Uniform bodies of air 

An air mass is a large body of air that has similar temperature and moisture properties throughout. The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below. Maritime tropical air masses (mT), for example, develop over the subtropical oceans and transport heat and moisture northward into the U.S.. In contrast, continental polar air masses (cP), which originate over the northern plains of Canada, transport colder and drier air southward.

Once an air mass moves out of its source region, it is modified as it encounters surface conditions different than those found in the source region. For example, as a polar air mass moves southward, it encounters warmer land masses and consequently, is heated by the ground below. Air masses typically clash in the middle latitudes, producing some very interesting weather.

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