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Question:
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.



Replies:
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.

Allsopp



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