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Name: Michael
Status: educator
Grade: 9-12
Location: CT
Country: USA
Date: Fall 2012

My question involves air masses and mid latitude cyclones. My understanding is air masses form because they stay stationary over certain areas. What begins to make them move where they form fronts? Is it the way air circulates around a low pressure system? Or do these mid latitudes cyclones form at fronts? I am trying to explain what causes these different air masses to move. What moves high and low pressure systems?

Hi Michael,

Remember the temperatures and pressure. Warm/humid air and cooler air masses are in motion due to energy differences in absorptive areas on Earth. The rush of cool and warm can be felt on our skin at the beach, the sea breeze. at morning and evening. As the warm/humid air and cold air masses converge, they begin an upward twisting as they mix. Generally, this causes rain and low pressure with cooling. If the warm water continues to fuel its energy into a cooler air mass, the twisting may become well organized into a cell. Continued organization and energy building yields a cell, then a tropical depression and on to a hurricane.

The energy creating the motion, the twisting, is a function of heat-cool cycling (the Carnot Cycle) and mixing.

Peter E. Hughes, Ph.D. Milford, NH


An air mass is a volume of air that has a common characteristic, such as polar air masses are cold and dry, tropical air masses are warm and humid, and other air masses run the scale in between.

Air masses are driven to move by different heating patterns over the surface of the earth. For example, a plowed farmer’s field or prairie will heat the air over it more so than say a forest. Then the hot air rises because it is less dense forming a low air pressure area. But as the hot air rises, it cools and eventually falls back to the surface forming a higher air pressure area. These differences in air pressure drive these air masses to move and the leading edges of these moving air masses are called fronts. For example, cold air advancing on warm air is called a cold front, and warm air advancing on cold air is called a warm front.

Here is an excellent article that provides more information.

Sincere regards, Mike Stewart

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