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. The Earth’s atmosphere contains large air masses with similar temperature and moisture. These air masses affect weather and climate in different areas. Meteorologists study them to predict the weather accurately.

Air masses can move thousands of kilometers and can reach up to 16 kilometers high. The idea of air masses was developed by Bjerkens and Bjerknes during World War I and was formalized in the 1930s for short-term weather forecasts (24-36 hours). Understanding air masses helps explain weather, especially in higher and mid-latitudes. The troposphere, the layer of air around the Earth, is not uniform,  it is made up of distinct air parcels called 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

Air masses come from specific regions where they gain temperature and moisture. These regions can be over land or water and are affected by factors like latitude and wind. There are two main types of air masses:

Maritime air masses: These form over oceans or lakes and are humid because of evaporation. When they move inland, they lose moisture as rain.

Continental air masses: These form over land and reflect the land’s characteristics. For example, polar continental air masses are cold in winter, while tropical continental air masses are hot and dry.

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:

1. Tropical (T): Warm and moist air masses from tropical regions near the equator.
2. Polar (P): Cold and dry air masses from polar areas near the North and South Poles.
3. Arctic (A): Extremely cold air masses from the Arctic regions.
4. Equatorial (E): Hot and humid air masses from near the equator and tropical oceans.
5. Continental (c): Dry air masses that form over land. They can be warm (cT) or cold (cP).
6. Maritime (m): Moist air masses that form over oceans. They can be warm (mT) or cold (mP).

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

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. Air masses can change due to sinking (subsidence) or rising (ascent) air, which affects their moisture levels through precipitation. There are two main ways these changes happen.

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).
• The path is 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

Air masses shape global climate and weather patterns by moving and interacting with each other. Their characteristics affect temperature, precipitation, and atmospheric stability in different areas.

For example, when a warm, moist maritime air mass meets a cold continental air mass, it can create a front, leading to clouds, rainfall, and temperature changes, which helps in predicting weather events.

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.

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