Air-Masses, Meaning, Types, Formation, Classification, Diagram

Air Masses

Air Masses: 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).

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.

• Continental interiors of Siberia.
• Vast regions of the Sahara.
• Continental regions of Canada.
• The Atlantic Ocean.
• Pacific Ocean.
• 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).

Tropical air masses are warm, while polar air masses are cold.

Air Masses Classification Map

Here is the map for the Classification of Air-Masses:

Important Air Masses

Here are the Important Air Masses of the world.

Air-Masses Modification

Modification of air mass involves heating or cooling of air mass that changes the moisture content of the air mass either by decreased evaporation or increased evaporation. Modification of air mass may also be caused by the process of subsidence or ascent that also alters the moisture content by precipitation. Two types of modification of air masses mostly occur.

Thermodynamic Modification

The modification of air mass that results from the thermodynamic action of heating and cooling as the air mass travels from one source region to another source region is called a thermodynamic modification. Thermodynamic modification depends upon

• The initial characteristics of the air mass.
• The nature of the surface it visits (either land or water).
• Path followed by the air mass.
• Time is taken by the air mass to reach the destination surface.

Mechanical Modification

The modification is also known as a dynamic modification. Mechanical modification of air mass involves vertical uplift or downward subsidence due to mountain/orographic barriers, cyclonic conditions, anti-cyclonic conditions, turbulence and eddies.

Based on the Above Modifications Two Types of Air Masses Arise

Cold Air Mass (k)

The temperature of the air mass is colder than the surface temperature it visits. Example: Arctic Ocean, Siberia, Northern Canada.

Warm Air Mass (w)

The temperature of the air mass is warmer than the surface temperature it visits. Examples: Sahara Desert, Tropical Oceans.

Air-Masses Impact on the Weather

After the formation of air masses by acquiring the properties of the source region they move out and interact with new regions with different surface properties. In the process, the air mass modifies the weather of the visited region and they themselves too get modified. The interacting air masses result in the creation of fronts and frontal weather patterns.

Air Masses UPSC

Cloud forms are distinct from air masses. As a result, meteorology studies them separately. These masses span thousands of miles across the Earth’s surface and typically have the same temperature and moisture levels. The cold and drier continental polar air mass from Siberia in the winters brings cold conditions while moving over Central Asia and heading towards South Asia. While moving over relatively warmer regions of Central, air mass becomes unstable.  In contrast, when the continental polar air mass from Siberia moves over a relatively colder surface i.e. the Arctic region, the air mass covers into an air mass which is warmer and more stable.

Thus, an air mass passing over warmer surface converts to a relatively colder and unstable air mass, and air mass passing over a colder surface always converts to a warm and stable air mass.

A solid understanding of this subject is required to help individuals pass competitive exams. On that note, we have covered this topic exclusively for students preparing for the UPSC and IAS exams.

Air Masses FAQs

Q) What is an Air-Mass?

Ans. Air Mass is a large volume of air in the atmosphere that is mostly uniform in temperature and moisture.

Q) What is a weather front?

Ans. Fronts are the physical boundaries that separate air masses.

Q) What are the four types of Air-Masses?

Ans. Meteorologists classify air masses based on where they form over the Earth. Air masses are classified into four types: arctic, tropical, polar, and equatorial. Arctic air masses, which are extremely cold, form in the Arctic region. Tropical air masses, which are moderately warm, form in low-latitude areas.

Q) What are superior air masses?

Ans. These occur when the atmosphere’s downward motion creates dry air.

Q) What causes an air mass?

Ans. When the atmosphere comes into contact with a large, relatively uniform land or sea surface for a long enough period of time, the temperature and moisture properties of that surface are acquired.

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