Ask A Scientist© Environmental Science Archive

Author:      pioneerlearners
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.

Response #:  1 of 1
Author:      allsopp
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.