Tornado Development



   tornado, noun.  A rotating column of air usually accompanied by a funnel shaped downward extension of a cumulonimbus cloud and having winds whirling destructively at speeds of up to 300 miles per hour. (American Heritage Dictionary of the English Language)

   Tornadoes are weather at its most violent and the very sight of one inspires fear and awe.  The May 3, 1999 severe weather outbreak in Oklahoma and Kansas was a showcase of the destructive power of tornadoes.  The following scenario displays the life cycle of a classic "killer tornado".  Many of strong tornadoes of the May 3, 1999 outbreak experienced most if not all of these stages.  


The Supercell Thunderstorm

   Before a tornado can be discussed, the environment in which they form must introduced.  The strongest tornadoes are often produced from large, single celled, and rotating thunderstorms termed supercell thunderstorms.  They occur in an environment of strong vertical wind shear and tend to propagate to the right of mean atmospheric flow.  Storms rotate due to a mesocyclone and contain a rotating updraft as well as a forward flank and rear flank downdraft.  Supercells tend to be long lived, lasting on the order of hours, and can generate multiple tornadoes which themselves can last on the order of tens of minutes.  Individual tornadoes have been known to last for over an hour.  Other severe weather associated with supercells include large hail and lightning.

Supercell Image

 

     The above view of a supercell shows typical surface conditions associated with the storm.  The likely position of tornadoes associated with supercells is in the region where the storms updraft meets the rear flank downdraft of the storm.  This Region is characterized by the "hooked" region seen in radar imagery and is marked T in the above diagram.  The area is normally located in the southwest portion of the storm.  This hook is indicative of storm rotation caused by the mesocyclone which itself causes the supercell to rotate.  Weaker tornadoes can also be associated with the gust front which is caused by the downdrafts of the storm and evaporation of precipitation. Another diagram is shown here in profile:

    Notice the downdrafts shown with the blue arrows as well as the rotating updraft shown in red.  Also observe how the three dimensional profile relates to the surface conditions in the first image of storm surface conditions.

(Schematics found at http://outreach.ocs.ou.edu/teachers/graphics/SevereWx.html)   

    Follow these links for thorough discussions of Radar Imagery and Satellite Imagery in association with supercells and tornadoes. 


Development or Organization stage

     Tornadoes generated by supercell thunderstorms typically form out of a wall cloud, which is a mass of cloud extending down from the rain free base of the supercell.  Appearance of and rotation in this cloud structure is usually a sign of an impending tornado.  A funnel cloud may soon appear and descend to the surface as a spike from the wall cloud.  It is important to realize that the funnel cloud appears due to condensation in the vortex of the tornado.  It is quite possible that tornadic rotation could be occurring at the ground before the funnel fully descends, and this can be evident through a debris cloud or damage at the surface underneath a funnel cloud that is not extended from the wall cloud to the surface. 

Examples of Wall Clouds:

Wall Cloud Image 1

Wall Cloud Image 2

Many other great examples of wall clouds  can be found at http://webserv.chatsystems.com/~tornado/SkyPix/skypixwc.htm 

Examples of Tornadoes Late in the Developing Stage:

Developing Stage Image 1

Developing Stage Image 2

Developing Stage Image 3


Mature Stage

   After tornado touchdown the funnel rapidly begins to expand in width.  At this point the tornado has entered the mature stage, where the it achieves its maximum size and strength.  This stage in the life cycle can last on the order of tens of minutes with the tornado cutting a long path of destruction across the landscape.  Throughout this period the tornado may fluctuate in intensity somewhat, but generally stays very destructive. 

Examples of Tornadoes in the Mature Stage:

Mature Stage Image 1

Mature Stage Image 2

   Some tornadoes can even be comprised of multiple suction vortices which rotate inside a parent tornado.  This form of tornado can be particularly destructive because the wind speed of the suction vortices can combine with the speed of the parent tornado to create a very powerful and destructive wind.


Shrinking Stage

   Once a tornado has continued through the mature stage for some time it may begin to shrink and become more horizontally tilted along with dissipation of the wall cloud.  This signifies the shrinking stage.  Although the tornado has dissipated in intensity somewhat it still has the potential to do considerable damage.  The damage path of the tornado may be smaller but the wind speed and damage to structures it causes may not be any less than in the mature stage.

Example of Tornado in the Shrinking Stage:

Shrinking Stage Image


Decaying Stage

   At the end of a tornado's life the wall cloud from which the tornado appeared may have completely vanished and have left only a very thin and contorted appearance of a funnel cloud.  A tornado at the beginning of the stage may still be quite potent even though it appears to be dying.  Gaps may appear in the funnel as it ropes through the sky underneath the supercell.  The death of the tornado is signified by the disappearance of the funnel and debris cloud at the surface below the storm.

Examples of Tornadoes in the Decaying Stage:

Decaying Stage Image 1

Decaying Stage Image 2


Link to Tornado Analysis

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