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.