A large body of air characterized by temperature (pressure) and humidity from its source region. Air masses can be 1000km wide or more. Air masses acquire their properties from where they originated and place these on areas they pass by. Air masses are named after the region from which they originated (Tropics or Polar) and or if they originated on land(Continent) or sea(Maritime). From these, four air masses are classified:
These originate on land from the tropics.
They are warm and dry (no moisture).
Originate mainly from arid regions e.g. Sahara, Kalahari.
They may bring the hottest and driest conditions in their wake.
Tropical Maritime (mT)
Originate in oceans and seas from the tropics e.g. over Mid Atlantic and North Indian Ocean.
They are warm and carry large amounts of moisture therefore bringing rainfall in their wake(path). A good example affecting Zimbabwe is the Zaire air
which migrate from the Atlantic and picks more moisture from the surrounding rainforest in Congo and joins with the ITCZ over Zimbabwe.
Originate on land from the polar areas and usually dry e.g. Canada, Scandinavia
Polar Continental (cP)
Theses air masses are associated with very low temperatures and bitterly cold winds in winter, and wind chill can be significant.
If they pick moisture in their path they may produce freezing rainfall, snow or even hail.
Polar Maritime (mp)
Originate from polar regions e.g. Canada, Greenland, Antarctica. The air mass is cold but carry large amounts of moisture. If these air masses move over warm waters they may become unstable
which can produce torrential rainfalls.
Arctic air masses are very cold (especially in winter) and bring cold conditions across Scandinavia. May become unstable if they hover along warm waters which can create torrential rainfalls and hail. They originate from continental areas (cA) such as Alaska, Canada or Greenland or from the Arctic ocean itself as (mA)Read here, Air masses affecting Zimbabwe