Air Masses, Fronts, Cyclones and Thunderstorms

1. Air masses and their source regions

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• An air mass is a large body of air with little change in temperature and moisture from side to side. It forms when air stays a long time over one uniform surface, such as a vast ocean or plain, and takes on that surface's character (Class 11, Atmospheric Circulation and Weather Systems).
• A source region is the uniform surface where an air mass forms. As an air mass moves, it is heated or cooled from below and gains or loses moisture, so its stability changes.

Type	Source region	Character and weather
Maritime tropical (mT)	Warm tropical and subtropical oceans	Warm, moist and unstable. Brings humid heat and heavy rain (Gulf of Mexico air into the eastern USA).
Continental tropical (cT)	Subtropical hot deserts (Sahara, Arabia, Thar)	Hot and dry. Brings heat waves.
Maritime polar (mP)	Cold high-latitude oceans (North Pacific, North Atlantic)	Cool and moist. Brings cloudy, showery weather to west coasts.
Continental polar (cP)	Snow-covered high-latitude continents (Siberia, northern Canada)	Cold, dry and stable. Brings cold waves far to the south.
Continental arctic (cA)	Permanent ice of the Arctic and Antarctica	Extremely cold and dry. Brings severe cold outbreaks.

• NCERT exercise: the Siberian Plain is a source region. Equatorial forests, the Himalaya and the Deccan are not.
• Fuller schemes, beyond NCERT, add maritime equatorial (mE) air.
• Indian links: modified cP air brings north India its winter cold waves. cT air brings the summer loo. mT air brings the monsoon rains (indian-climate-monsoon).

2. Fronts

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• A front is the boundary zone between two air masses that differ in temperature and density. Fronts occur mainly in the middle latitudes. They show steep changes in temperature and pressure, bring sudden weather changes, and lift air so that it forms cloud and rain.
• Frontogenesis is the formation or strengthening of a front. Its opposite, the weakening of a front, is frontolysis.
• The polar front is a semi-permanent front near the subpolar lows (about 60°), where cold polar air meets warm subtropical air. Most extra-tropical cyclones grow as waves along it.

Front	What happens	Weather
Cold front (blue triangles on maps)	Advancing cold air pushes under warm air and lifts it steeply	Cumulus and cumulonimbus clouds, short heavy showers and thunderstorms, then a sharp drop in temperature and clearing skies
Warm front (red semicircles)	Advancing warm air glides gently up over retreating cold air	Cirrus, then cirrostratus, altostratus and nimbostratus over hundreds of km; long, steady rain ahead of the front
Stationary front (alternating symbols)	Neither air mass advances	The boundary barely moves; a wave cyclone often starts here
Occluded front (purple)	A faster cold front overtakes the warm front and lifts the warm air fully off the ground	Marks the dying stage of the cyclone

3. Extra-tropical (temperate) cyclones

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• An extratropical cyclone is a low-pressure system of the middle and high latitudes that forms along the polar front (Class 11, Atmospheric Circulation and Weather Systems, Fig. 9.9; this is the Norwegian model of about 1920).
• How it forms (Northern Hemisphere):
• It starts on a stationary front, with warm air to the south and cold air to the north.
• Pressure falls along the front. Warm air moves north and cold air moves south, which starts an anticlockwise circulation.
• A warm front develops ahead and a cold front behind, with a warm sector of warm air between them.
• Warm air glides up ahead of the warm front, giving a sequence of clouds and rain. Behind, the cold front lifts the warm air and builds cumulus clouds.

• The faster cold front catches up with the warm front. The front occludes and the cyclone dies out.

• Character: often 1,000–2,000 km across. They form over both land and sea and move west to east with the westerlies. They bring several days of changeable weather. These are the "depressions" of north-west Europe. Western disturbances, which bring winter rain and snow to north-west India, are lows of this kind (indian-climate-monsoon).

• Bomb cyclone (beyond NCERT): a mid-latitude cyclone whose central pressure falls by at least 24 hPa in 24 hours (Sanders and Gyakum, 1980). It brings violent winds, blizzards or heavy rain, typically off the US east coast in winter.

4. Tropical cyclones: what they are and how they form

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• A tropical cyclone is an intense low between 30°N and 30°S. It is 500–1,000 km across and 12–14 km high. It works like a heat engine fuelled by latent heat (heat given out when water vapour from the sea condenses into cloud) (Class 11, Natural Hazards and Disasters).
• Cyclone is also the general word for any intense low with winds spiralling inward, anticlockwise in the Northern Hemisphere. Regional names: cyclones (Indian Ocean), hurricanes (Atlantic), typhoons (western Pacific and South China Sea), willy-willies (north-western Australia).
• Conditions for tropical cyclone formation:
• A large warm sea surface. NCERT gives above 27°C; the modern threshold is about 26.5°C through roughly the top 50 m.
• Enough Coriolis force (the turning effect of the earth's rotation) to stop the low from filling up. So none form within 0–5° of the equator.
• Weak vertical wind shear.
• A pre-existing weak low or low-level cyclonic circulation.

• Upper-level divergence, which carries the rising air away. Class 11, Natural Hazards and Disasters also adds an unstable troposphere.

• Vertical wind shear is the change of wind speed or direction with height. When shear is weak, the storm's warm core can build straight up. When shear is strong, the storm is torn apart. This is why few cyclones form during the south-west monsoon.

• Rapid intensification (NOAA): a rise in maximum sustained winds of at least 30 knots (about 55 km/h) in 24 hours. It cuts warning time. Examples: Ockhi (2017) and Tauktae (2021).

5. Structure, movement and landfall; tropical vs extra-tropical cyclones

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• Eye of the cyclone: the calm, often cloud-free centre, with sinking air and the lowest pressure. It is usually 20–50 km across. NCERT's "150–250 km" refers to the whole circulating system, not the eye.
• Eye wall: a ring of towering cumulonimbus clouds around the eye. Here moist air spirals up to the tropopause, giving the strongest winds (up to about 250 km/h) and torrential rain.
• Rain bands: spiral bands of cloud reaching outward from the eye wall. They bring bursts of rain and gusts well before the centre arrives.
• Over the Bay of Bengal and Arabian Sea, storms are 600–1,200 km across and move about 300–500 km a day.
• Landfall is the place and time where the cyclone's centre crosses the coast. After landfall the moisture supply is cut, so the storm weakens. The coast still takes the worst winds, rain and storm surge, an abnormal rise in sea level pushed by onshore winds and low pressure.
• Recurvature of cyclones: storms that move poleward turn north-east into the westerlies. NCERT notes that cyclones crossing about 20°N generally recurve and are more destructive.

Feature	Extra-tropical	Tropical
Fronts	Clear fronts	None
Area	Larger	Smaller
Origin	Land or sea	Warm seas only; dies over land
Winds	Weaker	Much stronger, more destructive
Movement	West to east	East to west, then recurves
Energy source	Temperature contrast	Latent heat
Eye	Absent	Present

6. Classifying and naming cyclones

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• A depression is a low with closed isobars. Normal sea-level pressure is about 1,013 mb.
• IMD classification of cyclonic disturbances (north Indian Ocean, by maximum sustained surface wind):

Category	km/h	knots
Low-pressure area	below 31	below 17
Depression	31–49	17–27
Deep depression	50–61	28–33
Cyclonic storm	62–88	34–47
Severe cyclonic storm	89–117	48–63
Very severe	118–166	64–89
Extremely severe	167–221	90–119
Super cyclonic storm	222 or more	120 or more

• Saffir-Simpson scale (Atlantic and eastern Pacific hurricanes): categories 1–5. Category 3 and above are "major hurricanes". Category 5 means winds of 252 km/h or more. Do not mix scales: IMD averages wind over 3 minutes, while NHC uses 1 minute.
• Naming of tropical cyclones: RSMC New Delhi (run by IMD) names north Indian Ocean storms once they reach cyclonic-storm strength.
• Naming began in 2004 with 64 names from 8 countries.
• In 2020 the WMO/ESCAP Panel adopted a list of 169 names, 13 from each of its 13 members: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen.
• Names are used in turn and never repeated.
• Naming makes warnings clearer, especially when two storms exist at the same time.

7. Cyclones in India: where and when

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• India's peninsular shape exposes it to both the Bay of Bengal and the Arabian Sea (Class 11, Natural Hazards and Disasters).
• Cyclones in the Bay of Bengal and Arabian Sea: the Bay produces roughly four times as many cyclones as the Arabian Sea. The reasons:
• Its water is warmer.
• River inflow makes its surface water fresher and layered, so the warm top layer stays warm.
• It receives the remains of western Pacific typhoons.

• Its shallow, funnel-shaped head makes surges worse.

• The Arabian Sea is cooler, because of upwelling (cold water rising from below) and dry air from Arabia. Its cyclones are now rising in number and strength. In 2019 the Arabian Sea had more cyclones than the Bay (verify current trend).

• Seasons are bimodal (two peaks): pre-monsoon (April–May) and post-monsoon (October–December). October–November is the peak for the Bay. Strong wind shear keeps cyclones rare in the monsoon months.
• NCERT's genesis latitudes for Bay storms are garbled ("16°–2°N"); read them as about 16–21°N, west of 92°E. By July, storms form near 18°N, west of 90°E, near the Sundarbans.
• Post-monsoon cyclones form over the Andaman Sea and southern Bay. They bring much of the Coromandel coast's rain but often devastate the Godavari, Krishna and Kaveri deltas.
• Most exposed coasts: Odisha, Andhra Pradesh, Tamil Nadu and West Bengal; Gujarat on the west. IMD tracks each storm. Class 7, Climates of India places NDRF battalions at 12 locations (verify current count).

8. Indian cyclones: case studies

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Cyclone	Sea, date	Landfall	Peak	Key lesson
Odisha super cyclone (1999)	Bay, 29 Oct 1999	Near Paradip	Super cyclonic storm, winds over 250 km/h	About 10,000 deaths. Led Odisha to set up its state disaster management authority (OSDMA) and cyclone shelters.
Cyclone Ockhi (2017)	Formed near Sri Lanka, late Nov 2017	Did not cross the mainland; passed through the Lakshadweep Sea	Rapid intensification	Little warning; many Kerala and Tamil Nadu fishermen killed or missing at sea.
Cyclone Fani (2019)	Bay, 3 May 2019	Near Puri, Odisha	Extremely severe	IMD warnings and evacuation of over a million people kept deaths low. Praised by the UN. (Class 7, Fig. 3.13)
Cyclone Amphan (2020)	Bay, 20 May 2020	West Bengal–Bangladesh coast near the Sundarbans	Super cyclonic storm	First in the Bay since 1999. Hit Kolkata during the COVID-19 lockdown. Mangroves buffered the surge.
Cyclone Biparjoy (2023)	Arabian Sea, June 2023 (about 13 days long)	Near Jakhau port, Kachchh, Gujarat, 15 June	Extremely severe (verify)	A "zero casualty" approach with mass evacuation kept deaths very low.

• The trend: deaths have fallen sharply since 1999 thanks to better forecasts and evacuation. But damage to property is still huge (verify figures in IMD reports).

9. Thunderstorms, lightning, dust storms, tornadoes and waterspouts

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• Thunderstorm: a short-lived, violent local storm from a well-grown cumulonimbus cloud. It is caused by intense convection (warm air rising strongly) on hot, humid days.
• Strong updrafts make the cloud grow taller.
• Where the cloud reaches heights below freezing, hail forms.

• Later, downdrafts bring cool air and rain to the ground.

• Dust storm: a thunderstorm with too little moisture can raise a dust storm. Examples are the pre-monsoon andhis of north-west India. The May 2018 storms in Rajasthan and Uttar Pradesh killed over a hundred people (verify).

• Lightning: a sudden electrical discharge inside a thundercloud or between a cloud and the ground. It kills more people in India than any other natural hazard: about 2,000–3,000 a year in NCRB data (verify current). Most victims are farmers in Bihar, Odisha, Jharkhand, Madhya Pradesh, Uttar Pradesh and West Bengal. IITM's Damini app warns of nearby strikes.
• Tornado: a violently rotating funnel that descends from a severe thunderstorm "like the trunk of an elephant". It has very low pressure at its centre and causes narrow but massive destruction.
• Tornadoes are most common in the middle latitudes, especially "Tornado Alley" in the US Great Plains.
• They are rated on the Enhanced Fujita scale, EF0–EF5.

• The deadliest recorded tornado struck Bangladesh (Daultipur–Salturia, 1989).

• Waterspout: a tornado over a sea or lake, usually weaker.

• NCERT's closing idea: these storms turn potential and heat energy into kinetic energy (energy of motion), and the atmosphere returns to a stable state.

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Exam angles

Prelims — high-yield facts and traps

• Air-mass sources: mT is tropical oceans; cT is hot deserts; mP is cold high-latitude oceans; cP is snow-covered continents (Siberia); cA is Arctic and Antarctic ice. The Siberian Plain is a source region; the Himalaya and Deccan are not.
• Cold front: steep slope, cumulonimbus, short heavy showers. Warm front: gentle slope, cirrus to nimbostratus, long steady rain. Occluded front: the final stage. Stationary front: the starting stage.
• "Extra-tropical cyclones have an eye" is FALSE. "Tropical cyclones form over land" is FALSE. "Tropical cyclones form at the equator" is FALSE (none within 0–5°).
• Formation conditions: sea surface above about 26.5–27°C, Coriolis force, weak vertical shear, a pre-existing low, upper divergence.
• The eye is calm with sinking air. The strongest winds and heaviest rain are in the eye wall, not the eye.
• IMD scale: depression 31–49 km/h, cyclonic storm from 62 km/h, super cyclonic storm from 222 km/h. Saffir-Simpson category 5 starts at 252 km/h.
• Naming: RSMC New Delhi, WMO/ESCAP Panel of 13 countries, 169 names (2020), never repeated.
• Regional names: hurricane (Atlantic), typhoon (western Pacific), willy-willy (north-western Australia).
• Bay storms outnumber Arabian Sea storms by roughly 4:1. The Bay's peak is October–November. Landfalls: Fani at Puri, Amphan near the Sundarbans, Biparjoy at Jakhau (Kachchh), 1999 storm near Paradip.
• Bomb cyclone: pressure falls by at least 24 hPa in 24 hours. Rapid intensification: winds rise by at least 30 knots in 24 hours.

Mains — GS-I themes

1. Why do tropical cyclones originate over seas, and why do the eye wall's winds and rain peak where they do? (NCERT exercise)
2. Compare tropical and temperate cyclones in origin, structure, movement, energy source and impact.
3. Why does the Bay of Bengal produce more and deadlier cyclones than the Arabian Sea, and why is the Arabian Sea changing?
4. From 1999 to Fani and Biparjoy: how has India's cyclone preparedness improved, and what gaps remain (rapid intensification, urban flooding, fishermen at sea)?
5. Lightning as an under-recognised disaster in India: causes, distribution and mitigation.
6. Is climate change making tropical cyclones stronger? Discuss with rapid intensification and storm-surge risk.

Current-affairs hooks

• Each season's named cyclones, their landfall points and IMD categories (verify current).
• Arabian Sea warming and rising cyclone activity; new names from the 2020 list coming into use.
• Lightning deaths, the Damini app, and states declaring lightning a state-specific disaster.
• Pre-monsoon dust storms and nor'westers in north and east India.
• Bomb cyclones in North America; record Atlantic hurricane seasons.
• Storm-surge forecasting by IMD and INCOIS; mangrove restoration in the Sundarbans after Amphan.