Air Mass & significance of Air Masses – Geography UPSC Notes

A large volume of air in the atmosphere that is mostly uniform in temperature and moisture is referred to as air mass. Air masses can travel thousands of kilometres in any direction and can reach the stratosphere—16 kilometres (10 miles) into the atmosphere. Air mass was a concept developed during World War I by Bjerkens & Bjerknes. It was founded in the 1930s to predict short-term forecasting (around 24-36 hours). The air mass concept helps in a better understanding of weather phenomena, especially in high and mid-latitudes. The troposphere, while a continuous body of mixed gases that surrounds the planet, is far from a uniform blanket of air. It is instead made up of many large parcels of air that are distinct from one another. These large parcels are known as air masses.

Air Masses Definition

Air Masses can be defined as a large body of air, usually, 1600kms or more, characterized by homogenous physical properties like temperature, and moisture content at a given height. A parcel of air must meet three criteria in order to be recognised as a distinct air mass:

It must be quite large. A typical air mass is over 1600 kilometres (1000 miles) across and several kilometres deep (from the Earth’s surface to the air mass’s top).

In the horizontal dimension, it must have uniform properties. This means that the physical characteristics of the air mass are relatively homogeneous at any given altitude, primarily temperature, humidity, and stability.

It must travel in a group. It must be distinct from the surrounding air, and when it moves, it must retain its original properties rather than being torn apart by differences in airflow.

Air Masses Formation

The uniform temperature and humidity of air masses is due to their formation over source regions. Furthermore, the low wind speeds keep these large volumes from moving. As a result, while they remain stationary over a region, they pick up the conditions of that region, such as temperature or humidity. Then, as wind speeds increase, they move to other areas while retaining the states of their original region. They can also collide with different air masses in other regions, resulting in storms.

Air-Masses Source

Properties of the air masses are derived from the Earth’s surface called a source region. Air masses tend to retain their identity even after moving away from their source regions. Such original identity is retained only in the upper part of air masses while the lower surface is modified due to the heat transfer by the process of convection. Air masses develop only in the regions having:

Extensive uniform topography.

Divergent airflow, thus providing a condition of high pressure.

The area should have a comparatively gentle and divergent airflow so that air stays in that region for a longer period.

Thus, high barometric pressure is ideal for the development of air masses.

Source regions are broadly divided into continental (c) and maritime (m) source regions. Source regions of air mass are either continental interiors in temperate latitudes or large-maritime regions in higher latitudes. Six major source regions are identified.

1. Continental interiors of Siberia.
2. Vast regions of the Sahara.
3. Continental regions of Canada.
4. The Atlantic Ocean.
5. Pacific Ocean.
6. Southern Indian Ocean.

If the air mass moving over a region causes stability or dry conditions then it is known as stable air mass (s). If it promotes precipitation then it is known as unstable air mass (u).

Air Masses Classification

Temperature and humidity characteristics are used to classify air masses. The air masses are broadly divided into polar and tropical air masses. Both the polar and the continental air masses can either have maritime or continental source regions. The following types of air masses are recognized:

Maritime tropical (mT)

Continental tropical (cT)

Maritime polar (mP)

Continental polar (cP)

Continental Arctic (cA)

Influence of Air Masses on World Weather

• 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 precipitation over 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 zone between different air masses and the weather associated with these disturbances is determined by the characteristics of the air masses involved.

Classification of Air Masses

• 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.

Continental Polar Air Masses (CP)

• Source regions of these air masses are the Arctic basin, northern North America, Eurasia and Antarctica.
• These air masses are characterized by dry, cold and stable conditions.
• The weather during winter is frigid, clear and stable.
• During summer, the weather is less stable with the lesser prevalence of anticyclonic winds, warmer landmasses and lesser snow.

Maritime Polar Air Masses (MP)

• The source region of these air masses is the oceans between 40° and 60° latitudes.
• These are actually those continental polar air masses which have moved over the warmer oceans, got heated up and collected moisture.
• The conditions over the source regions are cool, moist and unstable. These are the regions which cannot lie stagnant for long.
• The weather during winter is characterized by high humidity, overcast skies and occasional fog and precipitation.
• During summer, the weather is clear, fair and stable.

Continental Tropical Air Masses (CT)

• The source regions of the air masses include tropical and sub-tropical deserts of the Sahara in Africa, and of West Asia and Australia.
• These air masses are dry, hot and stable and do not extend beyond the source.
• They are dry throughout the year.

Maritime Tropical Air Masses (MT)

• The source regions of these air masses include the oceans in the tropics and sub-tropics such as the Mexican Gulf, the Pacific and the Atlantic Oceans.
• These air masses are warm, humid and unstable.
• The weather during winter has mild temperatures and overcast skies with fog.
• During summer, the weather is characterized by high temperatures, high humidity, cumulous clouds and convectional rainfall.

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 masses or tropical moist air masses 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 moved southern ward and came over the middle of the North American continent.