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Air masses UPSC | Climatology| Physical Geography | Geography for UPSC IAS PCS NET JRF

 Air masses

  • Air mass is a large parcel of air whose horizontal variation in physical properties such as temperature, humidity, etc. is almost negligible or very little.
  • Air masses are spread over an area of thousands of kilometers. The region where the air masses originate is called the source (origin) region of the air mass.

As per the source region, the following are the major types of air masses on the earth:

  • Maritime Tropical(mT) air mass
    • ( Source region: Warm Tropical and subtropical ocean;)
  • Continental tropical (cT) air mass
    • ( Source region: Subtropical Hot desert)
  • Maritime Polar(mP) air mass
    • ( Source region: Relative cold high latitude ocean;)
  • Continental Polar(cP) air mass
    • ( Source region: Cold snow-covered Continents in high latitudes)
  • Continental arctic(cA) air mass
    • ( Source region: Permanent ice cover continents in the arctic and antarctic)

  • Hot and humid air mass:
    • It is formed over the ocean in tropical water.

Formation of Air Masses:

  • When air stays longer over a homogeneous region such as a plain or large ocean (preferably an equal insolation region), they acquire the prevailing surface temperature (cold/warm) and humidity properties.
  • The low wind speed helps the large area of air to remain stable for a longer period of time and helps to obtain the characteristics of the source area such as temperature, and humidity.
  • The formation of air masses requires homogeneous surfaces (eg oceans, large plains, deserts) and homogeneous insolation.

Based on the moisture, there are two types of Air masses:

  • Dry Air mass
  • Moist Air mass

Dry air masses usually originate on continents. The "continental tropical (cT)" air mass is an example of the dry air mass.

Wet air masses are generated over the oceans. The "maritime tropical(mT)"  air mass is an example of a humid air sign.

Based on the temperature, there are two types of Air masses:

  • Cold Air masses
  • Warm Air masses

Cold Air masses:

  • If the land is warmer than the atmosphere above it; Air mass over such surface is called cool air mass.
  • It is unstable as convection current forms [ air rises] easily.

Warm Air mass:

  • The land is cooler as compared to the air mass present over it is called warm air mass. Warm air masses can be further classified into two types:
    • Maritime air mass
    • Continental air mass

The main function of Airmasses:

  • Air masses move from one region to another, with the movement they also transfer heat, cold, and moisture from the source region to the destination region.
  • According to the monsoon wind theory, the arrival of the monsoon is due to the transfer of oceanic tropical (mT) air masses to the Indian continents.
  • Temperate cyclones and fronts are formed as a result of the interaction of two different air masses.
Air mass modification:
Modification or transformation of air mass occurs when air masses move from their source region to another region. Where the air mass moves, there is an exchange of heat, cold, and moisture with the underlying surface.

The following are important factors responsible for the modification of air mass :
  • Nature of the underlying surface.
  • Stability or instability of air masses.
  • The apparent movement of the Sun.
Nature of the underlying surface:
Underlying surfaces are the main deciding factors of the properties of air masses. The air mass which is above the tropical ocean, they are moist and warm. The air mass is above the continent and higher latitudes are generally dry and cold.
  • If the destination region is warmer than the air mass, then the temperature of the air masses increases. In this way, the cold air mass gets converted into warm air mass.
  • If the destination region is colder than the air mass, then the temperature of the air mass decreases, in this way the warm air mass gets converted into the cold air mass.
  • If the air mass moves from the continent to the ocean, then its moisture increases, and on the contrary, if the air mass moves from the ocean to the continent, then its moisture decreases.
Stability or instability of air masses:
  • Stable air masses (such as continental polar air mass ) do not move and thus they do not modify easily. 
  • Unstable air masses such as tropical maritime air mass or tropical moist air mass move frequently and they are modified.
The apparent movement of the Sun:
  • When the Sun moves from the Southern Hemisphere to the Northern Hemisphere, all the Southern Hemisphere air masses also follow the Sun's motion and move to the North. The same happens when the Sun moves from the Northern Hemisphere to the Southern Hemisphere.
For example, 
  • In the summer season, tropical maritime air masses of the Atlantic ocean move over the Indian subcontinent.
  • In the winter season, the Continental polar airmass of Alaska and Canada move southern ward and came over the middle of the North American continent.

Try to solve the following questions:
  • What are the important factors responsible for airmass modifications? ( UPSC 2021 geography optional paper 1, 10 marks, 150 words)

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