Formation, Movement and Classification of Air Masses


Air mass is an extremely large body of air in the atmosphere whose properties – temperature, humidity and lapse rate, which is the decrease of atmospheric temperature with height, are largely uniform over an area which can be several hundred kilometers across the surface of the earth. Climate science defines air mass as a relatively huge bulk of air that is distinctive by its homogeneity of temperature and moisture content.

Air Mass is a voluminous body of air found in the lower regions of the atmosphere. Air masses are identifiable particularly for their unvarying qualities of moistness and heat at any specific altitude. Air Masses persist as distinct and discernible even when they become mobile.

According to NOAA,

An air mass is a large body of air with generally uniform temperature and humidity. The area over which an air mass originates is what provides it’s characteristics. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below. As such, air masses are associated with high pressure systems.

Two different air masses can be separated and the line of distinction is called a front. It is along with these fonts that weather formation occurs.

When we refer to air masses we are indicating at those massive air packages that can spread over an area approximately 1,600 kilometers. They exercise a considerable influence on the climatic conditions of the region over which they lodge and carry with them distinctive climatic features of their source region.

Formation of Air Masses

Air Masses are most common in the tropics, subtropics, and high latitudes. The zones from which air masses grow are called “source regions.” These are generally tracts of ocean, desert or snow-covered plains. The large surfaces with uniform temperatures and humidity, where air masses originate are called source regions. Uneven warming and cooling of the earth’s surface by the Sun gives rise to air masses.

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Warm air masses

The warm air masses form over the equator or desert areas where solar radiation is maximum. In clear, almost cloudless days, the heat is reflected back to the atmosphere. The air becomes light and spreads.

Cold air masses

Cold air masses form near the poles where solar radiation is at a minimum. On cloudless days, the snow cover near the Poles, reflect sunlight away, preventing the earth to warm up. When this persists for a long period of time, cold air masses form over a large area.

Movement of Air Masses

Warm wind is light and tends to rise. Cold air is heavy. Areas with the warm light wind have a low-pressure zone. The cold wind is heavy and creates high pressure. Wind flows from high-pressure air masses to low-pressure areas.

When wind speed is low the air remains stationary over a particular landscape and in the process gathers the natural climatic conditions of that region-heat or cold. When the winds move air masses, they carry the weather conditions along to a new region. An air mass on the move begins to transform as it passes over new landscapes, although retaining enough of its original qualities that alter local weather.

When this air mass reaches a new area, it often clashes with another air mass with different temperatures and humidity. This can create a severe storm.

Classification of Air Masses

The source regions and their climatic specifications classify the world’s major air masses. An air mass is named by the combination of its humidity and temperature specificity. The type of temperature that an air mass acquires is derived from the latitude of origin; temperatures generally decrease poleward.

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Air masses originating near the equator are Equatorial. These are considered hot air masses. At a higher latitude the air masses are called tropical air masses. These are considered warm air masses. The Polar air masses are at a still higher latitude. These range from cool to cold, depending on the position of the sun.


The Arctic is the highest latitude of origin of any air mass. This air mass is considered very cold. Their source of origin is Arctic Ocean, Siberia, Northern Canada, Southern Ocean.

Maritime Polar

Maritime Polar air masses have their source region over cold ocean currents or high latitude ocean waters. This air mass can produce widespread rain or snow, fog, drizzle, cloudy weather and long-lasting light to moderate rain.

Continental Polar

Continental Polar air masses are cold to cool and dry. Continental Polar air masses form over Canada and Siberia. These air masses bring cold air during the winter and cool, relatively clear, rather pleasant weather in the summer.

The air mass is stable and usually obstructs cloud formation. Air masses can also prevent the vertical movement and may cause high pollution levels, especially near and downstream from large industrial areas. As the Continental Polar air move south across the warmer land, the lower portion of the air mass may become sufficiently light by warming to cause formation of clouds within the air mass.

Maritime Tropical

Maritime Tropical air mass results from the warm waters of the Gulf of Mexico and Gulf Stream. This air mass is characterized by hot, humid conditions. These air masses form in almost all seasons across the south, southeastern and eastern United States. It is associated with cloudiness and precipitation. Maritime tropical air masses are also known as trade air masses.

Continental Tropical

These are the hot, dry air masses that originate over northern Mexico and the southwestern United States. This air mass moves into the United States through New Mexico, Arizona, and western Texas and frequently migrates eastward to north-eastward making Texas climate hot and dry.

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Effect of Air Masses on Weather

In a particular area, the occurrence of particular air masses helps to ascertain the climate of that region.

This in turn affects the types of vegetation, that can be found there and what can be cultivated successfully there.

Droughts are the result of a hot, dry air mass. This can destroy natural vegetation and kill trees. These regions have an increase risk of devastating wildfires.

At the boundaries between air masses, the clash of masses of air with different characteristics can lead to dynamic weather like hail, tornadoes, high winds or ice storms.

Air masses are important natural occurrence which may have the following features:-

  • It is within the transition zones that surface low pressure and fronts are most often found.
  • Dry air is denser than moist air. So, cold and dry air masses are stable because they have a higher density and higher average molecular bulk.
  • Warm moist air masses are drifting due to their low density. They expand because they have a lower molecular weight.
  • Low pressure forces air mass movement because it is light and contains less moisture. These are generally sourced inland areas.
  • The mid-latitude zones are unique. They can experience several different air mass types over the course of a year.
  • Tropical and Polar areas tend to have more uniform weather throughout the year, although the tropics can experience a wet rainy season, a dry season and mild winters. The temperature around the poles depends almost completely on the angle of the sun which varies from season to season.
  • Latitude, altitude, types of ocean currents, sunshine hours, sunshine angle, natural vegetation, temperature of the soil, snow cover, prevailing wind, determine the character of an air mass.

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