Pressure versus Temperature and cloud formation
As we are teaching the formation of clouds we have been faced with a difficult
scenario. One source explains that as air pressure decreases temperature also
decreases. Another source explains that as temperature rises air pressure
decreases. Could you help us to explain this to our fifth grade science
students? A general overview of the relationship between air pressure and
temperature would also be appreciated.
Air pressure decreases with height because as you move up through the
atmosphere, there is less and less air above you pushing down. Because
pressure decreases with height, air expands as it rises. When the air
expands, it uses up energy by pushing the surrounding air outward. The
molecules in the air lose energy and slow down. So air cools as it rises and
warms as it sinks. Rising air cools at a rate of 5.5 degrees F for every 1000
feet. How does this affect cloud formation? Warm air can hold more water
vapor (gas) than cold air. When warm moist air rises, it cools. Eventually
it cools to the point where it is saturated with water vapor. At this point
the water vapor begins to condense out as tiny droplets of liquid water . . .
clouds form. If the air sinks, it warms and the liquid water evaporates
causing clouds to dissipate.
Air moves out away from the center of high pressure. Air must come down from
above to replace the air moving away from the high. Thus there is a general
sinking or subsidence of air in a high pressure area. High pressure generally
brings dry fair weather.
Air is pushed toward a low pressure center. The converging air is forced
upward. Rising air (if there is sufficient water vapor) will result in clouds
and possibly precipitation. Low pressure is generally associated with cloudy
or stormy weather.
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Update: June 2012