Editing Mesocyclone (section)

== Tornado formation ==
[[File:Funnels Over Falcon.jpg|thumb|A tornado developing under a wall cloud within a mesocyclone near [[Falcon, Colorado]].]]
{{See also|Tornado}}

Tornado formation is not completely understood, but often occurs in one of two ways.<ref name=severe/><ref name=tornadoFAQ/>

In the first method, two conditions must be satisfied. First, a horizontal spinning effect must form on the Earth's surface. This usually originates in sudden changes in wind direction or speed, known as wind shear.<ref name="nssl.noaa.gov"/> Second, a cumulonimbus cloud, or occasionally a cumulus cloud, must be present.<ref name="nssl.noaa.gov"/>

During a thunderstorm, updrafts are occasionally powerful enough to lift the horizontal spinning row of air upwards, turning it into a vertical air column. This vertical air column then becomes the basic structure for the tornado. Tornadoes that form in this way are often weak and generally last less than 10&nbsp;minutes.<ref name="nssl.noaa.gov"/>

The second method occurs during a supercell thunderstorm, in updrafts within the storm. When winds intensify, the force released can cause the updrafts to rotate. This rotating updraft is known as a mesocyclone.<ref name=Thinkquest-2003-10/>

For a tornado to form in this manner, a [[rear-flank downdraft]] enters the center of the mesocyclone from the back. Cold air, being denser than warm air, is able to penetrate the updraft. The combination of the updraft and downdraft completes the development of a tornado. Tornadoes that form in this method are often violent and can last over an hour.<ref name="nssl.noaa.gov"/>