Air Mass

• Air over a uniform area for a long time takes on the area’s characteristics
• Uniform areas include vast oceans, plains, and plateaus
• Air with distinctive temperature and humidity is called an air mass
• Air masses are large bodies of air with little horizontal variation in temperature and moisture
• Air masses are part of the global wind system and associated with wind belts
• Air masses extend from the surface to the lower stratosphere
• Air masses can span thousands of kilometers

Source regions

• Homogenous surfaces where air masses form are called source regions
• High-pressure belts in subtropics and around poles are the main source regions
• Source regions establish heat and moisture equilibrium with overlying air mass
• Upper level maintains physical characteristics for a longer period when air mass moves away from source region
• Air masses are stable with stagnant air that does not facilitate convection
• Conduction and radiation in stagnant air are not effective

Conditions for the formation of Air masses

• Source region should have extensive, gentle, divergent air circulation at high pressure
• Areas with high pressure but little pressure difference or gradient are ideal source regions
• Mid-latitudes are not major source regions due to cyclonic and other disturbances

Conditions for the origin of Air masses

• Homogeneous Surface
• Isotropic surface
• Lack of turbulence in the air
• Lack of convection in air
• Subsiding air with high pressure
• Atmospheric stability
• Kinetic energy of wind and friction

Size and dimension

• Extend till Tropopause
• Width is hundreds of km
• Height varies b/w 8-12 km
• Latitudinal extent varies from 3000-6000 km

Classification of Air Masses

• Classification of Air Masses based on –
  • Nature of Surface
    • Continental
    • Marine
  • Source region
    • Polar
    • Tropical
  • Temperature
    • Cold
    • Warm
  • Atmospheric conditions
    • Stable
    • Unstable
• 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.

Air masses based on Source Regions

• There are five major source regions for air masses
• These regions include warm tropical and subtropical oceans, subtropical hot deserts, relatively cold high latitude oceans, very cold snow-covered continents in high latitudes, and permanently ice-covered continents in the Arctic and Antarctica
• Five types of air masses are recognized:
  • Maritime tropical (mT), 
  • Continental tropical (cT), 
  • Maritime polar (mP), 
  • Continental polar (cP), and 
  • Continental arctic (cA)
• Tropical air masses are warm, while polar air masses are cold
• Heat transfer processes that warm or cool the air take place slowly
• 

Cold Air Mass

• A cold air mass is one that is colder than the underlying surface and is associated with instability and atmospheric turbulence. (because of moisture and very low temperature)
• Cold source regions (polar air masses)
  • Arctic Ocean – cold and moist
  • Siberia – cold and dry
  • Northern Canada – cold and dry
  • Southern Ocean – cold and moist

Warm Air Mass

• A warm air mass is one that is warmer than the underlying surface and is associated with stable weather conditions.
• Warm source regions (tropical air masses)
  • Sahara Desert – warm and dry
  • Tropical Oceans – warm and moist

Influence of Air Masses on World Weather

• Properties of an air mass that influence weather include vertical distribution temperature and moisture content
• Air masses carry atmospheric moisture from oceans to continents and cause precipitation over landmasses
• Air masses transport latent heat, removing the latitudinal heat balance
• Migratory atmospheric disturbances such as cyclones and storms often originate at the contact zone between different air masses
• Weather associated with these disturbances is determined by characteristics of the air masses involved

• Continental Polar Air Masses (CP)
  • Source regions: Arctic basin, northern North America, Eurasia, and Antarctica.
  • Characteristics: dry, cold, and stable conditions.
  • Weather during winter: frigid, clear, and stable.
  • Weather during summer: less stable with the lesser prevalence of anticyclonic winds, warmer landmasses, and lesser snow.
• Maritime Polar Air Masses (MP)
  • Source regions: oceans between 40° and 60° latitudes.
  • Characteristics: continental polar air masses that have moved over the warmer oceans, got heated up, and have collected moisture.
  • Conditions over the source regions: cool, moist, and unstable.
  • Weather during winter: high humidity, overcast skies, and occasional fog and precipitation.
  • Weather during summer: clear, fair, and stable.
• Continental Tropical Air Masses (CT)
  • Source regions: tropical and sub-tropical deserts of Sahara in Africa, and of West Asia and Australia.
  • Characteristics: dry, hot and stable and do not extend beyond the source.
  • Dry throughout the year.
• Maritime Tropical Air Masses (MT)
  • Source regions: oceans in tropics and sub-tropics such as the Mexican Gulf, the Pacific, and the Atlantic oceans.
  • Characteristics: warm, humid, and unstable.
  • Weather during winter: mild temperatures, overcast skies with fog.
  • Weather during summer: high temperatures, high humidity, cumulous clouds, and convectional rainfall.