UPSC Mains 2016 GS Paper 1 Q13: model answer

A cold wave that sweeps from Canada to Texas in a few days, dropping temperatures by 20°C, is an air mass in motion. An air mass is a large body of air, hundreds to thousands of kilometres wide, with fairly uniform temperature and moisture.

Source regions and types

• Air masses form over large, uniform surfaces where air stays long enough to acquire their character.
• Continental polar (cP): cold and dry (Siberia, Canada).
• Maritime polar (mP): cool and moist (North Pacific, North Atlantic).
• Continental tropical (cT): hot and dry (Sahara, Thar).
• Maritime tropical (mT): warm and moist (Gulf of Mexico, Indian Ocean).
• Arctic and equatorial types at the extremes.

Role in macro-climatic changes

• Fronts form where air masses meet, causing mid-latitude cyclones, rain and snow.
• Movement of air masses brings seasonal change: cold waves, heat waves, monsoon onset.
• They transfer heat and moisture from tropics to poles, balancing global energy.
• Shifts in their paths, linked to the jet stream, cause long dry or wet spells.

Examples

• Western disturbances bring winter rain to north India.
• Polar outbreaks into the USA (2021 Texas freeze).
• Hot, dry cT air from the Thar and Iran drives north India's May heat waves and the loo.

Air masses are the building blocks of weather and climate; climate change is now altering their paths and frequency.