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Precipitation (cont'd) Rain is the most common form of precipitation
in Missouri. It is also the easiest form of precipitation
to understand. Quite simply it is the liquid water that falls
from clouds. But, have you ever wondered what causes rain
to fall? An easy example to demonstrate rain is to
use a mirror in a steamy bathroom. If you turn the shower
on hot the room steams up. The steam is very similar to a
cloud. They are both made of water vapor or tiny water particles.
When the steam comes in contact with the cold mirror it condenses.
That is the particles collect and "fog" the mirror.
The steam is very warm and the coldness of
the mirror causes it to cool. When the water particles cool
they begin to gather together to become bigger particles,
and eventually get big enough to form drops of water. After a few minutes in the steamy bathroom
enough water particles will collect to form water drops on
the mirror. At that point they get too big to stay up and
begin to run down the mirror. Those drops are the equivalent
of rain. When the water particles in a cloud cool they
condense and become bigger particles eventually becoming water
drops. The water drops become so big that they fall as rain.
To really understand this process we need
to understand a concept called Dew Point.
Remember that dew point is the temperature at which water
vapor in the air turns to liquid. The dew point is different
from day to day. It changes due to temperature and humidity.
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| figure 1 |
Lets look at what really happens to form rain.
Examine the following graphic:
- Begin at ground level and consider the air temperature
to be 56° and the dew point 45°.
- Air is forced upward by a cold front or other occurrence.
As the air rises the temperature decreases at approximately
5.5 degrees Fahrenheit per 1000 feet. The dew point does
not change.
- At the height where temperature and dew point become
equal, a cloud will begin to form.
- Dew point can never be lower than temperature. As the
air continues to rise the temperature will continue to fall.
The atmosphere will not be able to hold the moisture as
a gas anymore and raindrops will begin to form. As this
water falls from the cloud the dew point will decrease along
with the temperature.
As moisture is removed through raindrops or
other precipitation, the amount of overall moisture will drop.
Think of dew point as an invisible bucket of water. As some
of the water is poured out of the bucket (as rain) the amount
of water in the bucket would be less (less water in the atmosphere
will mean a lower dew point).
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