Overview
Air masses are large bodies of air with little horizontal variation in temperature and moisture. Each one takes its character from the ocean or plain over which it forms. Where two different air masses meet, the boundary zone between them is a front. The formation of a front is frontogenesis and its weakening is frontolysis. These ideas explain why the middle latitudes have such changeable weather, how a temperate cyclone grows, and why north-west India receives rain in winter.
Air Masses: Definition, Characteristics and Source Regions
What Is an Air Mass? Formation and Characteristics
An air mass forms when air rests over one uniform surface, a vast ocean or a wide plain, for long enough to take on its temperature and humidity. Such an air mass covers thousands of square kilometres. The longer the air stays over the surface, the more completely it takes on the surface's character.
- Temperature and moisture: These two properties identify an air mass. Both stay nearly the same across it.
- Colder or warmer: Colder air masses are called polar or arctic, and warmer ones are called tropical.
- Dry or moist: Continental air masses are dry, and maritime air masses are moist.
- Density: Cold air is denser than warm air. This is why cold air pushes under warm air wherever the two meet, and why the weather at a front is so active.
Source Regions of Air Masses: Five Kinds of Surface
The uniform surfaces over which air masses form are called source regions. There are five major ones, and each gives its air mass a name.
| Air mass | Source region | Character | Examples of source areas |
|---|---|---|---|
| Maritime tropical (mT) | Warm tropical and subtropical oceans | Warm and moist; also called trade air masses | The Caribbean Sea, the southern Gulf of Mexico and the tropical Atlantic east of Florida (for North America) |
| Continental tropical (cT) | Subtropical hot deserts | Extremely hot and dry | The Sahara (the major source), the Arabian Peninsula, central Australia, the south-western deserts of the United States |
| Maritime polar (mP) | Relatively cold high-latitude oceans | Cold and moist | The North Pacific and the North Atlantic |
| Continental polar (cP) | Very cold snow-covered continents in high latitudes | Cold and dry | Interior Canada (for North America) |
| Continental arctic (cA) | Permanently ice-covered Arctic and Antarctica | Deeply cold, colder than polar air; dry | Northern Siberia in winter |
Types of Air Masses: Classification and Modification
Classification of Air Masses: The Bergeron Letters
The Bergeron classification is the most widely accepted way of naming air masses. It uses up to three letters.
- First letter, moisture: A small c marks continental air, which is dry, and a small m marks maritime air, which is moist.
- Second letter, source: A capital T stands for tropical, P for polar, and A for arctic or Antarctic. Some schemes add M for monsoon, E for equatorial and S for superior air, which is dry air formed by strong sinking motion in the atmosphere.
- Third letter, stability: The letter k means the air mass is colder than the ground below it, and w means it is warmer.
An air mass from the desert south-west of the United States in summer is cT, and one from northern Siberia in winter is cA. A polar air mass blowing over the warm Gulf Stream is written cPk, because it is colder than the water beneath it.
Two further conventions are used. A change from one type to another is written with a hyphen between the old code and the new. Layered air masses are written with a slash, for example mT/cP when warm moist air overrides a polar air mass.
Air Mass Transformation Over Oceans and Land
An air mass does not keep its character for ever. Once it moves away from its source region, the vegetation and water bodies below it can change it quickly. Heat from warmer waters can modify an air mass over distances as short as 35 to 40 kilometres.
Cold air over a warm sea shows the idea well. Such air is stable, but when it travels over warmer ocean water it gains moisture and loses its stability. An arctic air mass blowing out over the Gulf of Alaska turns maritime polar and is written cA-mPk, which tells a forecaster that its dryness and extreme cold have started to change.
Air Masses over India
Winter Air, Western Disturbances and the Rabi Crop
India's winter weather is shaped by dry air from the north-west and by disturbances from the west. In winter the weather is pleasant, with feeble high pressure over the northern plain. Winds blow out from this north-western high-pressure zone towards the lower pressure over the Indian Ocean in the south. Around a high, the circulation is anticlockwise in the southern hemisphere and clockwise in the northern, and this anticyclonic circulation over land, together with the low humidity of the outflowing winds, is why most of India has no winter rain.
The cold itself has three causes. Punjab, Haryana and Rajasthan have a continental climate far from the sea, snow falls on the nearby Himalayan ranges, and around February cold winds from the Caspian Sea and Turkmenistan bring cold waves, frost and fog.
The pleasant weather is broken at intervals by western disturbances. These are shallow cyclonic depressions that start over the east Mediterranean Sea and travel east across West Asia, Iran, Afghanistan and Pakistan before they reach north-western India. On the way they pick up more moisture from the Caspian Sea in the north and the Persian Gulf in the south. They are driven by the westerlies, the prevailing winds that blow from west to east between 30 and 60 degrees latitude in each hemisphere, and they are embedded in the westerly jet stream.
- Rain and snow: The rain reaches Punjab, Haryana, Delhi and western Uttar Pradesh, and it falls as snow in the lower Himalayas. The average winter rainfall in Delhi is only around 53 millimetres, yet it greatly helps the rabi crop.
- A large share in the hills: About 30 per cent of the annual precipitation of Jammu and Kashmir, Ladakh, Himachal Pradesh and Uttarakhand falls in winter, mostly with western disturbances.
- After they pass: Cold wave conditions and dense fog can follow over the Indo-Gangetic plains.
- Summer river flow: Winter snow in the Himalayas helps to sustain the flow of Himalayan rivers in summer.
Hot-Weather Storms Where Dry and Moist Air Meet
Air masses also explain the violent storms of the hot-weather season. In the north-west a trough of low pressure lies over the Thar desert and extends towards Patna and the Chotanagpur plateau, and dry, hot winds blow across the plains in the afternoon. Moisture-laden winds are attracted towards the edge of the trough. A sudden contact between dry and moist air masses gives local storms of great intensity, with violent winds, torrential rain and even hailstorms.
Several well-known local winds and storms belong to this season.
- Nor'westers: These are dreaded evening thunderstorms in Bengal and Assam. In Bengal they are called Kalbaisakhi, meaning "the calamity of the month of Baisakh", and in Assam Bardoisila. The showers help tea, jute and rice.
- Loo: Hot, dry and oppressive winds blow over the northern plains from Punjab to Bihar, strongest between Delhi and Patna.
- Dust storms: Evening dust storms are common in May in Punjab, Haryana, eastern Rajasthan and Uttar Pradesh, and they give a short respite from the heat.
Weather Fronts: Where Air Masses Meet
Features and Map Symbols of a Front
A front is a zone of transition between air masses of different density, and so usually of different temperature or moisture. It is a zone and not a line, but on the weather map it is drawn as one line with symbols. A moving front is named after the advancing air mass, so a cold front is one where colder air is advancing.
- Where they occur: Fronts occur in the middle latitudes and are marked by a steep gradient of temperature and pressure.
- What they do: Fronts bring abrupt changes in temperature and make air rise, which forms clouds and precipitation.
- A sure sign: Some fronts bring no rain and little cloud, but there is always a shift in the wind.
- Pressure: Fronts are found along troughs of lower pressure.
- Dry line: A dry line is a boundary where moisture, not temperature, changes sharply.
Meteorologists also use two terms for the weather around a front. An anafront is unstable, with air rising rapidly along the boundary to give heavy precipitation. A katafront is weaker, with smaller changes in temperature and moisture and limited rain.
Types of Fronts: Cold Front and Warm Front
A cold front is the leading edge of a cooler air mass that replaces warmer air at ground level. The colder, denser air forms a steeply sloping boundary under the warmer, moister air and lifts it. If enough moisture is present, the result is a narrow band of showers and thunderstorms. Temperature differences across the boundary can exceed 30 °C.
A cold front can move up to twice as fast as a warm front. In the northern hemisphere the wind shifts from south-west to north-west as it passes, and pressure falls steadily as it nears, then rises sharply once it has gone. On the map it is drawn as a blue line with triangles pointing in the direction of travel.
A warm front is the leading edge of an advancing warm air mass. The lighter warm air cannot push the cold air aside, so it rises up a gentle slope of about 1 in 200 over the cold air, in a process called overrunning. The lifting is slow but steady, so the clouds ahead of the front are mostly layered, and rain increases gradually as the front approaches. Fog can occur before it passes, and warming and clearing follow quickly. On the map it is drawn as a red line with semicircles pointing in the direction of travel.
Stationary and Occluded Fronts: Stalled and Merged Boundaries
A stationary front is a boundary where neither air mass is strong enough to replace the other, so each advances into the other at under 5 knots at the ground. It tends to stay in the same area for hours to days and may wave as atmospheric waves move along it. The winds on the two sides often blow nearly parallel to the front in opposite directions.
The weather is usually overcast with rain or snow. A stationary front may turn into a cold or warm front, may dissipate after several days, or may degenerate into a shear line, which separates regions of different wind speed. On the map it is drawn as alternating blue spikes pointing towards the warmer air and red domes towards the colder air.
An occluded front forms when a cold front overtakes a warm front, usually around a mature low-pressure area such as a cyclone, and the warm air is lifted off the ground. The point where the three fronts meet is the triple point.
There are two kinds of occlusion. In a cold occlusion the overtaking air is colder than the cold air ahead of the warm front, so it ploughs under both. In a warm occlusion the overtaking cold air is less cold than the cold air ahead of the warm front, so it rides over that air and lifts the warm air. Thunderstorms are possible, but the passage of an occlusion is usually linked with drying of the air. On the map it is drawn as a purple line with triangles and semicircles together.
| Front | How it forms and moves | Weather it brings | Map symbol |
|---|---|---|---|
| Cold front | Colder air advances and undercuts warmer air on a steep boundary; up to twice as fast as a warm front | Narrow band of showers and thunderstorms; sharp temperature change, can exceed 30 °C; wind shift | Blue line with triangles |
| Warm front | Warm air advances and rises up a gentle slope, about 1 in 200, over colder air | Layered cloud, steady rain that increases as the front nears, often fog; quick warming after | Red line with semicircles |
| Stationary front | Neither air mass advances; each moves into the other at under 5 knots | Overcast with prolonged rain or snow; can last hours to days | Alternating blue spikes and red domes |
| Occluded front | A cold front overtakes a warm front and lifts the warm air off the ground | A wide variety, thunderstorms possible, usually followed by drier air | Purple line with triangles and semicircles |
Frontogenesis and Frontolysis
Frontogenesis: How Fronts Form
In meteorology, frontogenesis is the tightening of the horizontal temperature gradient between two air masses until a sharp boundary appears, so that a front forms. It occurs as a developing wave in the atmosphere draws cold air from the poles and warm air from the equator together. The result is a cold front and a warm front.
Horizontal deformation and horizontal shear are the two key mechanisms. In a mid-latitude cyclone, winds on the west side carry cold air and winds on the east side carry warm air, and this horizontal shear squeezes the temperature contrast along a line. Deformation, the stretching and squeezing of the air flow, concentrates the temperature gradient of cold polar air and warm equatorial air over a large scale. In all, eight mechanisms influence the gradient, namely horizontal deformation, horizontal shear, vertical deformation, differential vertical motion, latent heat release, surface friction, turbulence and mixing, and radiation.
Frontolysis: How Fronts Decay
The opposite process, frontolysis, is the dissipation or weakening of a front. It is a decrease in the temperature gradient between colliding air masses, and it can also be caused by a shift in the wind pattern. The areas where air masses diverge are areas of frontolysis, in contrast to the areas of convergence where frontogenesis occurs. On weather maps, a front that is weakening is drawn as a dashed line with the front markers on every second dash.
When the density contrast fades, for example after air flows out over a uniformly warm ocean, a front can degenerate into a shear line, most common over the open ocean.
Fronts and Temperate Cyclones
Temperate Cyclone Life Cycle on the Polar Front
Fronts are the birthplace of extra-tropical cyclones, also called temperate or mid-latitude cyclones, which together with anticyclones drive the weather over much of the middle latitudes. The idea was worked out by meteorologists of the Bergen School, founded in 1917 by Vilhelm Bjerknes, and the polar front theory is attributed to Jacob Bjerknes. The life cycle is still called the Norwegian cyclone model. The polar front is the boundary between cold polar air and warm tropical air, near 60 degrees latitude, and the westerlies steer the cyclones that grow on it.
- Stationary stage: Along the polar front, warm air blows from the south and cold air from the north, and the front is initially stationary.
- A wave forms: When pressure drops along the front, the warm air moves north and the cold air moves south. This sets up an anticlockwise cyclonic circulation, and the front splits into a warm front and a cold front.
- Mature stage: A warm sector, a pocket of warm air, lies wedged between the cold air ahead and behind. The warm air glides over the cold air ahead of the warm front, and a sequence of clouds brings rain. At the cold front, warm air is pushed up and cumulus clouds develop.
- Occlusion: The cold front moves faster, overtakes the warm front and lifts the warm air completely off the ground, so that the front is occluded.
- Dissipation: With no warm air to feed it, the cyclone weakens and dissipates. It becomes a cold-core low, and the frontal boundary remains until the next disturbance aloft forms a new low.
Extra-Tropical and Tropical Cyclones Compared
Extra-tropical cyclones occur between about 30 and 60 degrees latitude and differ from tropical cyclones in several ways. An extra-tropical cyclone grows on a front between unlike air masses, while a tropical cyclone forms over warm seas without any front and weakens once it reaches land. Both kinds of cyclone turn anticlockwise in the northern hemisphere and clockwise in the southern, because of the Coriolis effect.
| Feature | Extra-tropical cyclone | Tropical cyclone |
|---|---|---|
| Fronts | Has a clear frontal system | Has no fronts |
| Where it forms | Over land and sea | Only over the seas; dissipates on reaching land |
| Area and wind speed | Covers a much larger area; weaker winds | Smaller area; much stronger wind and more destructive |
| Direction of movement | West to east | East to west |
Air Masses and Macro-Climatic Change
Air masses redistribute heat and moisture across the globe, and their fronts set the rhythm of weather far from where they form. Their role in macro-climatic change works at several scales.
- Carriers of heat and moisture: Places under warm air masses have higher temperatures and places under cold air masses have lower ones. Arctic air can turn moist over a warm sea, so the weather it brings depends on the surface it has crossed.
- Rain and storms: Fronts lift air to give cloud and precipitation, and the extra-tropical cyclones that grow on them drive the weather of the middle latitudes.
- Seasons in India: Dry winter air, the western disturbances of winter and the hot-season storms where dry and moist air meet shape India's seasons.
- The jet stream: Cyclones form near a jet streak, a fast section of the upper-level jet stream, and the westerlies steer them, so the position of the jet stream affects where fronts and storms go.
Air masses also shape the climate of whole regions. In the humid subtropical climate of the eastern parts of continents, the air masses are generally unstable and cause rainfall throughout the year.
Previous Year UPSC‑CSE Questions
Each Prelims question is explained option by option, and each Mains question comes with an answer outline.
- UPSC Prelims 2015Consider the following statements:
- The winds which blow between 30° N and 60° S latitudes throughout the year are known as westerlies.
- The moist air masses that cause winter rains in North-Western region of India are part of westerlies.
Which of the statements given above is/are correct?
How to approach this Prelims question
Approach: Test each statement against the westerly belt and the source of India's winter rain.
- Step 1: Statement 1 is wrong. The westerlies blow between 30° and 60° latitude in each hemisphere, not across the equator from 30° N to 60° S.
- Step 2: Statement 2 is correct. The winter rain of north-west India comes from western disturbances, which are driven by the westerlies.
Trap to watch: The range 30° N to 60° S looks like the westerly belt but runs across both hemispheres.
Why each option is right or wrong:
- (a) 1 only: Wrong. Statement 1 is false, because the westerlies lie between 30° and 60° in each hemisphere.
- (b) 2 only: Correct. Western disturbances bring the winter rain and are carried by the westerlies.
- (c) Both 1 and 2: Wrong. Statement 1 is false.
- (d) Neither 1 nor 2: Wrong. Statement 2 is correct.
Answer: (b) 2 only. Western disturbances are carried by the westerlies; the belt is 30° to 60° in each hemisphere.
- UPSC Prelims 2001Assertion and Reason:
- Assertion (A): Anti-cyclonic conditions are formed in winter season when atmospheric pressure is high and air temperatures are low.
- Reason (R): Winter rainfall in Northern India causes development of anticyclonic conditions with low temperatures.
How to approach this Prelims question
Approach: Judge the assertion and the reason separately, then ask whether the reason explains the assertion.
- Step 1: The assertion is true. In winter the northern plain has high pressure and low temperatures, and the outflowing winds are anticyclonic.
- Step 2: The reason is false. Rain does not cause an anticyclone; the anticyclonic circulation lowers the chance of rain, and the winter rain of the north-west comes from western disturbances.
Trap to watch: The reason reverses cause and effect.
Why each option is right or wrong:
- (a) Both A and R are individually true, and R is the correct explanation of A: Wrong. The reason is false.
- (b) Both A and R are individually true, but R is NOT a correct explanation of A: Wrong. The reason is false, not just a poor explanation.
- (c) A is true, but R is false: Correct. Winter brings high pressure and cold; rain does not create the anticyclone.
- (d) A is false, but R is true: Wrong. The assertion is true.
Answer: (c) A is true, but R is false. Anticyclonic conditions keep winter dry; western disturbances bring the rain.
- UPSC Mains 2016 GS‑IDiscuss the concept of air mass and explain its role in macro-climatic changes.
Answer outline
The outline sets out what the question asks, the main arguments, relevant examples and the direction of the conclusion; it is not a full model answer.
Introduction: An air mass is a large body of air with little horizontal variation in temperature and moisture, taking its character from its source region.
Body (sub-themes to develop):
- Types: Five air masses, mT, cT, mP, cP and cA, from five kinds of source region, named by the Bergeron letters.
- Modification: Air masses change over new surfaces, as arctic air turns maritime polar over a warm sea.
- Fronts: Where air masses meet, warm air is lifted to give cloud and rain; cold, warm, stationary and occluded fronts bring different weather.
- Cyclones: Temperate cyclones grow on the polar front and drive the weather of the middle latitudes.
- India: The dry winter air, western disturbances that bring winter rain to the north-west, and the hot-weather storms where dry and moist air meet.
Conclusion: Air masses carry heat and moisture far from their source regions and, through their fronts, set the rhythm of the seasons.
Sources
- NCERT: Fundamentals of Physical Geography (Class XI), Atmospheric Circulation and Weather Systems
- NCERT: India Physical Environment (Class XI), Climate
- NOAA National Weather Service JetStream: Air masses
- NOAA National Weather Service JetStream: Norwegian cyclone model
- India Meteorological Department (Srinagar): Western disturbances
- PIB (Ministry of Earth Sciences): Impact of western disturbances, February 2021
- Wikipedia: Air mass
- Wikipedia: Weather front
- Wikipedia: Frontogenesis
- Wikipedia: Frontolysis
- Wikipedia: Cold front
- Wikipedia: Warm front
- Wikipedia: Stationary front
- Wikipedia: Occluded front
- Wikipedia: Extratropical cyclone
- Wikipedia: Norwegian cyclone model
- Wikipedia: Bergen School of Meteorology
- Wikipedia: Polar front
- Wikipedia: Westerlies
- Wikipedia: Western disturbance
- UPSC: Civil Services Examination question papers
Editorial Disclaimer
This article draws on the NCERT geography textbooks, NOAA, the India Meteorological Department, the Press Information Bureau and the other works listed under Sources. Source regions on the map are representative, not exact boundaries.
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