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2. What is a tornado?
Once rotation has begun it extends through the depth of the cloud in the region
of updraft. Within a short period of time the updraft becomes concentrated
along the axis of the rotation and a column extends from the cloud toward
the ground. A measure of the strength of the tornado is often the diameter
of the funnel that extends downward. A wide funnel that drops directly to
the ground will draw in air from a wide area to feed the storm. The pressure
fall near its centre will be immense (particularly given its small size).
By contrast, narrow funnels that twist like a rope are usually relatively
weak. There are two reasons why the funnel is visible. First, as air enters
the low pressure area it expands, cools and water vapour condenses into
droplets. Second, and more commonly, the funnel lifts dust and debris into
the atmosphere and this swirls around the vortex. Water spouts are tornadoes
that form over water and are visible because a column of water is sucked
upwards into the cloud - can you imagine the force required to rise a column
of water several hundred metres into the air? Not surprisingly, tornadoes
are associated with 'freak weather' - rainfalls of fish and frogs, chickens
with their feathers plucked off, straw driven into wood, etc..
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