of the Earth's surface; in the upper atmosphere, it has no effect at all and wind speeds are much higher as a result.
If wind enters a convergent or divergent weather pattern, such as in a strong or forming storm, the combined forces already at work in the weather system can quickly and dramatically increase wind speeds. This is the reason that cyclones and hurricanes generate such strong winds.
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