Local weather conditions are brought in by prevailing winds. But weather conditions depend upon characteristics of the air itself. The air that is carried by prevailing winds moves as a unit. It is really a body of air that is moving. This body of air that is carried by the prevailing winds is called an air mass.
An air mass is large. It may extend for hundreds of kilometers. Also, the air mass is nearly uniform throughout. This means that all the air has about the same amount of moisture. It also means that all the air is at nearly the same temperature.
The characteristics of a particular air mass depend upon where the air mass forms. In general, air masses form either over land or over water and near a polar region or near the tropics. Four terms used to describe such air masses are continual, maritime, polar, and tropical. A continental air mass is one that forms over dry land (the continent). A maritime air mass is one that forms over an ocean. (Mare is the Latin word for sea or ocean.) A polar air mass is one that forms near the North or South Pole. A tropical air mass is one that forms near the Tropic of Cancer or the Tropic of Capricorn.
Each different air mass has its own general characteristics. A continental air mass will contain dry air whereas a maritime air mass will contain moist air, picking up moisture when it forms over an ocean. A polar air mass will contain cold air. A tropical air mass will contain warm air. A continental polar air mass will be dry and cold. A maritime tropical air mass will contain both characteristics also (moist and warm).
