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Figure 1-15.—Roll cloud formation on cumulonimbus. | Up
Aerographers Mate, Module 01-Surface Weather Observations | Next
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from the base of the CB cell. As the funnel develops, it
will gradually assume the more common cone-shaped
appearance. When the funnel cloud (fig. 1-16) extends
downward from the CB base to a point where its circular
rotation reaches the ground, it is then called a tornado
(fig. 1-17). Funnel clouds and tornados contain
destructive, concentrated, cyclonic winds in excess of
150 knots. The force of the wind is amplified by the
rapid change in the direction of the winds as the tornado
passes over an area. Estimates based on damage equate
the force of tornados to straight-line winds of near 500
miles per hour. Funnel clouds can be seen due to the
visible moisture from the parent cloud. Under a funnel
cloud, the rapidly circulating winds may be invisible
until the circulation picks up dust and debris from
earth’s surface.
Waterspouts (fig. 1-18) develop over warm ocean
or bay waters more frequently than overland. They
have been observed from the bases of rapidly building
towering cumulus cells, often without any precipitation
occurring. They are generally weaker than tornadoes,
but still contain dangerous, destructive winds.
Figure 1-17.—Tornado.
When conditions are favorable for tornado
development, waterspouts may be assumed to be as
strong as a tornado.
But when conditions are NOT
favorable for tornadic development, then any
waterspouts that form are usually less powerful. A
Great Lakes freighter, S.S. Edmond Fitzgerald, was lost
Figure 1-16.—Funnel clouds.
Figure 1-18.—Waterspout.
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