Air Mass Meaning
In Meteorology, the air mass is the large body of air having nearly uniform characteristics of temperature and humidity at any given level of altitude. Such a mass has a distinct borderline and may outstretch hundreds or thousands of kilometers horizontally and sometimes as high as the troposphere ( about 10-18 km [ 6 -11] miles above the Earth’s surface. An air mass is formed whenever an atmosphere gets in touch with a large, relatively stable land or sea surface for long enough periods to receive the temperature and moisture of that surface.
Air masses are commonly classified concerning the latitudes, and their continent or maritime source regions. Colder or freezing air masses are termed polar or arctic, whereas warmer air masses are deemed tropical. Maritime and monsoon air masses are moist whereas continental and superior air masses are dry.
Air Mass Definition
“An air mass is defined as the large mass of air whose physical features such as moisture, temperature, lapse rate, and content are approximately uniform horizontally for hundreds of kilometers”.
According to A.N, Straheler, and A.H. Straheler (1978), a large body of air in which an upward rise of temperature and moisture are fairly uniform over a large area is known as an air mass.
Types of Air Masses
The most common types of air masses are maritime polar(mP), maritime tropical (mT), continental tropical (cT), continental polar (cP), and continental arctic (cA).
Maritime Polar (mP) Air Mass- The maritime polar air mass is cold and humid originating from the oceans in the polar latitudes.
Maritime Tropical (mT) - The maritime tropical air mass is warm and humid originating from the oceans in the tropics.
Continental Polar (cP) - The continental polar air mass is cold and dry originating from land regions in the polar latitudes.
Continental Tropical (cT) - The continental tropical air mass is hot and dry originating from land in the tropics.
Continental Arctic (cA) - The continental arctic air mass is dry originating from the North pole.
Continental Antarctica (cAA) - The continental antarctic air mass is extremely cold and dry originating from land at the south pole.
Look at the image given below to observe from where these different types of air masses typically originate.
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Air Mass Creation
Air masses are developed when air is present to the surface over an extended period. This generally occurs at high temperatures with light winds. The area where air masses develop is known as the source area. Air masses over the warmer surface develop much faster than the colder surface because there is a weaker disturbance in the stable air over the cold surface. When the air masses get shifted from their source region, they change over time due to the surface and the area over which the air masses flow.
Air Masses Movement
Air masses do not stay over their source location for a long period. As the weather pattern changes slightly, air mass gets shifted to a new location. Two things occur as the air mass moves towards a new location. First, as the air mass shifts over different surface characteristics, it begins changing. This process is known as air modification. For example, a maritime polar air mass that moves from the Pacific oceans over the mountains in the western continental US will sometimes dry as it crosses over the mountains, wash away its moisture, and warm over the land surface until it becomes the continental tropical air mass. The second thing that occurs when air mass moves is that they can hit the other air masses. When two air masses hit, a boundary known as the front is developed.
Did You Know?
Maritime tropical air masses are often referred to as the trade air mass.
Continental Polar masses are dry and cold because of their continental source regions.
The continental polar mass that affects North America develops over interior Canada.
Colder air masses are known as polar or arctic whereas warmer air masses are known as tropical.
Five air masses that affect the United States around the time of a typical year are maritime polar, maritime tropical, continental polar, continental tropical, and continental arctic.
The stability of air mass can be represented using the third letter either ‘k’ ( air mass colder than the surface below it ) or ‘w’ ( air mass warmer than the surface below it). An example of this can be a polar air mass blowing over the Gulf airstream, represented as cPk.
FAQs on Air Mass
1. What is an air mass in geography?
In geography, an air mass is defined as a vast, extensive body of air in the troposphere that has largely uniform temperature and humidity in its horizontal dimensions. These characteristics are acquired from the surface over which it originates, known as the source region. The longer an air mass remains over its source region, the more it will adopt the properties of that surface.
2. On what basis are air masses classified?
Air masses are classified based on two primary criteria: the temperature and the moisture content of their source region. This is represented by a two-letter code:
Moisture Content (lowercase letter): 'c' for continental (dry, from land) and 'm' for maritime (moist, from oceans).
Temperature (uppercase letter): 'T' for tropical (warm), 'P' for polar (cold), and 'A' for arctic (extremely cold).
For example, an air mass classified as 'mT' is a maritime tropical air mass, meaning it is warm and moist.
3. What are the major types of air masses and their characteristics?
The major types of air masses are classified by their source regions, each bringing distinct weather patterns:
Continental Polar (cP): Cold, dry, and stable. Originates over large, high-latitude landmasses like Canada or Siberia.
Maritime Polar (mP): Cool, moist, and unstable. Forms over cold ocean currents in the upper latitudes.
Continental Tropical (cT): Hot, dry, and stable. Forms over low-latitude deserts like the Sahara or Northern Mexico.
Maritime Tropical (mT): Warm, moist, and very unstable. Originates over warm oceans like the Gulf of Mexico or the Indian Ocean.
Continental Arctic (cA): Extremely cold and dry. Forms over the ice- and snow-covered regions of the Arctic.
4. What happens when two different air masses meet?
When two different air masses meet, they do not readily mix due to their density differences. Instead, they form a boundary zone called a front. The interaction at these fronts is a primary cause of weather changes. For example, when a warm air mass rises over a cold air mass (a warm front), it often produces gentle, widespread rain. Conversely, when a dense cold air mass forcefully lifts a warm air mass (a cold front), it can lead to thunderstorms and heavy precipitation.
5. Why is the source region so crucial for determining the characteristics of an air mass?
The source region is crucial because an air mass develops its defining properties of temperature and humidity through prolonged contact with the surface below. For an area to be a good source region, it must be extensive and geographically uniform, allowing the air to sit and stagnate. Over time, the air achieves thermal and moisture equilibrium with the surface. For instance, air sitting over a tropical ocean becomes warm and humid (mT), while air over a vast, frozen landmass becomes cold and dry (cP).
6. How does a Continental Polar (cP) air mass differ from a Maritime Tropical (mT) air mass in its weather impact?
A Continental Polar (cP) air mass and a Maritime Tropical (mT) air mass create vastly different weather due to their opposing characteristics:
Continental Polar (cP): Originating over cold land, it is cold and dry. In winter, it brings clear skies and frigid temperatures. Its low humidity means there is little moisture for cloud formation or precipitation.
Maritime Tropical (mT): Originating over warm oceans, it is warm and moist. It is the primary source of humidity and precipitation for many regions. When this air mass moves over land, it can lead to heavy rainfall, thunderstorms, and humid conditions, especially in summer.
7. How does an air mass get modified as it moves away from its source region?
An air mass gets modified when it travels over a surface with different characteristics than its source region. This process, known as air mass modification, can happen in two main ways: a cold air mass moving over a warm surface, or a warm air mass moving over a cold surface. For example, a cold, dry cP air mass moving over a warmer ocean will pick up heat and moisture from below. This makes the lower layers of the air mass unstable, often leading to the formation of cumulus clouds and precipitation.
8. Which air masses primarily influence the weather patterns in India?
The weather patterns in India are primarily influenced by the seasonal interplay of different air masses:
Maritime Tropical (mT): During the summer, a warm and extremely moist mT air mass from the Indian Ocean moves inland, causing the Southwest Monsoon and bringing widespread heavy rainfall.
Continental Tropical (cT): In the pre-monsoon season (spring and early summer), hot and dry cT air masses from the arid regions of Rajasthan, Pakistan, and the Middle East cause severe heatwaves across North and Central India.
Continental Polar (cP): During winter, cold and dry cP air masses from Central Asia and Siberia move south, causing cold waves and bringing cool, pleasant weather to Northern India.
