Causes of Turbulence

There are many different types of turbulence that a pilot needs to consider:

Wind Shear:	A sudden, drastic shift in wind speed and/or direction. It may occur at any altitude in a vertical or horizontal plane.  It is associated with temperature inversions. With fronts, the most critical period is either just before or just after the frontal passage. With a cold front, wind shear occurs as the front passes and a small time afterwards. Warm fronts create much stronger wind shears than cold fronts. It only occurs before the passage of the warm front and can last up to six hours. Once the warm front has passed, the danger ceases to exist.
Microburst:	An intense, localized downdraft of brief duration which spreads out in all directions when it reaches the surface. Microbursts create severe horizontal and vertical wind shears which pose serious hazards to aircraft, particularly those near the surface. Sometimes they cover areas of less than two and a half miles at the surface and usually last no longer than fifteen minutes. Any convective cloud can produce this phenomenon. Microbursts are commonly associated with heavy precipitation in thunderstorms but can also occur in virga, which are streamers of precipitation that trail beneath a cloud but evaperate before they hit the ground.
Mechanical Turbulence:	When obstacles like buildings or rough terrain interfere with the normal wind flow, turbulence develops. This is often experienced in the airport environment when the wind blows around or over hangers.
Mountain Waves:	When stable air moves across a ridge and the wind is 40 knots or greater. The wave pattern may extend up to 100 miles or more downwind, and the crests may extend well above the highest peaks. The Rotor  is an area of rotary circulation which forms below the mountain peaks. Both the rotor and the waves can create violent turbulence.

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