Air masses are large bodies of air that have relatively uniform temperature and humidity. They are formed when air remains over a particular area for a long period of time, taking on the characteristics of that area.

Here are the main sources of air masses:

1. Polar Regions:

* Arctic (A): Cold, dry air masses originating over the Arctic ice and snow.

* Antarctic (A): Similar to Arctic air masses, but even colder and drier.

2. Tropical Regions:

* Tropical (T): Warm, moist air masses originating over tropical oceans.

3. Continental Regions:

* Continental Polar (cP): Cold, dry air masses originating over large landmasses in high latitudes, like Canada and Siberia.

* Continental Tropical (cT): Hot, dry air masses originating over large landmasses in low latitudes, like the Southwestern United States and Northern Mexico.

4. Maritime Regions:

* Maritime Polar (mP): Cool, moist air masses originating over the oceans in high latitudes.

* Maritime Tropical (mT): Warm, moist air masses originating over the oceans in low latitudes.

Important Note: The characteristics of an air mass can change as it moves across different surfaces, particularly in terms of temperature and moisture. For example, a cP air mass moving over a warm ocean surface will become warmer and more humid.