Vertical draft

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An updraft or downdraft (air pocket) is the vertical movement of air as a weather related phenomenon. Commonly, one of two forces causes the air to move. Localized regions of warm or cool air will exhibit vertical movement. A pocket of warm air will typically be less dense than the surrounding region, and so will rise until it reaches air that is either warmer or less dense than itself. The converse will occur for a mass of cool air, and is known as subsidence. This movement of large volumes of air, especially when regions of hot, wet air rise, can create large clouds, and is the main cause of thunderstorms. Drafts can also be created by low or high pressure regions. A low pressure region will attract air from the surrounding area, which will move towards the center and then rise, creating an updraft. The reverse will naturally occur in a high pressure region, as air moves away from the high pressure center.

[edit] Notable effects

Updrafts and downdrafts are some of the main causes of in-flight turbulence in planes, and are one reason airlines try to avoid regions of bad weather. Extreme cases, known as downbursts and microbursts are responsible for many airplane crashes. The crash of Delta Air Lines Flight 191 on their final approach leg before landing in 1985 was presumably caused by a microburst, and prompted the Federal Aviation Administration to research and deploy new storm detection radar stations at some of the major airports.

Downbursts can cause extensive localized damage, similar to that caused by tornadoes. Downburst damage can be differentiated from that of a tornado because the resulting destruction is circular and radiates away from the center. Tornado damage radiates inward, towards the center of the damage.