There are actually five main air masses, not four, and they are classified based on their source region and temperature characteristics. Here's a breakdown:

1. Arctic (A): Forms over the Arctic Ocean and Greenland. Very cold and dry.

2. Polar (P): Forms over high-latitude land and ocean areas (e.g., northern Canada, Siberia, the Arctic Ocean). Cold and dry, but less extreme than Arctic air.

3. Tropical (T): Forms over tropical and subtropical oceans. Warm and moist.

4. Continental Polar (cP): Forms over large landmasses in high latitudes (e.g., Canada, Siberia). Cold and dry.

5. Continental Tropical (cT): Forms over large landmasses in low latitudes (e.g., southwestern United States, northern Mexico). Hot and dry.

Note: Maritime (m) is added as a prefix to air mass designations to indicate an origin over the ocean, making them generally moister than their continental counterparts. So, for example, Maritime Polar (mP) air would form over the North Atlantic Ocean, while Maritime Tropical (mT) air would form over the Caribbean Sea.