Cold air masses develop over the northern portions of the Atlantic and Pacific Oceans and over the snow-covered lands of north-central Canada. Warm air masses can form in any season over the waters of the southern North Atlantic, the Gulf of Mexico/Caribbean Sea, and the southern North Pacific, and in summer, they can form over the deserts of the southwestern United States and northwestern Mexico. Image portrays the principal recognized source regions for air masses that affect North America.
Air masses rarely form over the irregular terrain of mountain ranges. Ideal source regions are – ocean surfaces and extensive flat land areas that have a uniform covering of snow, forest, or desert. Such regions must be extensive, physically uniform, and associated with air that is stationary or anticyclonic. The formation of air masses is usually associated with what are called source regions: regions of Earth’s surface that are particularly well suited to generate air masses. Stable air is more likely to remain stagnant for a few days than unstable air, so regions with anticyclonic (high pressure) conditions commonly form air masses. This stagnation needs to last for only a few days if the underlying surface has prominent temperature and moisture characteristics. An air mass develops its characteristics when it stagnates or remains over a uniform land or sea surface long enough to acquire the temperature/humidity/stability characteristics of the surface below. It must be distinct from the surrounding air, and when it moves it must retain its original characteristics and not be torn apart by differences in airflow. This means that at any given altitude in the air mass, its physical characteristics:- primarily temperature, humidity, and stability are relatively homogeneous. It must have uniform properties in the horizontal dimension. 
A typical air mass is more than 1600 kilometers (1000 miles) across and several kilometers deep (from Earth’s surface to the top of the air mass).
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