An air mass is a large body of air characterized by similar temperatures and moisture levels throughout its depth. As air masses move from their source regions, their characteristics change and influence the weather over new areas. The weather in much of North America is influenced by continental polar and maritime tropical air masses, which form over land and ocean surfaces and transport their characteristics inland.

1. Air Masses 20.1

2. Air Mass A large body of air that is characterized by similar temperatures and amounts of moisture at any given altitude.

3. Key Concept An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given altitude.

4. Key Concept As it moves the characteristics of an air mass change and so does the weather in the area over which the air mass moves.

5. Key Concept In addition to their overall temperature, air masses are classified according to the surface over which they form.

6. Key Concept Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses.

7. Key Concept Only occasionally do cT air masses affect the weather outside their source region.