Air Masses

An air mass moving over a warmer surface is heated from below. Its lower layers become less stable, relative humidity falls and convection becomes easier. Moving over a colder surface makes the lower layers colder, more stable and relatively more humid.

Polar maritime

mPc begins cool and stable over the North Atlantic, then becomes unstable as it crosses warmer water or land. It gives cumulus and cumulonimbus, showers, hail and good visibility away from showers. At night over land the cloud can decay, leaving an inversion and radiation fog.

Tropical maritime and continental air

mTw is warm and moist. As it moves north its temperature reduces and stability rises, favouring low stratus, stratocumulus, drizzle, poor visibility and advection fog. cTw is warm, dry and stable, often clear but vulnerable to dust haze. cPc is cold and dry over land, but becomes unstable and showery after a sea crossing.

A front is the boundary zone between air masses of different temperature. The denser cold air remains below while warmer air rises across the frontal surface. The polar front separates polar and tropical air masses and is the birthplace of many temperate depressions.

Warm and cold fronts

At a warm front, warm air overtakes and rises over cold air. Cloud commonly appears in the order Ci, Cs, As, Ns and St, with widespread steady precipitation and poor visibility ahead and during passage. A cold front undercuts warm air, produces a steeper ascent and is associated with showers, cumulonimbus, squalls and rapid weather change.

Joshi connects fronts with extra tropical cyclones and western disturbances affecting India. The front is not merely a line on a chart: its clouds, wind change, visibility and precipitation are products of rising air along the boundary.

Know the route history before using a label. The same polar maritime air can be unstable and showery by day, but become stable with fog after night cooling. A continental polar flow may be clear over land or turn showery after a cold sea crossing.