It is a large body of air whose physical properties like temperature, moisture content and lapse rate are uniform horizontally for hundreds of kilometres. It might be so extensive that it might cover an entire continent and it may vertically extend through troposphere. The air masses are classified according to the source regions.
FIVE MAJOR SOURCE REGIONS
- Warm tropical and subtropical oceans.
- The subtropical hot deserts.
- The relatively cold high latitude oceans.
- The very cold snow-covered continents in high latitudes;
- Permanently ice-covered continents in the Arctic and Antarctica.
TYPES OF AIRMASSES ARE RECOGNISED
- Maritime tropical (mT)
- Continental tropical (cT)
- Maritime polar (mP)
- Continental polar (cP)
- Continental arctic (cA)
CLIMATIC SIGNIFICANCE OF AIR MASSES
- Cause precipitation: Ex. maritime-tropical airmass over Atlantic Ocean causes precipitation east of Rocky Mountains.
- Modify temperature: Airmasses developed over North Atlantic brings moderating effect on NW coast of Europe.
- Modify Weather: They modify the weather of regions where they visit. Ex. cP airmass produces ‘lake snow’ effect at the shores of great lakes in North America.
- Cyclones & anti-cyclones: Convergence of opposite air masses create fronts and lead to temperate cyclones.
- Droughts: Hot dry airmass can increase the evaporation and lead to drought situation.