Maarg Manthan · Topic 5.12

Air Masses, Fronts and Extra-Tropical Cyclones

Geography › Climatology · Topic 5.12

An air mass is a large body of air with nearly uniform temperature and moisture, which takes its character from the region where it forms, called its source region. Air masses are named by the source: continental or maritime, and tropical, polar or arctic. A front is the sloping boundary between two air masses that differ in temperature and humidity. Along fronts, such as cold, warm, stationary and occluded fronts, the middle-latitude (extra-tropical or temperate) cyclones form, and bring most of the rain and storms of those latitudes.

7 min read · UPSC Prelims · Geography · Climatology

At a Glance

Air mass Large body of uniform air
Source regions Cold or warm, land or sea
Front Boundary between two air masses
Fronts Cold, warm, stationary, occluded
Cyclone Forms along the polar front
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows the winds and moves to the weather systems of the middle latitudes. It introduces air masses and fronts, which the topic on tropical cyclones does not need, and which explain temperate cyclones in detail later. Previous topic: westerlies, polar easterlies Seasonal Monsoons, land and sea breezes Local hot winds Loo, sirocco, khamsin, foehn, chinook…">Seasonal and Local Winds: Land and Sea Breezes, Mountain and Valley Winds. Next topic: Cumulonimbus Tornado Rotating funnel reaching the ground Scale Fujita scale for tornadoes Tornado Alley Great Plains of…">Thunderstorms and Tornadoes.

What Is an Air Mass and How Is It Classified?

In short: an air mass is a huge block of air that takes on the temperature and moisture of the surface where it rests.

When air stays for days over a large, uniform surface, such as a cold snow-covered plain, a hot desert or a warm ocean, it takes the temperature and humidity of that surface. This area is its source region. Good source regions are those where the winds are light and the pressure is high, such as subtropical and subpolar highs, as in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell. The mid-latitudes are not good source regions because the air there is continually mixed by storms. Air masses are classified in two ways: by latitude, which gives temperature, and by surface, which gives humidity.

Air mass	Symbol	Source region	Character
Continental polar	cP	Northern Canada, Siberia	Cold and dry; very cold clear winter weather
Maritime polar	mP	Cold oceans in the North Atlantic and North Pacific	Cool, moist and unstable; showers on west coasts
Continental tropical	cT	Deserts such as the Sahara, Arabia and the Thar	Hot and dry; the origin of the loo in India
Maritime tropical	mT	Warm tropical oceans	Warm and moist; brings the monsoon rain to India when it is drawn onto the land
Continental arctic or Antarctic	cA	Ice sheets and polar regions	Extremely cold and dry
Equatorial	mE	Near the equator	Hot, very humid and unstable

An air mass is changed when it moves, and its weather depends on the difference between its temperature and that of the ground it crosses. A cold air mass moving over a warm surface is heated from below and becomes unstable, with showers; a warm air mass moving over a cold surface is cooled from below and becomes stable, with fog and low cloud, as in Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations.

What Is a Front and How Does It Form?

When two air masses of different temperature and density meet, they do not mix quickly. The warm, light air is pushed above the cold, dense air along a sloping boundary, called a frontal surface, and the line where it meets the ground is the front. The formation or strengthening of a front is called frontogenesis, and its weakening or disappearance is frontolysis. The idea was developed by the Norwegian meteorologists Vilhelm and Jacob Bjerknes and their colleagues around the end of the First World War, who likened the clash of air masses to opposing armies on a battle front. The main zone is the polar front, where the polar easterlies meet the westerlies, between about 40 and 60 degrees in each hemisphere. Fronts are common in the middle latitudes and rare in the tropics, where air masses are similar in temperature.

What Are the Kinds of Fronts and Their Weather?

Front	Description	Slope	Weather
Cold front	Cold air advances and pushes under warm air	Steep, about 1 in 50 to 1 in 100	Warm air is forced up rapidly; cumulonimbus clouds, thunderstorms and short, heavy showers; temperature falls and pressure rises after passing; wind veers
Warm front	Warm air advances and rises over retreating cold air	Gentle, about 1 in 100 to 1 in 200	Cirrus, then altostratus and nimbostratus; widespread, long and steady rain or drizzle; temperature rises after passing
Stationary front	Neither air mass advances	Nearly flat	Cloud and drizzle for days; wind blows parallel to the front
Occluded front	A faster cold front overtakes a warm front and lifts the warm air off the ground	Complex	Mixed weather with the heaviest rain at the beginning, then dissipation; marks the old age of a temperate cyclone

A cold front moves faster than a warm front, so it catches up with it and forms an occlusion. A warm front brings gradual warning, as clouds thicken over a day, while a cold front arrives suddenly, as in Thunderstorms and Tornadoes. These fronts are the structure of a temperate cyclone, as in Temperate (Extra-Tropical) Cyclones: Air Masses, Fronts, Origin and Development.

How Do Air Masses and Fronts Matter in India?

India is in the tropics and has no true fronts for most of the year. The continental tropical air mass from the northwest creates the loo in summer, and the maritime tropical air mass brings the southwest monsoon. In winter, western disturbances bring cold fronts and cold air into the northwestern plains, and the dry continental polar air from Central Asia is blocked by the Himalaya, which is one reason why India is warmer than other places at its latitude. As an analytical point, the monsoon can also be understood as a seasonal replacement of one air mass by another, and the burst of the monsoon is regarded by some as the arrival of a front-like boundary between the dry continental and the moist maritime air.

What Is a Temperate Cyclone?

In short: a temperate cyclone is a travelling low pressure wave on the polar front, with a cold front and a warm front.

Temperate cyclones are the storms of the middle latitudes. They are known as extratropical cyclones because they form outside the tropics, and as wave cyclones because they begin as a wave on the front. The theory of their origin, the polar front theory, was put forward by the Norwegian school of Vilhelm Bjerknes and others around 1920. Unlike tropical cyclones, which draw energy from warm seas, temperate cyclones get energy from the contrast of temperature between the cold and the warm air, and form over both land and sea. They are the dominant cause of rain and snow in Europe, North America and East Asia, and are most vigorous in winter, when the temperature contrast is highest.

Why Is This Topic Important?

Concepts Air mass and front types asked
Link Foundation of temperate cyclones
India Loo, monsoon and western disturbances

What Came Next?

The next topic covers thunderstorms and tornadoes, which are small, intense storms. Next topic: Thunderstorms and Tornadoes.

Key People and Terms

Air mass Large body of uniform air
Source region Place where an air mass gets its properties
Front Boundary between two air masses
Frontogenesis Formation of a front
Occlusion Cold front overtaking a warm front
Polar front Meeting of polar and subtropical air

Exam Corner

Points to Remember

Air masses are labelled by latitude and surface: cP, mP, cT, mT.
A cold front has a steep slope and sharp showers.
A warm front brings long, steady rain.
An occluded front marks an ageing cyclone.
Bjerknes developed the front concept.

Do Not Confuse With

Cold front and warm front Advancing cold air versus advancing warm air
Stationary and occluded front No movement versus overtaking
Frontogenesis and frontolysis Formation versus dissolving

Memory Hook

Cold front: steep, fast, showers; warm front: gentle, slow, drizzle; the cold catches the warm and occludes.

Mains Angle

Questions compare fronts. These points add depth.

Weather Showers versus steady rain
Slope Steep versus gentle
India Few fronts in the tropics

A Question You May Face

An original practice question, not a past paper question.

“What are air masses and fronts? Explain the weather associated with cold and warm fronts.”

How to Answer

Air masses Source regions and types.
Fronts Formation.
Weather Cold versus warm.
India Tropical air masses.

GS Relevance

Prelims: types of air masses, fronts and their weather, extra-tropical cyclone. GS Paper 1: atmospheric disturbances.

Frequently Asked Questions

What is an air mass?

An air mass is a large body of air, hundreds to thousands of kilometres across, with nearly uniform temperature and humidity at any height. It gets these properties by staying for a time over a uniform surface called the source region, such as a desert, an ocean or an ice sheet.

How are air masses classified?

Air masses are classified by the latitude of their source, as arctic, polar, tropical or equatorial, and by the nature of the surface, as continental, which is dry, or maritime, which is moist. This gives types such as continental polar, maritime polar, continental tropical and maritime tropical.

What is a front?

A front is the boundary between two air masses of different temperature and density. The warm, lighter air is forced above the cold, denser air along a sloping surface, which causes clouds and rain. Its formation is called frontogenesis, and fronts are common in the middle latitudes.

What is the difference between a cold front and a warm front?

A cold front forms when cold air advances and wedges under warm air, with a steep slope, giving cumulonimbus clouds and short heavy showers. A warm front forms when warm air rises over cold air on a gentle slope, giving long steady rain from nimbostratus clouds.

What is an occluded front?

An occluded front forms when a faster-moving cold front overtakes a warm front and lifts the warm air off the ground. It marks the mature or dying stage of a temperate cyclone, with mixed weather and rain that eventually weakens as the temperature difference disappears.

PYQ Practice — Statement Analysis

1 A cold front has a steeper slope than a warm front.

True

Cold air wedges under warm air sharply.

2 A maritime tropical air mass is cold and dry.

False

It is warm and moist.

3 An occluded front forms when a cold front overtakes a warm front.