Thursday, 25 August 2016

Frequently, you may hear of a cold air mass moving southward from Canada or of Gulf (of Mexico) air dominating the Midwest. What is an air mass, and how can we detect the presence of an air mass on the surface weather map? Air masses are large volumes of air that are relatively uniform in their horizontal temperature and humidity characteristics. They are generally identifiable by the "Highs" or "H's" written on maps that represent the centers of large high-pressure regions. These centers of high pressure are at the same time often locations of the centers of air masses.

Air masses form when air resides over a land or water surface with uniform surface properties for several days or more. The air gradually assumes the temperature and moisture characteristics of that underlying surface. Eventually, these broad expanses of "conditioned" air begin to move. Masses of air from northern latitudes are usually cold and termed "polar" while those from southerly latitudes are warm or "tropical". If the surface of the source region were land, the air would probably be relatively dry or "continental", whereas ocean air would be more humid or "maritime".

Coterminous U.S. weather is mainly prone to continental polar air masses from central Canada, maritime polar air masses from the northern Pacific Ocean, or maritime tropical air masses from the Gulf of Mexico and their transitions. While rare, continental tropical air would develop over the desert Southwest and the Mexican Plateau.


To be submitted on the lines for Thursday on the Investigations Manual, Chapter 1 Progress Response Form (found in Week 1 section of the Study Guide and also on RealTime Weather Portal).

  1. People living in Pennsylvania could reasonably expect relatively [(warm and humid) (cool and humid)] conditions when an air mass passes through from the Gulf of Mexico.
  2. The type of cold, dry air mass most likely to be found over North Dakota in January would be a [(continental polar) (maritime polar)] air mass.



Various weather elements, such as temperature and moisture, can be used to identify air masses. An "air mass" is a large body of air (hundreds of miles in extent) in which these weather elements are relatively uniform in their horizontal extent at each particular altitude. Consequently, the density is essentially uniform in the horizontal. While horizontal uniformity is an ideal condition, variations do exist, but they are usually gradual with no abrupt discontinuities within the same air mass.


In a general sense, the temperature of the air mass is determined by the latitude of the source region, while its moisture content is determined by the type of surface over which the air mass developed. When air stagnates over the snow-covered polar latitude continents in winter, the near surface air rapidly loses heat through radiation during the long winter nights. Although the entire air column can cool, the greatest cooling occurs near the surface. In the tropics and subtropics, the intense sunlight heats the surface during the day. The air immediately above the surface is heated upon contact.


Air masses are usually classified using the following scheme devised during the early 20th century that contains a one or two letter code:




Maritime Polar (mP)

Polar and subpolar oceans poleward of approximately50 degrees

Prominent in all seasons.
Cool, moist (damp).

Continental Polar (cP)

Continents in vicinity of Arctic and Antarctic Circles or Antarctica

Prominent in winter.
Winter: Very cold and dry
Summer: cool and dry.

Maritime Tropical (mT)

Tropical, Trade wind belt and major subtropical ocean basins.

Prominent in all seasons.
Moist and warm.

Continental Tropical (cT)

Tropical continents, especially tropical and subtropical deserts, chiefly Sahara and Australian deserts

Prominent in summer.
Hot and very dry.

Arctic (A)

Arctic or Antarctic
especially the Polar anticyclone

Found all seasons, prominent in winter.
Bitterly cold and very dry in winter.

Equatorial (E)

Equatorial belt

Prominent in all seasons.
Hot and high humidity.


When the air mass travels away from the source region, the air mass will remain for a time as a recognizable entity, retaining thermal and moisture properties. However, with time and new underlying surface conditions, the air mass does undergo a slow process called "airmass modification". The amount of modification depends upon the route that the air mass traveled, the nature of the underlying surface and the time spent enroute. Usually, polar air masses moving over warmer surfaces are modified more rapidly than tropical air moving over cold ground. Heating of the polar air mass from below enhances vertical air motions that mix the air mass, and increase the likelihood that clouds with a puffy appearance (called cumulus type clouds) will form. Conversely, cooling of the air by contact with a colder surface suppresses vertical motions restricting cloud formation.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2016, The American Meteorological Society.