Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 304

6
Tornadoes
''In this review no consideration has been given to the tornado, which
energetically is only a detail in the severe storm. However, its importance as a
hazard and the interest of the problems which it poses make it desirable to
indicate its probable place in the cumulonimbus model.''
F. H. Ludlam (1963)
6.1 BASIC OBSERVATIONAL ASPECTS OF TORNADOES
Tornadoes are rapidly rotating columns of air that make contact with the ground.
In general we consider them to be intense, columnar vortices in contact with the
ground that are capable of inflicting damage. They are either connected to or are
situated underneath a cumuliform, buoyant convective cloud above ( Figure 6.1 ) .
Tornadoes are sometimes, but not always, visible as a funnel cloud when the
pressure deficit in them is low enough that water vapor condenses and cloud
particles form ( Figure 6.2 ). When the air is too dry or the pressure not low
enough for a condensation funnel to form or to extend down to the ground, the
tornado may be visualized as a column of rotating dust or debris ( Figure 6.3 ) or
not visualized at all if there are no visible particles available to be lofted. In some
instances they are encased in precipitation (''rain-wrapped'') and either not visible
at all or visible only from a restricted viewing angle ( Figure 6.4 ).
Lasting anywhere from just a few seconds to an hour or more, most tornadoes
last only 10min. The diameter of a tornado is usually 200m, but can vary
from as narrow as 10mtoaswideas 1.5-2 km. Most tornadoes are distinctly
narrower than mesocyclones, which are 2-5 km in diameter. Tornadoes are the
most violent and destructive of all the severe weather phenomena that localized
convective storms produce.