Air Masses - From The Source

Last month, we defined what air masses were and looked at some of their basic properties. Those distinguishing properties derive from the birthplace of the individual air mass, the geographical region where they form. Once an air mass leaves its birthing grounds, it is slowly altered by the environment through which it moves, yet retains much of its original characteristics. Eventually, an air mass becomes so altered that it is assimilated into the character of the region it settles in. At this time, the old air mass dies, completely integrated with the characteristics of its new home.

Breeding Grounds

Areas dominated by extensive high air pressure and light winds are the ideal breeding grounds for air masses. High polar latitudes are good source regions as are the subtropical belts around 30 degrees latitude. Open ocean, large deserts and extensive continental plains at high or low latitudes provide optimum terrain for producing air masses; mountainous regions do not. Mid-latitude oceans can be source regions under certain conditions because their surfaces have very uniform characteristics.

Air masses are designated as hot or cold, wet or dry, based on the characteristic properties picked up in their breeding ground, The terms are relative as the absolute levels wax and wane with the solar seasons. For example, a cold air mass in summer may have the same core temperature as a warm air mass in winter.

In general, wet air masses (maritime: m) are air masses forming over the oceans, and dry air masses (continental: c) are those forming over the continents. Warm air masses (either maritime or continental) generally form in the tropical or subtropical zones (T). Cold air masses (either maritime or continental) form at higher latitudes, usually poleward of 50 degrees (P).

We can thus distinguish four basic combinations: hot and dry; hot and wet; cold and wet; and cold and dry. For example, a Polar Continental air mass (cP) is cold and dry, while a Polar Maritime air mass (mP) is cold and wet.

There are two special types of air masses. We can thus distinguish four basic combinations: hot and dry; hot and wet; cold and wet; and cold and dry. Equatorial air masses (E) -- those formed within 15 degrees of the Equator -- are all considered to be wet because the land under the equatorial zone is mostly covered in tropical rainforests that can add as much moisture to the air as the oceans. All Arctic (A), or Antarctic in the Southern Hemisphere (AA), air masses -- formed poleward of the polar circles -- are considered dry because their temperatures are so low that even at saturation over open ocean, the absolute humidity remains very low.

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