Air masses & fronts

A. The warm air is lifted off the surface and the overtaking cold air rides over the colder air ahead

In any occlusion the warm sector air is lifted off the ground. In a warm occlusion the air behind the cold front is less cold than the air ahead, so it rides up over it.

B. Rides up over the cold air

At a warm front the advancing warm air is lighter, so it climbs gently up over the wedge of retreating cold air - producing the long sequence Ci, Cs, As, Ns.

A. Suddenly becomes squally and veers

At a cold front the wind veers sharply, often with violent gusts and squalls, the temperature drops and the pressure starts to rise.

B. Western depression

When a western disturbance deepens enough to show two or more closed isobars at 2 hPa spacing, it is called a western depression.

B. About double

The dense cold air pushes forward quickly; a cold front typically travels about twice as fast as a warm front, which is why it eventually catches up and forms an occlusion.

A. A steeper slope and moves faster

Cold front slope about 1:50 to 1:80 against 1:150 for a warm front, and about double the speed. Steep and fast gives a narrow band of violent weather; shallow and slow gives wide, long-lasting weather.

A. Warm

A warm front announces itself from far ahead: first Ci, then Cs, As, Ns and finally St as the frontal surface comes down to the ground.

B. Warm front

The front is named after the air that is advancing: warm air replacing cold air is a warm front.

B. Cold occlusion

In a cold occlusion the coldest air is the overtaking air behind the cold front, so it wedges under everything ahead of it.

C. Cold and dry

Polar = cold, continental (land) = dry: polar continental air is cold and dry.

B. Extra-tropical depressions

Fronts need two contrasting air masses, which meet along the polar front in mid-latitudes. Extra-tropical depressions form on it; tropical depressions and heat lows form in a single air mass and have no fronts.

A. Ahead

With a cold front, fog and poor visibility are found ahead of the front; behind it the fresh, cold air brings good visibility.

B. Warm air advancing over retreating cold air, with a gentle slope

Warm air replaces cold air along a shallow slope of about 1 in 150, which is why its cloud and rain spread far ahead of the surface front.

C. Backs and strengthens

As a warm front approaches the pressure falls and the gradient tightens, so the wind backs and freshens; at the front it veers.

A. Precipitation ceases quickly and visibility improves

Cold-front weather is a narrow band. Once it has passed, the rain stops, the cold air brings good visibility, and scattered showers may follow.

A. Warm and moist

Low latitude = tropical (warm); over the sea = maritime (moist). Tropical maritime air is warm and moist.

C. Winter

WDs travel east with the subtropical westerlies, which lie over north India in winter - five to seven a month then.

A. Shows little or no movement

When the winds on both sides blow nearly parallel to the front, it hardly moves. Its weather can sit over one area for days.

B. Occluded Fronts

Western disturbances reach India as occluded fronts. They start as frontal depressions far to the west and are usually occluded by the time they arrive, often showing only as an upper-air trough.

A. Parallel to the front, in opposite directions

A front stays put when neither air mass can displace the other. That happens when the flow on each side runs ALONG the boundary rather than across it, in opposing directions, so neither pushes it. The polar front is the classic example.

B. Cold front

Cold fronts move at about twice the speed of warm fronts; a stationary front hardly moves at all.

C. Underlying surface

An air mass takes its temperature and moisture from the land or sea it sits over in its source region - hot or cold, dry or moist.

A. Polar maritime

High latitude = polar; over the sea = maritime. Polar maritime air is cool and moist.

A. Front

A front is the boundary zone where two air masses of different temperature and humidity meet - the polar front, for example, separates polar from tropical air. It is named warm or cold by which air is replacing the other.

B. Cold front

Fronts are named after the air that is doing the replacing. Cold air pushing in under and replacing warm air makes a cold front; warm air replacing cold makes a warm front.

A. Cold Front

A cold front lifts warm air sharply, giving CB, roll clouds and Sc or Ac with embedded CB along a narrow band. Layered cloud from Ci down to Ns belongs to the warm front.

A. Ahead of a warm front

At a warm front the warm air rises gently over the cold air across a wide area, producing layered cloud and steady rain over a broad belt ahead of the surface front. A cold front gives a narrow band of showers.

C. Polar Front

Extra-tropical depressions form on the polar front, where cold polar air meets warm tropical air in the middle latitudes.

B. A low with a warm front radiating out Southwards followed by a cold front .

A mature extra-tropical depression is a low with a warm front running out from it, followed by a cold front, the two joining in an occlusion near the centre as the cold front catches up.

A. A large body of air whose temperature and humidity are nearly uniform horizontally

An air mass takes on the temperature and moisture of the surface it lingers over, and stays nearly uniform across hundreds of kilometres. The boundary between two air masses is a front.

B. Warm and Moist

Air that forms over the equatorial oceans takes on the heat and moisture of the warm sea surface, so an equatorial air mass is warm and moist.

C. Remain same

Ahead of a warm front the cold air stays in place under the sloping frontal surface, so the surface temperature changes little as the front approaches. The marked rise comes after the front has passed, when the warm sector air arrives.

A. Backs and strengthens

As the front approaches, pressure falls and the wind backs - turns anticlockwise - and strengthens. It veers AFTER the front has passed. Ahead: backs; after: veers.

B. Unstable, with cumulus and showers

Heating an air mass from below steepens its lapse rate and makes it unstable. The warmed surface air rises, and if there is enough moisture it forms cumulus and showers. The opposite case - warm air over a cold surface - is stabilised, and gives fog or stratus instead.

A. Cold front

A front is named after the air mass that is doing the advancing. Cold air pushing in under warm air and displacing it is a cold front; warm air riding up over retreating cold air is a warm front. The polar front is the name of a particular boundary - between polar and tropical air - not of this motion.

A. More stable, favouring fog and stratus

Cooling the bottom of an air mass reduces the temperature difference between the surface and the air above it, so the lapse rate becomes smaller and the air more stable. A stable, moist air mass cannot convect - so its moisture shows up as fog, mist or a sheet of stratus rather than as cumulus.

C. Cold and dry

An air mass takes its character from its source region. Polar means cold, continental means dry, so continental polar air is cold and dry. Maritime polar would be cold and moist, and a tropical source gives the warm versions of each.