TORNADOES

  Tornadoes are not the largest of storms, but they are definitely the
  most violent.  There is no other weather occurrence that produces more
  destructive power as it passes by.  It sweeps the ground clear of all
  moveable objects.  It is a violently circulating column of air, small in
  diameter, and reaches from a thundercloud to the ground.  A tornado starts
  as a funnel cloud which is a rotating column of air inside a cumulonimbus
  cloud and begins to reach for the ground.  When this rotating air reaches
  the ground, it is an official tornado.  As the tornado matures, the funnel
  reaches its greatest diameter.  It is usually vertical, most of the time
  it is touching the ground, although many times skipping occurs along its
  path.  This is when the worst damage occurs, and many large pieces of
  debris can be seen within its circular motions.  As a tornado starts
  to shrink, the funnel becomes smaller and tilts from its vertical angle.
  As the tornado decays, it becomes long, distorted, and rope-shaped.  The
  end is near for this wind of destruction.

  In the Northern Hemisphere, tornadoes nearly always turn counterclockwise,
  although officially documented instances of clockwise tornadoes are on
  record.  Most tornadoes occur when temperatures are above 65 degrees F.,
  (18 degrees C.), and the dew point temperature is at 50 degrees F.,
  (10 degrees C.) or higher.  The vast majority of tornadoes travel from
  southwest to northeast, travel an average of 35 MPH, is an average width
  of 121 yards, and has an average life of 14 minutes.  Thus, this average
  tornado would travel about 8.5 miles.  It is interesting to note that even
  though professionals know what type of clouds typically create tornadoes,
  exact time and location is difficult, if not impossible, to predict with
  absolute accuracy.  There has never been any wind speed measuring
  device created that can withstand the tortures of a tornado.  However,
  it is possible to estimate the wind speeds using less direct methods.
  There have been many films and videos of the large debris flying around
  in tornadoes.  Engineers have determined the force required to move these
  pieces of debris; thus, finding the approximate wind speed.  More
  recently Dopler radar has created more accurate findings.

  Some definitions: A 'funnel cloud' is a potential tornado which never
  touches the ground.  'Dust devils' and 'sand devils' are common in dry
  desert areas, and are circulating columns of dust or sand.  These can extend
  up several hundred feet, and last but a few minutes.  These are not to be
  confused with tornadoes as they are caused by local heating of the earth's
  surface, while tornadoes are created in large thunderclouds.