Environmental Risks and Disasters
Lecture Notes 4 (1/30/2008)
The phenomenology of tornados
According to a meteorological dictionary a tornado is `a violently rotating column
of air, pendant from a cumulonimbus cloud, and nearly always observable as a
funnel cloud or tuba'. This definition includes several characteristics: the funnel
shape, the rotation, and the association with thunderstorms (the cloud type).
Technically, a tornado has to be in contact with the ground and with a cloud in
order to be a tornado.
Tornados are the most violent of the so-called vortex storms, which also include
dust-devils, water spouts, hurricanes and mid-latitude cyclonic storms. Localized
whirlwinds such as dust devils are not associated with clouds and are not
tornados. Hurricanes are orders of magnitude larger in size, and are energetically
driven by different processes than a tornado.
The funnel of a tornado is usually observed as being dark. This is simply the
effect of the water droplets and dust that are trapped in the vortex and blocking
the light. Close to the ground, the tornado stirs up dust and debris creating a
The size of a tornado (meaning the size of the funnel on the ground) can vary
quite a bit. The widest tornado observed had a width of approximately 2 miles.
More commonly, they are about 300 meters wide, though they can be as narrow
as tens of meters. Most tornados are relatively short lived, lasting on average 10-
15 minutes and traveling some 10 km. Some last only for a fraction of a minute,
affecting only a localized spot, others can travel for hours and for more than 100
km. Most tornados can be outdriven in a car, if there is a road going in the right
Much of what we know about tornados has been deduced by carefully analyzing
the damage caused by the tornado. The reason for this is simply that very few
instruments can survive the tremendous winds that exist within the tornado
funnel. A famous tornado expert was Theodore Fujita, who was the first to
characterize many aspects of the tornado phenomenon. He developed the scale
that is currently used to describe the damage caused by a tornado, and to estimate
indirectly the speed of the wind in the tornado.
Tornados generate the highest wind velocities that we observe in any storm.
Direct measurements are very difficult to make. In 1999, there was a wind
velocity measurement of 318 mph made in Oklahoma using a doppler-radar
instrument, which does not have to be in the wind to make a measurement. This
measurement was, however, not at ground level, but measured off the side of the
funnel at some elevation. It is thus not clear how strong the winds can be on the