When a large body of air remains over an area for several days or weeks, the body ofair may change its thermal and moisture properties according to the land or water underit. The air exchanges heat with the land or water, either warming or cooling, until itstemperature closely matches that of the surface below it. The body of air also gains orloses moisture depending on the temperature and moisture content of the surface. A body ofair that resembles the characteristics of the surface under it is called an airmass.

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Five basic types of air masses affect Michigan"s weather. They can bring anything fromscorching heat to bone-chilling cold depending on the type of air mass. The five are. Continental Arctic (cA): Extremely cold temperatures and verylittle moisture. These usually originate north of the Arctic Circle,where days of 24 hour darkness allow the air to cool to sometimes record-breaking lowtemperatures. Such air masses often plunge southward across Canada and the USA duringwinter, but very rarely form during the summer because the sun warms the Arctic. Continental polar (cP): Cold and dry, but not as cold asArctic air masses. These usually form farther to the south and oftendominate the weather picture across the USA during winter. Continental polar masses doform during the summer, but usuallyinfluence only the northern USA. These air masses are the ones responsible for bringingclear and pleasant weather during thesummer to the North. Maritime polar (mP): Cool and moist. They usually bringcloudy, damp weather to the USA. Maritime polar air masses form over the northern Atlanticand the northern Pacific oceans. They most often influence the Pacific Northwest and theNortheast. Maritime polar air masses can form any time of the year and are usually not ascold as continental polar air masses. Maritime tropical (mT): Warm temperatures with copiousmoisture. Maritime tropical air masses are most common acrossthe eastern USA and originate over the warm waters of the southern Atlantic Ocean and theGulf of Mexico. These airmasses can form year round, but they are most prevalent across the USA during summer.Maritime tropical air masses areresponsible for the hot, humid days of summer across the South and the East. Continental Tropical (cT): Hot and very dry. They usuallyform over the Desert Southwest and northern Mexico during summer. They can bring recordheat to the Plains and the Mississippi Valley during summer, but they usually do not makeit to the East and the Southeast. As they move eastward, moisture evaporates into the air,making the air mass more like a maritime tropical air mass. Continental tropical airmasses very rarely form during winter, but they usually keep the Desert Southwestscorching above 100 degrees Fahrenheit during summer.

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