Types of Air masses

Definition of Air Mass

An air mass is a huge body of air having homogeneous temperature and moisture (humidity) condition over a large area. The properties of an air mass convey the climatic condition of its source region. There are different types of air masses based on source region.

Source regions of air masses are usually a high pressure zone spreading across thousands of square kilometers with similar climatic conditions where the air remains stable for a sufficiently long period of time to acquire the temperature and humidity properties of that area. The wind from such source regions flows outwards towards zones of relatively low pressure.

Classification of Air masses

Air masses are classified based on temperature and moisture (humidity) combined with source region

Source region	Maritime (m) air mass: originating from large water bodies i.e. seas and oceans	Continental (c): originating from continents
Polar (P)	mP (Polar Maritime)	cP (Polar Continental
Arctic (A)	The Arctic is a permafrost region so maritime conditions do not prevail but due to melting of snow during summer as observed recently, there may be maritime air mass conditions in future.	cA (Arctic Continental)
Tropical (T)	mT (Tropical Maritime)	cT (Tropical Continental)

 

Based on above table, we can classify air masses into Five Types

1. Polar Maritime (mP)

• Polar Maritime air mass originate from seas surrounding polar region i.e. Southern Ocean, North Atlantic Ocean and North Pacific Ocean.
• It is a cool, moist and unstable air mass.
• They move westward and collide with Polar Continental air masses to form mid-latitude/frontal cyclones and shower rains.

2. Polar Continental (cP)

• Polar Continental air mass originates from cold continental regions such as North Canada, Greenland, Siberia, Antarctica etc.
• These are cold, dry and stable air masses.
• They move southward and bring frigid conditions into mid-latitude areas in the absence of a physical barrier such as the Himalayas.
• GK: These cold winds from poles are called Blizzard.

3. Arctic Continental (cA)

• Arctic Continental air mass originates from the Arctic Ocean surrounding the North Pole.
• These air masses are extremely cold, dry and stable.
• Although, Arctic is an ocean, no maritime conditions prevail due to the permafrost nature of the ocean.
• Like Polar Continental air mass, arctic air mass also moves southwards and bring dry and frigid weather to mid-latitudes.

4. Tropical Maritime (mT)

• Tropical Maritime air masses originate from warm oceans and seas of tropical regions i.e. Bay of Bengal, Arabian Sea, Caribbean Sea, Central Pacific Ocean etc.
• These air masses are primarily found in the eastern coasts of continents because the western coasts have cold ocean currents which alter the nature of tropical maritime air mass.
• These are warm, moist and unstable air masses and create cyclonic conditions in tropical regions.
• These air masses move westward and poleward. They bring rains to warm and dry continental areas.

What happens to warm, unstable air?

5. Tropical Continental (cT)

• These air masses originate, primarily, over arid and desert regions in tropic e.g. Sahara, Thar, and Tropical Australia etc.
• Tropical continental air masses are very hot and dry air mass.
• Due to high temperature, the relative humidity is very low and fails to condense even in presence of strong convection current.
• These regions are located in mid-latitudes where the air from the upper troposphere subsides and gets heated adiabatically. Subsidence of air leads to hot, dry and stable atmospheric conditions.

It must be noted that the air masses represent the temperature and moisture conditions over a very large area. Therefore, minor and local variations in temperature and humidity may exist within these air masses. Similarly, when they travel from their source region towards other regions, their properties change slightly due to climatic conditions found in transit regions.