Air masses and their source regions

Detail

What an air mass is

• An air mass is a very large body of air. Its temperature and moisture change very little from side to side (horizontally).

• It can cover thousands of kilometres. One air mass can sit over a whole country or a big part of an ocean.

• How an air mass forms:

• Air stays still or moves very slowly for a long time (days to weeks) over one large, homogeneous surface (a surface that is the same everywhere), such as a vast ocean or a wide plain.
• The surface heats or cools the air from below and gives it moisture, or gives it none.

• In the end the air takes on the surface's character: hot or cold, wet or dry.

• Air masses usually form under anticyclones (high-pressure areas where air sinks and winds are light). The calm air stays in one place long enough to take on the surface's character.

Source region

• A source region is the large, uniform surface where an air mass forms, such as a warm sea, a hot desert or a snow-covered plain.
• What makes a good source region:
• It is large, so the air takes on one uniform character.
• It is uniform: all ocean, all desert or all snow, with no big mountains.

• It has light winds and sinking air, so the air stays long enough.

• The Siberian anticyclone is a semi-permanent high-pressure system centred over north-eastern Siberia in the colder half of the year. It is one of the main sources of polar air masses [7].

Naming code (two letters)

• First letter (lower case) = surface type, which sets the moisture:
• c = continental. Forms over land, so the air is dry [5].

• m = maritime. Forms over the ocean, so the air is moist [5].

• Second letter (capital) = latitude, which sets the temperature:

• A = Arctic. Forms over the Arctic or Antarctic and is very cold [5].
• P = Polar. Forms over higher latitudes, on land and sea. It is cold, but less cold than Arctic air [5].

• T = Tropical. Forms over lower latitudes, on land and sea. It is warm or hot [5].

• Rule to remember: tropical air masses are warm and polar ones are cold.

The five NCERT types

Type	Source region	Character	Weather it brings
Maritime tropical (mT)	Warm tropical and subtropical oceans (e.g. Gulf of Mexico, Indian Ocean)	Warm, moist, unstable	Humid heat and heavy rain. Example: Gulf of Mexico air moving into the eastern USA
Continental tropical (cT)	Subtropical hot deserts: Sahara (North Africa), Arabia (West Asia), Thar (Rajasthan, India and Pakistan)	Hot and dry	Heat waves
Maritime polar (mP)	Fairly cold high-latitude oceans: North Pacific, North Atlantic	Cool and moist	Cool, cloudy, showery weather on west coasts. North Pacific air reaches British Columbia (Canada). North Atlantic air reaches north-western Europe
Continental polar (cP)	Very cold, snow-covered high-latitude continents: Siberia, northern Canada	Cold, dry, stable	Cold waves that reach far to the south
Continental arctic (cA)	Permanent ice of the Arctic and Antarctica	Extremely cold and dry	Severe cold outbreaks

• Stable air resists rising, so few clouds form. Unstable air rises easily, so clouds grow tall and rain falls.
• cP air is cold at the bottom and sits heavily on the ground, so it is stable and brings clear, cold, dry weather.
• mT air is warm and wet at the bottom, so it rises easily. Clouds build and heavy rain follows.

Beyond NCERT: fuller schemes

• Maritime equatorial (mE): forms over warm equatorial oceans. It is very warm and very moist. The main world air masses are usually listed as cA, cP, cT, mP, mT and mE [6].

• cA and cP are cold and dry. cT is hot and dry. mP is cool and moist. mT and mE are warm and moist [6].

• Antarctic air (cAA): some schemes give the very cold air over the Antarctic ice sheet its own class, separate from Arctic air.

• "k" and "w" suffixes (from standard climatology texts such as Barry and Chorley):
• k (from German kalt, cold) means the air is colder than the surface it is passing over. The air is heated from below, becomes unstable, and gives showers. Example: mPk, polar sea air moving onto a milder coast.
• w (warm) means the air is warmer than the surface below. The air is cooled from below, becomes stable, and brings fog, low cloud or drizzle. Example: mTw, tropical sea air moving over cold land in winter.

How an air mass changes as it moves (air-mass modification)

• An air mass does not stay the same once it leaves its source region.
• Heated from below → becomes unstable:
• Cold air passes over warm land or a warm sea.
• Its lowest layer warms and rises.
• Clouds and showers form.

• Example: in winter, very cold, dry Arctic air can move over the ocean. It picks up warmth and moisture from the warmer water [5].

• Cooled from below → becomes stable:

• Warm air passes over cold land or a cold sea.
• Its lowest layer cools and stops rising.

• Fog and low stratus cloud form, and temperature inversion may develop (a layer where temperature rises with height instead of falling).

• Gains moisture over seas and lakes, and loses moisture as rain, especially when it is forced up over mountains.

• Why it matters: when two different air masses meet, they form fronts (boundary zones between air masses). Fronts lead to extra-tropical cyclones (storms of the middle latitudes). This is covered in the next section.

NCERT exercise: which surface is a source region?

• Correct answer: the Siberian Plain. It is vast, flat, uniform and snow-covered in winter, so it is a classic cP source region.
• Wrong options and why:
• Equatorial forests: the surface is not uniform, and rising air (convection) does not let the air stay in place.
• The Himalaya: high, broken mountains. A mountain range blocks and changes air masses rather than creating them.
• The Deccan: a plateau of mixed relief and land use, too varied and too small to be a source region.

Indian links

Winter: cold waves from modified cP air

• In winter, cold, dry air builds up over Siberia and Central Asia.
• Some of it reaches north India in a modified form. The Himalaya blocks most of it, so the air that gets through is changed on the way.
• Cold waves in India usually come in November–March. Cold winds push into north-western and central India when western disturbances pass over the region [3].

• Western disturbance: a low-pressure trough that moves eastward in the upper-air westerly winds, north of 20°N. It often reaches down to the lower atmosphere and brings cold air from northern latitudes into India [3].

• During a cold wave, minimum temperatures fall by more than 4°C. Spells usually last 3–5 days and are most common in January [3].

• Where: the plains of north-west India and the nearby parts of central India have the most cold wave days, an average of 6–8 days a year (data 1971–2019) [3].
• IMD cold wave criteria (current):
• A cold wave is considered only if the minimum temperature is ≤10°C in the plains or ≤0°C in the hills [2].
• Cold wave: minimum temperature is 4.5°C to 6.4°C below normal. Severe cold wave: more than 6.4°C below normal [2].
• Plains only: a cold wave can also be declared when the actual minimum is ≤4°C, and a severe cold wave when it is ≤2°C [2].

Summer: loo from cT air

• In summer, hot, dry cT air forms over the Thar and the deserts of Arabia and Pakistan.
• It blows across the north-western plains as the loo (a hot, dry, dusty wind that blows on summer afternoons over north and north-west India).
• Heat waves usually come in the pre-monsoon months, March–June. The most affected belt is central and north-west India, known as the heat wave zone [3]. In some years heat waves last into July [4].
• IMD heat wave criteria (current):
• A heat wave is considered only if the maximum temperature is ≥40°C in the plains, ≥37°C at coastal stations or ≥30°C in the hills [2].
• Heat wave: 4.5°C to 6.4°C above normal. Severe heat wave: more than 6.4°C above normal [2].
• Plains only: a heat wave can also be declared when the actual maximum is ≥45°C, and a severe heat wave when it is ≥47°C [2].
• NDMA's older wording uses steps of 5–6°C (or 4–5°C where the normal maximum is above 40°C) and an automatic declaration at 45°C [4].

Monsoon: rain from mT air

• In summer the land heats up and low pressure forms over north-west India.
• Warm, moist mT air from the Indian Ocean is pulled in as the south-west monsoon.
• This air is unstable. When it is forced up over the Western Ghats and the Himalaya, it brings heavy rain (see indian-climate-monsoon).
• The same warm, moist air over the Bay of Bengal and the Arabian Sea is the "fuel" for tropical cyclones such as Fani (2019) and Amphan (2020) in the Bay of Bengal and Biparjoy (2023) in the Arabian Sea. Cyclones are covered in a later section.

Prelims Hooks

• Source region match: Siberian Plain = source region (cP). Equatorial forests, the Himalaya and the Deccan are not source regions.
• Code logic: c = continental = dry; m = maritime = moist; T = warm; P = cold; A = extremely cold.
• cT source regions are the Sahara, Arabia and the Thar. cT air brings heat waves and the loo.
• mP air brings cool, showery weather to west coasts: North Pacific air to British Columbia, North Atlantic air to north-western Europe.
• mT air from the Gulf of Mexico brings humid heat and heavy rain to the eastern USA.
• Stability trap: cP air is stable (cold below, so it does not rise). mT air is unstable (warm and wet below, so it rises).
• k / w suffix: k = colder than the surface below, so it becomes unstable and gives showers. w = warmer than the surface below, so it becomes stable and gives fog.
• IMD cold wave (plains): minimum ≤10°C, and 4.5–6.4°C below normal or actual ≤4°C. Heat wave (plains): maximum ≥40°C, and 4.5–6.4°C above normal or actual ≥45°C [2].
• Indian cold wave season: November–March, peak in January, triggered by western disturbances [3].

Mains Points

• Air masses shape India's three seasons:
• Winter: modified cP air brings cold waves and fog.
• Summer: cT air brings the loo and heat waves.
• Monsoon: mT air brings rain.

• So the farm calendar, water supply and power demand all depend on which air mass is over India (GS-I).

• Air-mass modification links geography to weather:

• The Himalaya blocks much of the Siberian cP air, which keeps north India milder in winter than places at the same latitude in Central Asia.

• The Western Ghats force mT air to rise and drop its rain, which creates the wet windward side and the dry rain-shadow Deccan.

• Disaster management (GS-III):

• cT heat waves (March–June) and cP-linked cold waves (November–March, mostly north-west and central India) need different responses.
• These include IMD's threshold-based warnings [2] and NDMA's heat wave guidance and health advisories [4].

• A warming climate is making heat waves longer and more intense [4].

• Fronts and cyclones: the contrast between cold polar air and warm tropical air drives mid-latitude cyclones. The warm, moist mT air over the north Indian Ocean feeds tropical cyclones (Fani 2019, Amphan 2020, Biparjoy 2023). This makes air-mass study the base for cyclone forecasting and coastal preparedness.