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Moisture, air masses, fronts and cyclones

Climatology: the atmosphere, winds, weather systems and world climates · Part 3 / 4 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Humidity, dew point and latent heat; dew, frost, fog and the cloud types; forms of precipitation and convectional, orographic and frontal rain with the rain-shadow; air masses and fronts; tropical cyclones, their structure, regional names and storm surge, and how IMD grades and names north Indian Ocean storms; temperate cyclones, thunderstorms and tornadoes.

Part 2 explained how the wind blows; this part follows the water it carries. Evaporation lifts invisible water vapour from seas, lakes, soil and plants, and the wind spreads it far and wide. When moist air cools, the vapour turns back into droplets or ice, forming dew, fog and clouds, and when those droplets grow heavy enough, precipitation falls. Large bodies of air with their own temperature and moisture, called air masses, collide along fronts, and the most violent weather of all, cyclones, thunderstorms and tornadoes, draws its energy from this moisture.

Humidity is the amount of water vapour in the air. Absolute humidity is the actual mass of vapour in a given volume of air, usually stated in grams per cubic metre. Warm air can hold far more vapour than cold air, so meteorologists also use relative humidity: the vapour actually present as a percentage of what the air could hold at that temperature. This is why relative humidity changes through the day even when the amount of vapour stays the same: it falls in the warm afternoon and rises in the cool night. Air holding as much vapour as it can is saturated, with a relative humidity of 100 per cent. The temperature to which air must cool, with no change in its vapour content, to become saturated is the dew point; cool the air below it and condensation begins.

Changing water between liquid and vapour involves a hidden store of energy. When water evaporates it absorbs heat without getting warmer, and this latent heat travels with the vapour; when the vapour condenses, the same heat is released into the surrounding air. Evaporation therefore cools a surface, which is why sweat cools the body and water kept in an unglazed clay pot stays cool in summer. Condensation warms the air in which it happens, and in tall clouds this released heat makes the air rise faster still, which is the engine of thunderstorms and tropical cyclones. Condensation also needs something to form on: tiny particles of dust, sea salt and smoke act as condensation nuclei.

Condensation takes several forms depending on where and how the air cools. Dew forms on grass and other surfaces that cool below the dew point on clear, calm nights; if the dew point is below freezing, the vapour turns straight into ice crystals as frost. When a large mass of air near the ground cools below its dew point, droplets hang in the air as fog or, when visibility is less reduced, as mist; in India, IMD reports fog when visibility falls below one kilometre. Clouds are the same process high above the ground. They are grouped by shape and height into four basic types: thin, feathery cirrus high up, made of ice crystals; heaped cumulus with flat bases; layered, grey stratus; and dark, rain-bearing nimbus. Combinations such as cumulonimbus, the towering thunder cloud, and nimbostratus, which brings steady rain, are named from these.

Cloud droplets are tiny; precipitation falls only when they grow large enough to overcome rising air. Rain is liquid water; snow forms when the vapour freezes directly into ice crystals in air below freezing; sleet is a mixture of rain and snow, or raindrops that freeze into pellets while falling through a cold layer; and hail is made of balls of ice that grow in layers as strong updrafts in a cumulonimbus cloud carry them up and down until they are too heavy to be held. Rainfall is grouped by what lifts the air. Convectional rain comes when the ground is strongly heated, warm moist air rises, cools and builds towering clouds; it is typical of afternoons in the equatorial belt.

orographic rainfall occurs when moist wind meets a mountain barrier and is forced to climb. As it rises it cools, its vapour condenses and heavy rain falls on the windward slope. Having lost much of its moisture, the air crosses the crest and sinks down the leeward side, warming as it descends, so that side stays relatively dry; it is called the rain-shadow area. India offers a clear example: the south-west monsoon drenches the windward western slopes of the Western Ghats, while places just east of the crest, in the interior of Maharashtra and Karnataka, receive much less rain. The third type, frontal or cyclonic rain, falls where warm air is lifted over colder air along a front, and is typical of the middle latitudes. Worldwide, rainfall is heaviest near the equator and on windward coasts, and lowest on the western sides of continents in the subtropics, deep in continental interiors and near the poles.

An air mass is a huge body of air, often more than a thousand kilometres across, whose temperature and humidity are nearly the same at any given height. It takes on these qualities by resting for days or weeks over a large, uniform surface, such as a warm ocean, a hot desert or a snow-covered plain, called its source region. Air masses are classified by two questions: where did the air come from in latitude, and did it form over sea or land? Tropical (T) air is warm and polar (P) air is cold; maritime (m) air formed over the sea is moist, while continental (c) air formed over land is dry. So continental tropical air from the Sahara is hot and dry, maritime tropical air from warm oceans is warm and humid, continental polar air from Siberia or northern Canada is cold and dry, and maritime polar air is cool and moist.

Air masses of different temperature do not mix easily; they meet along a sloping boundary called a front, where the warmer, lighter air is pushed upwards. At a cold front, advancing cold air wedges under warm air and lifts it steeply, building cumulonimbus clouds that bring short, heavy showers, gusty winds and a sharp drop in temperature after the front passes. At a warm front, the advancing warm air rides slowly up and over the cold air that is giving way; the slope is gentle, so the clouds thicken gradually from high cirrus to low nimbostratus, and rain is lighter but lasts longer over a wider area. When neither air mass advances, the front is stationary. When a faster cold front catches up with a warm front, the warm air is lifted off the ground entirely and the front is occluded. Fronts are a feature of the middle and high latitudes; they are rare in the tropics, where neighbouring air masses differ little in temperature.

A tropical cyclone is an intense, rotating low-pressure storm that forms over warm tropical seas. It needs a large expanse of ocean with a surface temperature of roughly 26–27 °C or more, enough Coriolis force to start the spin (so not within about 5° of the equator), little change in wind speed and direction with height, and a pre-existing disturbance. Its energy comes from the latent heat released as moist air rises and condenses. At the centre lies the eye, a calm, often clear zone of sinking air, ringed by the eyewall, where winds are strongest and rain heaviest; spiral rainbands extend outwards. The same storm is called a hurricane in the Atlantic and eastern Pacific, a typhoon in the western North Pacific and a cyclone in the Indian Ocean; in Australia the old name willy-willy was used. Over land a cyclone weakens quickly, cut off from its moisture and slowed by friction. Its deadliest effect is usually the storm surge, a rise of sea level driven ashore by the winds and the low pressure, which floods low coasts.

In the north Indian Ocean, the Bay of Bengal produces many more cyclones than the Arabian Sea, and most form in two seasons: the pre-monsoon months of April to June and the post-monsoon months of October to December. The India Meteorological Department's Regional Specialised Meteorological Centre in New Delhi, designated by the World Meteorological Organization, tracks and forecasts these storms and issues advisories for the region. IMD grades low-pressure systems by sustained wind speed, from a depression up to a super cyclonic storm (see the table). A storm is named when it reaches cyclonic-storm strength. Names come from a list of 169 adopted in 2020, 13 contributed by each of the 13 member countries of the WMO/ESCAP Panel on Tropical Cyclones, including India, Bangladesh, Sri Lanka, Pakistan and Oman; names are used in order and not repeated.

Outside the tropics, temperate or extra-tropical cyclones form along the polar front, where cold polar air and warm tropical air meet and a wave develops on the boundary. They are much larger than tropical cyclones, can form over land as well as sea, have weaker winds, no eye, and carry a warm front and a cold front; steered by the westerlies, they travel from west to east. The western disturbances that bring winter rain and snow to north-west India are low-pressure systems of this mid-latitude kind. Anticyclones, by contrast, bring sinking air, clear skies and calm weather, and in winter they favour frost and fog. On a much smaller scale, a thunderstorm grows from a towering cumulonimbus cloud with strong updrafts, lightning, thunder, heavy rain and sometimes hail; the violent pre-monsoon storms of West Bengal and Assam are called nor'westers or kalbaishakhi. A tornado is a narrow, violently rotating column of air reaching from a thunder cloud to the ground; it covers a small area but has the fiercest winds on Earth and is most frequent in the central United States.

This part forms one chain. Evaporation loads the air with vapour and stores latent heat; cooling to the dew point, helped by condensation nuclei, turns the vapour into dew, frost, fog and clouds. Precipitation falls when droplets grow, and the air can be lifted by heating, by mountains or along fronts, giving convectional, orographic and frontal rain. Air masses carry the character of their source regions, and where unlike masses meet, fronts and temperate cyclones develop. Over warm tropical seas, released latent heat powers tropical cyclones, the most destructive weather of all. Part 4 shows how temperature and rainfall combine over the long term into the world's climatic regions.

Exam traps

• Relative humidity falls when air warms even if its vapour content is unchanged; absolute humidity does not change with temperature in that way.
• Evaporation absorbs latent heat (cooling); condensation releases it (warming) and powers thunderstorms and cyclones.
• Frost forms when the dew point is below freezing; dew when it is above freezing.
• Cirrus clouds are high and made of ice crystals; nimbus clouds are the rain-bearing ones; cumulonimbus is the thunder cloud.
• The rain-shadow is the leeward side, not the windward side; the interior east of the Western Ghats lies in rain-shadow during the south-west monsoon.
• Maritime (m) air masses are moist and continental (c) ones dry; tropical (T) is warm and polar (P) cold.
• A cold front gives short, heavy showers; a warm front gives long, steady, lighter rain. Fronts are rare in the tropics.
• Tropical cyclones have an eye and form only over warm seas, not near the equator; temperate cyclones have fronts, no eye, and can form over land.

Recap

• Relative humidity = vapour present as a percentage of what the air can hold; dew point = temperature at which air becomes saturated.
• Latent heat is absorbed in evaporation and released in condensation; condensation needs nuclei of dust, salt or smoke.
• Forms of condensation: dew, frost, mist, fog (IMD: visibility under 1 km) and clouds (cirrus, cumulus, stratus, nimbus).
• Precipitation: rain, snow, sleet, hail; rainfall types: convectional, orographic (with rain-shadow) and frontal.
• Air masses: maritime or continental, tropical or polar; they take their character from source regions.
• Fronts: cold (steep, short heavy rain), warm (gentle, long steady rain), stationary, occluded; typical of mid-latitudes.
• Tropical cyclones need warm seas (about 26–27 °C), Coriolis force and low wind shear; eye, eyewall, storm surge; hurricane, typhoon, cyclone.
• North Indian Ocean: more cyclones in the Bay of Bengal; RSMC New Delhi (IMD) names storms from a list of 169 names (2020) from 13 countries.
• Temperate cyclones form on the polar front, have fronts and no eye, move west to east; western disturbances are of this kind.
• Thunderstorms grow from cumulonimbus (nor'westers in Bengal and Assam); tornadoes are small but have the fiercest winds.

Practice questions

Sources

• NOAA National Weather Service, JetStream online school: moisture and humidity, clouds, precipitation, air masses, fronts, thunderstorms, tornadoes and tropical cyclones (US government) — https://www.noaa.gov/jetstream/clouds
• India Meteorological Department, Ministry of Earth Sciences: Regional Specialised Meteorological Centre for Tropical Cyclones, New Delhi; classification of cyclonic disturbances over the north Indian Ocean and the list of cyclone names (2020); public material on fog, thunderstorms and western disturbances — https://rsmcnewdelhi.imd.gov.in
• NASA Earth Observatory: the water cycle, latent heat and global precipitation (US government) — https://earthobservatory.nasa.gov/features/Water