Air Masses

  • Air mass, in meteorology, is a large body of air having nearly uniform conditions of temperature and humidity at any given level of altitude.
  • Such a mass has distinct boundaries and may extend hundreds or thousands of kilometres horizontally and sometimes as high as the top of the troposphere (about 10–18 km [6–11 miles] above the Earth’s surface).
  • An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface for a time sufficiently long to acquire the temperature and moisture properties of that surface.
  • The Earth’s major air masses originate in polar or subtropical latitudes.
  • The middle latitudes constitute essentially a zone of modification, interaction, and mixing of the polar and tropical air masses.

Air Masses

  • The homogenous surfaces, over which air masses form, are called the source regions.
  • The main source regions are the high pressure beltsin the sub tropics (giving rise to tropical air masses) and around the poles (the source for polar air masses).
  • Source Region establishes heat and moisture equilibrium with the overlying air mass.
  • When an air mass moves away from a source region, the upper level maintains the physical characteristics for a longer period. This is possible because air masses are stable with stagnant air which do not facilitate convection. Conduction and radiation in such stagnant air is not effective.
  • Source region should be extensive with gentle, divergent air circulation(slightly at high pressure).
  • Areas with high pressure but little pressure differenceor pressure gradient are ideal source regions.
  • There are no major source regions in the mid-latitudes as these regions are dominated by cyclonic and other disturbances.
  • Where an air mass receives it’s characteristics of temperature and humidity is called the source region.
  • Air masses are slowly pushed along by high-level winds, when an air mass moves over a new region, it shares its temperature and humidity with that region.
  • So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it.
  • Storms arise if the air mass and the region it moves over have different characteristics.
  • For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms.
  • When a warmer air mass travels over colder ground, the bottom layer of air cools and, because of its high density, is trapped near the ground.
  • In general, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles.
  • This brings heat to cold areas and cools down areas that are warm.
  • It is one of the many processes that act towards balancing out the planet’s temperatures.
  • Air masses are slowly pushed along by high-level winds.
  • When an air mass moves over a new region, it shares its temperature and humidity with that region.
  • So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it.

Air masses are classified based on their temperature and humidity characteristics. Broadly, the air masses are classified into polar and tropical air masses.

Both the polar and the continental air masses can be either of maritime or continental types.

The principal source regions of the earth may be classified according to the nature of the surface (land or water) and latitude of the region.

Classification of Air Masses

Thus the source regions are classified as under:

  • Surface
  • Continental
  • Maritime
  • Latitude
  • Arctic (located in the high latitudes)
  • Polar (between Arctic source regions and subtropical highs)
  • Tropical (occupy subtropical high pressure belts)
  • Equatorial (located around the equator between the trade winds of the northern and southern hemispheres)

The air masses may be classified as under:

Classification of Air Masses


    • Continental Polar airmass (winter time) cPK
      • Source regions: Central Canada and Siberia.
      • Extremely cold, dry, stable airmass (coldest wintertime airmasses)
      • Produce intense cold waves
      • No clouds in these air masses.
    • Continental Polar airmass (summer time) cPW
      • Source regions: Central parts of high latitude continents. Example Central Canada
      • Cool and dry airmasses
      • Steep lapse rates.
      • When cPK moves out to oceanic surface, it is modified into cPW air mass with haze, fog and low stratus clouds.
    • Maritime Polar airmass (winter time) mPK
      • mPK forms over open areas in the higher latitudes cool and moist few clouds in their source regions
      • Extensive precipitation is produced when forced to ascend mountain barriers
      • Lower layers moist and unstable and dry and cool in upper parts.
      • Produce squally weather.
    • Maritime Polar airmass (summer time) mPW
      • Cool and moist in the lower parts, but dry aloft
      • Temperature inversion is produced with moisture discontinuity Temperature slightly higher


  • Continental Tropical air mass
    • Source regions: Subtropical high pressure land areas
    • High temperature and low moisture content.
    • In United States, these air masses are only important in summer. They are both dry in winter and summer.
    • In summer they are very hot
    • Subsidence and stability found in the upper parts of these air masses in their source regions.
    • If cT air mass is aloft over warm moist air at the surface, atmosphere becomes convectively unstable and violent thunderstorms and tornadoes are produced.
    • Maritime Tropical air mass mT
    • Warm and moist and highly unstable having convective unstability.
    • Maritime Tropical air mass (winter time)
      • Source regions: Warm oceans in both the hemisphere Warm moist and unstable air masses
      • Steep lapse rate up to tropopause and moisture well distributed up to high levels.
      • When these air masses are lifted over fronts or high mountains, they produce heavy Precipitation
    • Maritime Tropical air mass (summer time)
      • Source regions located in the belt of great semi permanent highs of the tropical oceans including the Caribbean Sea.
  • The properties of an air mass which influence the accompanying weather are vertical distribution temperature(indicating its stability and coldness or warmness) and the moisture content.
  • The air masses carry atmospheric moisture from oceans to continents and cause precipitationover landmasses.
  • They transport latent heat, thus removing the latitudinal heat balance.
  • Most of the migratory atmospheric disturbances such as cyclones and storms originate at the contact zonebetween different air masses and the weather associated with these disturbances is determined by characteristics of the air masses involved.