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To
understand how floods work, you have to know something about how water
behaves on our planet. The total amount of water on Earth has remained
fairly constant for millions of years (though its distribution has
varied considerably in that time). Every day, a very small amount of
water is lost high in the atmosphere, where intense ultraviolent rays
can break a water molecule apart, but new water is also emitted from the
inner part of the Earth, by volcanic activity. The amount of water that
is created and the amount that is lost are pretty much equal.
Driven
by these air-current cycles, Earth's water supply moves in a cycle of
its own. When the sun heats the oceans, liquid water from the ocean's
surface evaporates into water vapor in the air. The sun heats this air
(water vapor and all) so that it rises through the atmosphere and is
carried along by wind currents. As this water vapor rises, it cools down
again, condensing into droplets of liquid water (or crystals of solid
ice). Collections of these droplets are called clouds. If a cloud moves
into a cooler environment, more water may condense onto these droplets.
If enough water accumulates in this way, the droplets become heavy
enough that they fall through the air as precipitation (rain, snow,
sleet or hail). Some of this water collects in large, underground
reservoirs, but most of it forms rivers and streams that flow into the
oceans, bringing the water back to its starting point.
Since
waterways are formed slowly over time, their size is proportionate to
the amount of water that normally accumulates in that area. When there
is suddenly a much greater volume of water, the normal waterways
overflow, and the water spreads out over the surrounding land. At its
most basic level, this is what a flood is -- an anomalous accumulation
of water in an area of land. Source: http://science.howstuffworks.com |
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