Indian Monsoon and Climate

Flagship: what this is and why CAPF cares

The monsoon is the seasonal reversal of wind that gives India about three-quarters of its annual rain in four months and decides whether the farm year succeeds or fails. CAPF tests it as a mix of one-liners (onset date, the wettest place), matching (local pre-monsoon name to region), and statement judgement (the El Nino and jet-stream links). The security value runs through the calendar: the monsoon and the high-altitude winter govern when the passes to Ladakh, Siachen and the Line of Actual Control are open or snowbound, dictating the resupply and rotation cycles of the Indo-Tibetan Border Police and the Army, while western disturbances bring the blizzards that have caused major non-combat casualties on the Siachen Glacier. The anchor text is NCERT Class XI, India: Physical Environment (the chapters on climate).

Factors controlling India's climate

Several factors together explain why India has a tropical-monsoon climate with strong regional variation.

• Latitude: the Tropic of Cancer cuts the country in half, so the south is tropical and the north is subtropical.
• The Himalayas: the wall blocks the cold central-Asian winds in winter, keeping India warmer than its latitude would suggest, and traps the monsoon to force its rain over the plains.
• Distance from the sea (continentality): the coasts are equable while the interior has a large temperature range.
• Altitude: the mountains are cold, the plains warm; temperature falls with height.
• Pressure and wind reversal: the seasonal pressure pattern drives the monsoon.
• Ocean currents and upper-air jets: the jet streams steer the systems, and ocean temperature (El Nino, the IOD) modulates strength.

The four seasons of the Indian year

The India Meteorological Department recognises four seasons, and CAPF sometimes tests their order and character.

The hottest pre-monsoon period sees the low-pressure trough deepen over the north-west; the highest temperatures are recorded in the Thar and the interior, while the coasts stay moderated by the sea.

Jet streams and upper-air controls (reference)

The withdrawal of the subtropical westerly jet north of Tibet in late spring is the upper-air trigger that lets the monsoon set in; its return south in autumn marks the retreat.

The two branches of the south-west monsoon

The south-west monsoon (June to September) enters India in two arms after crossing the equator and turning by the Coriolis effect.

• The Arabian Sea branch strikes the Western Ghats head-on, giving the windward (western) slopes very heavy orographic rain and leaving a sharp rain shadow on the Deccan leeward side (Pune and the interior get far less than the coast just to the west). Part of this branch sweeps up the west and over central India.
• The Bay of Bengal branch advances up the Ganga plain and is deflected north-west by the Himalayan wall. Striking the funnel-shaped Khasi Hills of Meghalaya, it dumps the world's heaviest rainfall on Mawsynram and nearby Cherrapunji (Sohra).

The monsoon normally arrives over Kerala around 1 June, covers most of the country by mid-July, and is the wet season for almost all of India except Tamil Nadu's interior.

The two branches compared:

Onset, advance and withdrawal

Onset sequence (ordering questions)

The monsoon advances from south to north and from the coasts inward, so an ordering question can ask which place gets the rain first. The usual sequence: Andaman Sea and the south Bay (late May), Kerala and the north-east almost together (about 1 June), then up both coasts, Mumbai and Kolkata by mid-June, the Ganga plain and Delhi by end June, and finally the dry north-west (west Rajasthan) by early July. Withdrawal runs in reverse, beginning over the north-west in early September and clearing the south by mid-October.

The retreating monsoon and winter rain

As the land cools in October and November the monsoon retreats from the north-west, and the wind reverses to the north-east monsoon (the winter monsoon). Blowing from land to sea, it is dry over most of India, but as it crosses the Bay of Bengal it picks up moisture and gives the Coromandel Coast (coastal Tamil Nadu and south Andhra) its main rains. This is why Tamil Nadu is wettest in winter, not summer, the most counter-intuitive Indian climate fact. The retreating season is also the cyclone season for the Bay of Bengal coasts. The transition month of October, with high temperature and humidity and oppressive weather before the rains fully clear, is sometimes called "October heat".

In the far north-west, winter rain and snow come not from the monsoon but from western disturbances, extratropical depressions that originate over the Mediterranean and Caspian region and are carried east by the subtropical westerly jet. They are vital for the rabi wheat crop of Punjab and Haryana and bring the heavy snow of the Himalayas.

Tropical cyclones

The Bay of Bengal and the Arabian Sea both spawn tropical cyclones, but the Bay is far more cyclone-prone, mainly in the pre-monsoon (April to May) and post-monsoon (October to November) seasons. They strike the Odisha, Andhra Pradesh, Tamil Nadu and West Bengal coasts hardest, and the Gujarat coast from the Arabian Sea. Cyclones are named from a regional list maintained by the regional meteorological centre. They bring storm surge, the chief killer, and they are the recurring natural-disaster scenario in which the National Disaster Response Force deploys with CAPF battalions in support. The link to wind systems and pressure is in climatology atmosphere and winds.

Local winds and pre-monsoon showers (matching)

Rainfall distribution and pre-monsoon showers

Rainfall is highly uneven. The wettest belts are Meghalaya (Mawsynram, Cherrapunji), the windward Western Ghats, and the north-east; the driest are western Rajasthan (the Thar) and Ladakh (a cold desert in the rain shadow of the Himalayas), with the Deccan interior in the Western Ghats rain shadow.

Rainfall extremes and gradients:

The variability is high and the rain is concentrated in a few months and a few heavy spells, which is why Indian agriculture is so exposed to a late or weak monsoon.

Temperature distribution

January is the coldest month, with temperature falling north and inland (the north-west plains are cool, the south stays warm because it is near the equator and the sea). May is the hottest month over most of the interior, with the highest readings in the Thar and the north-western plains, while the coasts are moderated by the sea. The annual range of temperature is large in the interior and the north-west and small near the coasts and the equator; this is why Chennai is warm all year while Delhi swings between cold winters and very hot summers. Pre-monsoon convective showers have famous local names: mango showers in Kerala and coastal Karnataka (help the mango crop and the coffee), Blossom showers in Karnataka coffee tracts, Kalbaisakhi or Norwesters in West Bengal and Assam (violent April-May thunderstorms, called Bardoli Chheerha in Assam for their help to tea and jute), and Loo, the hot, dry, dusty wind of the northern plains in May and June.

Western disturbances versus the monsoon (do not confuse)

Static facts to memorise

Climatic regions (Koppen, simplified)

India's climate is broadly tropical monsoon, but it ranges from cold mountain to hot desert. The Koppen system, which the exam references, classifies climates by letters: the first capital is the broad type (A tropical, B dry, C warm temperate, E polar/tundra, H highland), the second the rainfall pattern, the third the temperature. India shows the following:

The codes worth recognising are Amw (monsoon with a short dry season, the Malabar coast), Aw (savanna, most of the peninsula), BShw (semi-arid steppe), BWhw (hot desert, the Thar), Cwg (humid subtropical, the Ganga plain) and E/H (the cold and high Himalayan zones).

Why the monsoon matters

The monsoon is, in the phrase often quoted, the true finance minister of India: about three-quarters of the annual rain falls in the four monsoon months, the kharif crop depends on it directly, the rabi crop depends on the soil moisture and the reservoirs it leaves behind, and groundwater recharge, hydropower and drinking water all hinge on it. A timely, well-distributed monsoon lifts rural incomes and tempers food inflation; a late, weak or erratic one (often in an El Nino year) brings drought, crop loss and distress. This is the bridge to indian agriculture and cropping and soils and natural vegetation of india, since the rainfall band fixes both the crop and the natural vegetation.

Security and strategic angle

Climate is the operational calendar of the border forces. The high-altitude winter and the monsoon decide when passes such as Zoji La and the routes to Ladakh, Siachen and the LAC are open or snowbound, which fixes when the Indo-Tibetan Border Police and the Army can rotate troops and stockpile supplies; before all-weather tunnels, a forward post could be cut off for months. Western disturbances are not just farm rain in the north-west; they bring the avalanche-and-blizzard conditions that have caused the bulk of casualties on the Siachen Glacier, where the enemy is the weather more than the adversary. Cyclones spun up over the Bay of Bengal during the retreating monsoon are met by the National Disaster Response Force with CAPF battalions in support. The Thar belt along the western border sets the heat-and-dust posture of the Border Security Force in Rajasthan and Gujarat, where summer operations contend with the Loo and sandstorms. See climatology atmosphere and winds and india borders neighbours and strategic geography.

Wettest and driest, at a glance

How CAPF asks it

• One-liner: when the monsoon reaches Kerala (about 1 June), the wettest place (Mawsynram), which monsoon rains Tamil Nadu (the north-east).
• Matching feature to region: local pre-monsoon name to area (mango showers to Kerala, Kalbaisakhi to West Bengal, Loo to the northern plains).
• Statement-based: judge claims such as "An El Nino year tends to weaken the Indian monsoon" (correct) and "Tamil Nadu gets most of its rain from the south-west monsoon" (incorrect, the north-east).
• Cause-effect: rain shadow east of the Western Ghats; the Khasi Hills funnel and Mawsynram.

Authored practice:

1. Tamil Nadu receives most of its annual rainfall from the (a) south-west monsoon (b) north-east monsoon (c) western disturbances (d) Kalbaisakhi. Answer (b). The retreating north-east monsoon picks up moisture over the Bay of Bengal and rains the Coromandel Coast in winter.
2. Which upper-air feature is associated with the burst of the south-west monsoon over India? (a) subtropical westerly jet (b) polar front jet (c) tropical easterly jet (d) the Loo. Answer (c). The tropical easterly jet establishes over the peninsula in summer once the westerly jet withdraws north of Tibet.
3. The wettest place on earth, in Meghalaya, is (a) Cherrapunji (b) Mawsynram (c) Agumbe (d) Pasighat. Answer (b). Mawsynram edges out nearby Cherrapunji; both are fed by the Bay branch funnelled into the Khasi Hills.
4. Consider: (1) Western disturbances bring winter rain to north-west India. (2) A La Nina phase usually weakens the Indian monsoon. Which is or are correct? Answer: only statement 1. La Nina usually strengthens the monsoon; El Nino weakens it.
5. The hot, dry, dust-laden wind of the northern plains in May and June is called the (a) Kalbaisakhi (b) Loo (c) mango shower (d) Norwester. Answer (b). The Loo is a dry summer wind; the others are moist pre-monsoon showers or storms.
6. The south-west monsoon normally reaches Kerala around (a) 1 May (b) 1 June (c) 1 July (d) 15 June. Answer (b). From Kerala it covers the whole country by about mid-July.
7. Which sea generates the more frequent and destructive tropical cyclones affecting India? (a) Arabian Sea (b) Bay of Bengal (c) Andaman Sea (d) Laccadive Sea. Answer (b). The Bay of Bengal is far more cyclone-prone than the Arabian Sea.
8. Consider: (1) The Arabian Sea branch causes a rain shadow on the leeward Deccan. (2) The Bay of Bengal branch is funnelled by the Khasi Hills. Which is or are correct? Answer: both.
9. Mango showers, which help ripen the mango crop, occur mainly in (a) Punjab and Haryana (b) Kerala and coastal Karnataka (c) West Bengal and Assam (d) Rajasthan and Gujarat. Answer (b). The Bengal-Assam pre-monsoon storms are the Kalbaisakhi instead.

Common confusion

• South-west monsoon (summer, June to September, most of India) versus north-east monsoon (winter, October to December, Tamil Nadu).
• Subtropical westerly jet (winter, south of the Himalayas, steers western disturbances) versus tropical easterly jet (summer, over the peninsula, aids the burst).
• El Nino (weakens the monsoon) versus La Nina (strengthens it); a positive IOD can rescue a weak El Nino year.
• Mango showers (moist, pre-monsoon, Kerala-Karnataka) versus the Loo (hot, dry, northern plains).
• Mawsynram and Cherrapunji are both in Meghalaya; Mawsynram is now the wetter of the two.
• Western disturbances (Mediterranean origin, winter, north-west) are not part of the monsoon system at all.
• Arabian Sea branch (Western Ghats, Deccan rain shadow) versus Bay of Bengal branch (Ganga plain, Khasi Hills).
• Onset over Kerala is about 1 June; the whole country is covered by mid-July; withdrawal starts in the north-west in September.
• January is the coldest month and May the hottest; the largest temperature range is in the interior, not the coast.

Memory hook

• Seasons in order: "Winter, Summer, Monsoon, Retreat" (cold, hot, rainy, withdrawing).
• Two branches: Arabian Sea hits the Western Ghats, Bay of Bengal hits the Khasi Hills (Mawsynram).
• El Nino weakens, La Nina strengthens: "El Nino, El Less rain."
• Tamil Nadu is the winter-rain oddity; remember "TN gets the retreat."
• Jets: Westerly in Winter, Easterly in summer (when the monsoon Erupts).

Night before

• South-west monsoon June to September; reaches Kerala about 1 June; covers India by mid-July.
• Two branches: Arabian Sea (Western Ghats, rain shadow on the Deccan) and Bay of Bengal (Khasi Hills, Mawsynram).
• Tamil Nadu's main rain is the retreating north-east monsoon, October to December.
• Subtropical westerly jet in winter, tropical easterly jet in summer; El Nino weakens, La Nina strengthens.
• Western disturbances bring winter rain and snow to the north-west, helping the rabi crop and feeding Siachen blizzards.
• Mawsynram wettest; Thar and Ladakh driest; Loo is the hot summer wind, Kalbaisakhi the Bengal pre-monsoon storm.
• Four IMD seasons: winter, summer (pre-monsoon), south-west monsoon, retreating monsoon.
• Factors controlling climate: latitude, the Himalayas, the sea, altitude, pressure-and-wind, the jets.
• Bay of Bengal cyclones in the pre- and post-monsoon hit the eastern coasts; storm surge kills; the NDRF and CAPFs deploy.
• Wettest: Mawsynram and Cherrapunji (Meghalaya), Agumbe in the peninsula; driest: the Thar and Ladakh.
• The two branches compared decide where it rains: Western Ghats (Arabian Sea) and the Khasi Hills (Bay of Bengal).
• A monsoon break is a dry spell within the rainy season that can hurt the standing kharif crop.
• The jet streams steer the systems; ocean temperatures (El Nino, IOD) set the strength of the season.
• Onset over Kerala about 1 June, Mumbai about 10 June, Delhi end June, whole country by mid-July.
• Withdrawal: north-west from early September, peninsula by mid-October, then the north-east monsoon over Tamil Nadu.

Glossary

• Monsoon: a wind system that reverses direction seasonally, bringing India its summer rains.
• ITCZ (Inter-Tropical Convergence Zone): the equatorial low-pressure belt that migrates north in the Indian summer.
• Orographic rainfall: rain forced out of moist air rising over a mountain barrier (the windward Western Ghats).
• Rain shadow: the dry leeward side of a mountain that the rain has been wrung out before reaching (interior Deccan).
• Jet stream: a fast, narrow upper-air current; the subtropical westerly and tropical easterly jets steer Indian weather.
• Western disturbance: an extratropical depression from the Mediterranean bringing winter rain and snow to north-west India.
• El Nino: a warm Pacific phase that tends to weaken the Indian monsoon; La Nina is the opposite cool phase.
• Indian Ocean Dipole (IOD): an east-west sea-surface temperature contrast in the Indian Ocean that modulates the monsoon.
• Burst of the monsoon: the sudden onset of heavy rain when the system arrives.
• Loo: the hot, dry, dust-laden wind of the northern plains in summer.
• Kalbaisakhi / Norwester: violent pre-monsoon thunderstorms of West Bengal and Assam.
• Mango shower: pre-monsoon rain in Kerala and Karnataka that helps ripen mangoes.
• Blossom shower: pre-monsoon rain that helps the coffee flower in Karnataka.
• Bardoli Chheerha: the Assam name for the Norwester pre-monsoon storms that help tea and jute.
• ENSO (El Nino Southern Oscillation): the coupled Pacific ocean-atmosphere cycle of El Nino and La Nina.
• Monsoon trough / ITCZ over India: the inland low-pressure axis that pulls in the rain-bearing winds.
• Pre-monsoon season: the hot weather period (March to May) of dust storms and convective showers.
• Post-monsoon season: the retreating period (October to November) of withdrawal and Bay of Bengal cyclones.
• October heat: the oppressive hot, humid transition weather of October before the rains fully clear.
• Variability of rainfall: the year-to-year and place-to-place unevenness that makes Indian farming monsoon-dependent.
• Norwester: another name for the Kalbaisakhi, the violent pre-monsoon thunderstorm of eastern India.
• Monsoon onset: the date the rains arrive at a place, about 1 June over Kerala.
• Rainfall variability: how much the rain differs year to year, highest in the dry west.
• Cold desert: an arid high-altitude region in the rain shadow, like Ladakh.
• Storm surge: the wall of sea water pushed ashore by a cyclone, its chief killer.
• Annual range of temperature: the difference between the hottest and coldest month, large inland, small at the coast.
• Monsoon trough: the seasonal low-pressure axis over the Ganga plain in summer, the inland ITCZ.
• Mascarene High: the high-pressure cell in the southern Indian Ocean that drives the Arabian Sea branch.
• Somali Jet: the cross-equatorial low-level jet that funnels moisture toward the Indian west coast.
• Withdrawal (retreat) of the monsoon: the southward retreat of the rains from October.
• Continentality: the tendency of inland areas to have hotter summers and colder winters than coasts.
• Tibetan heating: the summer warming of the Tibetan Plateau that helps establish the tropical easterly jet.
• Equable climate: a climate with a small temperature range, typical of the coasts.
• Tropical cyclone: an intense low-pressure storm of the warm seas, more frequent over the Bay of Bengal.
• Koppen classification: a temperature-and-rainfall scheme using letter codes (A, B, C, E, H) for climate types.
• Inversion of the seasons: the way Tamil Nadu is wettest in winter (the north-east monsoon), against the national pattern.
• Convective rain: rain from rising heated air, as in the pre-monsoon Kalbaisakhi thunderstorms.
• Monsoon break: a spell of weak or no rain within the rainy season, harmful to standing crops.

Related notes

• climatology atmosphere and winds
• india physiography
• indian drainage system and rivers
• indian agriculture and cropping
• soils and natural vegetation of india
• india borders neighbours and strategic geography
• syllabus index