Climate of India

Climate of India

Nature and Classification of India's Climate

India's climate is broadly classified as the monsoon type or tropical monsoon climate, also described as the tropical deciduous biome climate, because of the characteristic seasonal reversal of surface winds by 180 degrees. India receives about 75 to 90 percent of its annual rainfall during the southwest monsoon season. The Koppen classification identifies multiple climate types within India: tropical wet (Am) along the western coast and northeast, tropical savanna (Aw) across most of peninsular India, semi-arid steppe (BSh) in parts of the Deccan plateau and Rajasthan, arid desert (BWh) in western Rajasthan and Kutch, humid subtropical (Cwa) across the Indo-Gangetic plain and central India, and highland climate (H) in the Himalayas. Despite this internal diversity, the unifying feature of Indian climate is the dominance of the monsoon system governing precipitation patterns for the entire country.

Factors Controlling India's Climate

Several factors operate simultaneously to produce India's distinctive climate. The location of India between 8 degrees N and 37 degrees N places most of the country within the tropics and subtropics, giving it high insolation and warm temperatures. The Himalayan mountain barrier in the north plays a decisive role by blocking cold polar continental air masses from Central Asia from entering India during winter, keeping temperatures much warmer than other countries at similar latitudes, and by forcing the moisture-laden southwest monsoon winds to rise and precipitate heavily on the southern slopes. Without the Himalayas, north India would be a cold desert. The Thar Desert and the low-pressure system that develops over the northwestern plains during summer is another critical factor: this thermal low attracts the southwest monsoon from the southern hemisphere across the equator. The position of the ITCZ (Intertropical Convergence Zone), which shifts northward over the Indian subcontinent in summer and is sometimes called the monsoon trough, determines the onset and withdrawal of the monsoon. The tropical easterly jet stream, flowing at around 150 hPa over peninsular India and originating due to the intense heating of the Tibetan Plateau, is essential for sustaining the southwest monsoon. The subtropical westerly jet stream, which disappears from India in summer and reappears in winter, brings western disturbances during the cold season. Distance from the sea creates a strong continentality effect in the interior and north India, producing extreme temperature ranges, while the peninsula and island territories experience maritime moderation. Ocean currents, particularly the warm Arabian Sea and Bay of Bengal waters, supply moisture to both branches of the monsoon.

Seasons of India

The India Meteorological Department (IMD) recognises four seasons in India: the cold weather season (December to February), the hot weather season (March to May), the southwest monsoon season (June to September), and the retreating monsoon or northeast monsoon season (October to November).

Cold Weather Season (December to February)

During winter, the sun is in the southern hemisphere and most of India, except the coastal belt, experiences high pressure due to cold and dense air. The high pressure and cold temperatures are most prominent in north India and the Himalayas. The southern branch of the subtropical westerly jet stream reappears over north India, flowing at around 25 to 30 degrees N. This jet stream carries extratropical (temperate) cyclones originating over the Mediterranean Sea, the Black Sea, and the Caspian Sea region into the northwestern subcontinent, producing precipitation known as western disturbances. Western disturbances are critically important for the rabi crop, particularly wheat, across Punjab, Haryana, Uttarakhand, Himachal Pradesh, and Jammu and Kashmir. They also cause snowfall in the Himalayas, which supports the perennial character of Himalayan rivers and thereby sustains canal irrigation across the Indo-Gangetic plain. In recent years, the westerly jet stream has also been implicated in transporting pollution from crop stubble burning in Punjab, Haryana, and Pakistan into the great plains, severely degrading air quality.

The surface winds during the winter season blow as northeast monsoon winds, originating from the high pressure system over Central Asia. These winds are largely dry because they originate over land. However, the northeast monsoon winds that pass over the Bay of Bengal pick up moisture and deliver heavy rainfall to the Coromandel Coast of Tamil Nadu and parts of Andhra Pradesh from October to December. This is why Tamil Nadu receives most of its rainfall during the retreating monsoon season when most of the rest of India experiences dry conditions. The perennial nature of the Kaveri River, the only major perennial river of peninsular India, is sustained in part by northeast monsoon rainfall, which also explains why the sugar industry in India has been shifting southward.

Winter temperatures in north India are modified by cold waves (sheets of cold continental air moving southward), fog, and frost. Punjab, Haryana, Uttar Pradesh, Bihar, and West Bengal experience dense fog during December and January, disrupting road, rail, and air transport. The mean January temperature in Delhi is around 14 degrees Celsius, in Amritsar around 7 degrees Celsius, and at Leh it falls well below freezing. Coastal stations like Mumbai and Chennai maintain temperatures above 20 degrees Celsius throughout winter due to maritime influence.

Hot Weather Season (March to May)

As the sun moves northward after the winter solstice, insolation increases progressively from the peninsula northward. The low pressure system over north India gradually intensifies. The pressure system is not yet well developed in March and April, and local and regional weather systems dominate. The hot weather season is characterised by a series of important local convective and orographic weather phenomena. The Loo is a hot, dry, and desiccating wind blowing across the great plains of north India during afternoon hours, with temperatures reaching 45 to 48 degrees Celsius in parts of Rajasthan, Haryana, and Uttar Pradesh; it causes heat stroke and death among vulnerable populations. Andhis are dust storms or thunderstorms that develop over the great plains due to intense surface heating creating convectional instability; they bring temporary relief from heat. Kal Baisakhi, known as Nor'westers, are violent pre-monsoon thunderstorms occurring in the late afternoon and evening over Bengal and Assam during April and May; they bring convectional rainfall that is beneficial for jute cultivation and is known as Kalbaisakhi in Bengali and Bardoli Cherra in Assam. Mango showers occur over Kerala and Karnataka, giving early rainfall that supports the ripening of mangoes and early blossoming of coffee plants, hence the name blossom shower for Karnataka. Cherry blossoms in coffee plantations are triggered by these pre-monsoon showers. Thunderstorms and hailstorms are common during April and May across interior peninsular India, causing extensive damage to kharif crops such as cotton, pulses, and oilseeds.

By the end of May, temperatures in the northwest and central India reach extreme values. The mean May temperature in Rajasthan can exceed 40 degrees Celsius. The lowest atmospheric pressure of the year is recorded over the northwestern plains and the Thar Desert by around June 21, the summer solstice, when the ITCZ shifts northward to approximately 25 to 30 degrees N over the Indo-Gangetic plain, which is also called the monsoon trough during this period.

Southwest Monsoon Season (June to September)

The southwest monsoon is the most important climatic event for India, contributing about 75 to 90 percent of annual rainfall and determining agricultural output, water availability, and economic conditions. As the thermal low deepens over the northwestern plains and the ITCZ shifts over India, the southerly branch of the subtropical westerly jet disappears from over India. The pressure gradient between the high pressure over the southern Indian Ocean and the thermal low over India pulls moisture-laden winds northward. These trade winds from the southern hemisphere cross the equator and, deflected by the Coriolis force, arrive over India from the southwest. The Arabian Sea branch of the southwest monsoon hits the Western Ghats first, causing very heavy orographic rainfall on the windward western slopes (Mahabaleshwar receives over 600 cm annually) while the leeward Deccan plateau falls in a rain shadow and receives much less. The Bay of Bengal branch travels northward along the eastern coast and then curves northwestward into the Ganga plain, while a portion penetrates into northeast India (Cherrapunji and Mawsynram in Meghalaya, receiving the highest rainfall in the world, benefit from the funnelling effect of the Brahmaputra valley and orographic lifting by the Shillong Plateau). The two branches meet and merge over the Indo-Gangetic plain.

The tropical easterly jet stream (TEJ), flowing at around 150 hPa between 5 and 15 degrees N, plays an essential role in sustaining the monsoon by creating divergence in the upper atmosphere over the Indian subcontinent, which promotes low pressure and convergence at the surface. The TEJ originates due to the intense differential heating of the elevated Tibetan Plateau relative to the surrounding ocean surfaces. The strength of the TEJ correlates positively with the intensity of monsoon rainfall.

Rainfall distribution during the southwest monsoon is highly uneven. The Western Ghats (Malabar Coast, Western Ghats mountains), northeast India (Meghalaya, Assam, Arunachal Pradesh), and the Andaman and Nicobar Islands receive the highest rainfall. The northwestern plains (Punjab, Haryana, western Rajasthan), the interior of the Deccan plateau (rain shadow region), and the Thar Desert receive the least. The isohyet of 100 cm annual rainfall runs roughly northeast to southwest through the country, dividing the heavy-rainfall east from the lighter-rainfall west. Rajasthan, except the eastern strip, receives less than 20 cm annually. The Western Ghats act as a major orographic barrier: as Aravalis are roughly parallel to the direction of the southwest monsoon, they do not intercept it, which is why western Rajasthan remains a desert while areas to the east of Aravalis receive some rainfall. The rain shadow effect of the Western Ghats creates a narrow strip of low rainfall (less than 75 cm) along the leeward side of the ghats, running through parts of Karnataka, Maharashtra, and Tamil Nadu.

The monsoon does not arrive simultaneously all over India. It typically arrives at the Kerala coast around June 1, advances northward and eastward, and covers the entire country by approximately July 15. The progress of the monsoon is tracked by the IMD through rainfall data, cloud cover, and wind direction. The monsoon is characterised by active phases of heavy rainfall interspersed with break conditions when the monsoon trough shifts northward toward the foothills of the Himalayas; during break monsoon conditions, the plains receive less rainfall while the Himalayan foothills and parts of northeastern India receive heavy rainfall and landslides are common.

Retreating Monsoon Season (October to November)

By mid-September, the low pressure system over northwest India begins to weaken as the sun moves southward after the autumnal equinox. The ITCZ withdraws southward, and the monsoon begins retreating from north India first, then progressively from peninsular India. The monsoon withdraws completely from the country by approximately December 1. This season is characterised by extremely high daytime temperatures combined with high humidity, creating oppressive conditions known as October Heat. The retreating monsoon season is also the season of very heavy rainfall along the eastern coast of India, particularly Tamil Nadu and Andhra Pradesh, due to tropical cyclones originating in the Bay of Bengal. The frequency of Bay of Bengal cyclones is highest during October and November, and these cyclones bring catastrophic flooding and storm surges to the Coromandel Coast, the Krishna-Godavari delta, and the Mahanadi delta regions of Odisha.

Western Disturbances

Western disturbances are extratropical cyclones that originate over the Mediterranean Sea, Black Sea, and Caspian Sea regions and travel eastward, brought into the Indian subcontinent by the subtropical westerly jet stream during the winter months (November to March). They bring rainfall to the plains of northwest India and snowfall to the Himalayas, Karakoram, and Hindu Kush ranges. Western disturbances are essential for the rabi agricultural season: the wheat crop in Punjab, Haryana, and western Uttar Pradesh depends critically on this winter rainfall. Snowfall in the Himalayas due to western disturbances is stored as snow and glacial ice, sustaining the perennial flow of rivers like the Indus, Jhelum, Chenab, Ravi, Beas, and Sutlej through the dry summer months and supporting canal irrigation infrastructure built during the colonial period. The arrival of a temperate cyclone (western disturbance) is signalled by cirrus clouds appearing in the sky. Precipitation occurs with the passage of warm fronts (gentle rainfall from nimbostratus clouds) and cold fronts (heavy rainfall from cumulonimbus clouds). From 2012 onwards, the frequency and intensity of western disturbances bringing significant rainfall has been decreasing, and the 2017-18 winter was the driest winter since 1901, attributed to global warming reducing atmospheric moisture and disrupting jet stream patterns.

Tropical Cyclones in India

Tropical cyclones are among the most dangerous atmospheric disturbances affecting India, forming over warm tropical ocean surfaces (sea surface temperature exceeding 27 degrees Celsius) and drawing energy from the latent heat of condensation of water vapour. They are known as cyclones in the Indian Ocean region (hurricanes in the Gulf of Mexico and Caribbean, typhoons in the South China Sea, Willy-Willies in Australia). A tropical cyclone is classified as a depression when wind speed is around 60 km/h, a cyclonic storm when wind speed is between 60 and 90 km/h, a severe cyclonic storm between 90 and 120 km/h, a very severe cyclonic storm between 120 and 165 km/h, and a super cyclonic storm beyond 165 km/h. The structure of a tropical cyclone features a central rainless and cloudless eye with descending air, surrounded by an eye wall of violent cumulonimbus clouds with the most intense winds and precipitation, and spiral rain bands extending outward. The average diameter ranges from 30 to 300 km and height reaches around 15 km.

Tropical cyclones require several conditions for formation: sea surface temperature above 27 degrees Celsius; higher Coriolis force, which is why they do not form within 5 degrees N or S of the equator; abundant moisture and moist heat (latent heat of condensation as the driving engine); presence of the ITCZ for initial convergence of warm moist air masses; anticyclonic conditions in the upper atmosphere promoting outflow; and absence of strong vertical wind shear that would disrupt the vertical structure. Tropical cyclones die when they make landfall because the supply of moisture and moist heat from the ocean surface is cut off. Tropical cyclones do not develop during the southwest monsoon season itself because the strong surface southwest monsoon winds and the strong upper-level tropical easterly jet stream create high vertical wind shear that inhibits cyclone formation and intensification.

The eastern coast of India is far more prone to tropical cyclones than the western coast for several reasons. First, Bay of Bengal cyclones after forming travel northwestward and then curve northward, making landfall on the eastern coast of India. Second, the Bay of Bengal records about 35 percent more cyclones than the Arabian Sea because the Bay of Bengal is enclosed, warmer, and receives enormous freshwater input from rivers (making surface waters less dense and not mixing well with cooler deep waters), sustaining high sea surface temperatures. The Arabian Sea is relatively cooler due to upwelling driven by strong monsoon winds, which inhibits cyclone formation and intensification. Third, the Bay of Bengal is connected to the Pacific Ocean, and remnants of typhoons that cross the South China Sea enter the Bay of Bengal and intensify cyclonic activity. The most cyclone-prone regions of the eastern coast are coastal Andhra Pradesh (Krishna-Godavari delta), the Mahanadi delta (Odisha), and coastal Tamil Nadu. Storm surges associated with intense cyclones cause catastrophic flooding of coastal agricultural land, destruction of mangrove ecosystems, and massive loss of life and infrastructure in these densely populated delta regions.

Tropical cyclones that form in the Arabian Sea tend to move northwestward toward Pakistan, Oman, and the Arabian Peninsula rather than hitting the western coast of India. However, exceptions occur (e.g., Cyclone Tauktae in 2021 severely impacted the Gujarat and Maharashtra coast). The naming of Indian Ocean cyclones began in 1999, with the India Meteorological Department (IMD) serving as the Regional Specialised Meteorological Centre (RSMC) for the North Indian Ocean, providing cyclone advisories to eight countries: Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka, and Thailand. Names are assigned in alphabetical order of these countries in rotation.

The intensity of tropical cyclones is increasing globally and in the Indian Ocean due to rising sea surface temperatures associated with anthropogenic climate change. Destruction of mangrove forests, which serve as natural buffers against storm surges, and unplanned coastal urbanisation have significantly increased the vulnerability of coastal communities to cyclone damage, as evidenced by the severe floods in Kerala (2018), Mumbai, and coastal Odisha and Andhra Pradesh in successive years.

El Nino, La Nina, and ENSO

El Nino is one of the most significant climate phenomena affecting India's monsoon variability. Under normal conditions, strong trade winds blow from South America (the Peru-Chile coast) toward Australia and Southeast Asia. These trade winds drive the warm surface water of the Pacific westward, causing cold upwelling of the Peruvian (Humboldt) current along the South American coast. This cold upwelling creates high pressure over Peru and Chile, promoting arid conditions there, while the warm water piled up near Australia and Indonesia creates low pressure and heavy rainfall over eastern Australia and Indonesia. This normal pattern of atmospheric circulation over the tropical Pacific is called the Walker Cell, with the ascending limb over Australia-Indonesia and the descending limb over Peru-Chile.

El Nino refers to the occasional development of anomalously warm ocean surface temperatures near the Peru-Chile coastline, typically occurring around Christmas (hence the name El Nino, meaning the Christ Child in Spanish). El Nino develops when the trade winds weaken or reverse direction. Without strong trade winds, surface warm water is no longer pushed westward, the cold Peruvian upwelling ceases, and warm tropical counter-currents replace the cold water along the South American coast. This warming of the central and eastern Pacific reverses the Walker Cell: ascending air over Peru-Chile creates low pressure there, while descending air over Australia, India, and Indonesia creates high pressure, causing droughts and forest fires in those regions. This see-saw reversal of atmospheric pressure between the eastern and western Pacific is called the Southern Oscillation (SO). Together, El Nino and the Southern Oscillation are described as ENSO (El Nino Southern Oscillation).

The effects of El Nino are global. In Peru and Chile, it brings devastating floods and destroys the fishing industry by stopping upwelling of the nutrient-rich Peruvian current, collapsing the anchoveta fishery and the guano fertiliser industry. In Australia and Indonesia, El Nino causes severe droughts and forest fires. Coral bleaching events on the Great Barrier Reef (as in 2018) and other reef systems are strongly associated with El Nino-driven warming of ocean surface temperatures. El Nino years are positively associated with droughts in India: approximately seven out of ten El Nino years produce below-normal monsoon rainfall or drought conditions in India. The monsoon-El Nino relationship operates through the suppression of the tropical easterly jet, weakening of the low pressure over India, and disruption of the moisture supply from the Bay of Bengal and Arabian Sea. The frequency and intensity of El Nino events are increasing with anthropogenic global warming.

La Nina is the opposite of El Nino and represents an extreme intensification of normal conditions. During La Nina, trade winds strengthen significantly, driving more warm surface water westward and intensifying cold upwelling along the Peru-Chile coast. The central and eastern Pacific cools anomalously. La Nina years are associated with above-normal monsoon rainfall in India, Australia, and Indonesia, and reduced rainfall in Peru and Chile. La Nina years, such as 2018, brought excess rainfall to India. The alternation between El Nino (warm phase), neutral conditions, and La Nina (cold phase) is the ENSO cycle, which has a periodicity of approximately 3 to 7 years. The Indian Ocean Dipole (IOD), another sea surface temperature anomaly pattern in which the western Indian Ocean warms relative to the eastern Indian Ocean, also independently influences Indian monsoon variability, and positive IOD events tend to compensate for El Nino-induced monsoon deficit.

Rainfall Distribution in India

Annual rainfall distribution in India is highly uneven both spatially and temporally. Cherrapunji (Sohra) in Meghalaya, receiving about 1,080 cm annually, and the nearby Mawsynram, which holds the record for the world's highest average annual rainfall at over 1,141 cm, receive enormous precipitation due to the combination of southerly winds funnelled up the Brahmaputra valley and orographic lifting by the Khasi Hills escarpment. The western slopes of the Western Ghats receive 200 to 400 cm annually while the leeward Deccan plateau receives only 50 to 75 cm. The Konkan and Malabar coasts receive 150 to 250 cm. The northeast states of Arunachal Pradesh, Meghalaya, Assam, Nagaland, Manipur, and Mizoram receive heavy to very heavy rainfall. By contrast, western Rajasthan (Jaisalmer, Barmer) receives less than 12 cm annually, making it hyper-arid. The Thar Desert, Kutch, and parts of the Deccan plateau in Karnataka and Andhra Pradesh receive less than 50 cm. The Bihar-Bengal-Odisha coastal belt, the Konkan coast, and the Western Ghats form the high-rainfall zones of India.

Temporal variability is also high: India's rainfall is concentrated in the four months of the southwest monsoon (June to September), and inter-annual variability is significant. Drought years correspond predominantly to El Nino years. Flood years are often La Nina years or years of positive IOD. From 2012 onward, an increase in dry days during both the southwest monsoon and winter season has been documented, with more intense but shorter rainfall events (cloud bursts) replacing the steady moderate rainfall of earlier decades, a pattern consistent with global warming intensifying the hydrological cycle unevenly.

Temperature Distribution in India

Temperature distribution in India shows strong latitudinal, continentality, and relief gradients. In summer (May to June), the highest temperatures are recorded in the Thar Desert and the northwestern plains, with temperatures in Rajasthan regularly exceeding 48 degrees Celsius. By contrast, the Western Ghats and the northeastern hills remain relatively cool even in summer, and the Himalayas experience snow. The peninsula's coastal regions are moderated by sea breezes and maritime air masses, rarely exceeding 36 to 38 degrees Celsius. In winter (December to January), the temperature gradient between north and south India is steep: Leh in Ladakh records minus 20 degrees Celsius or lower, Shimla drops below zero, while Kanyakumari remains above 25 degrees Celsius. The Himalayan region, including Jammu and Kashmir, Himachal Pradesh, and Uttarakhand, experiences severe cold, heavy snowfall, and frequent blizzards. The warm oceanic influence keeps coastal Tamil Nadu, Kerala, and the Andaman and Nicobar Islands well above 20 degrees Celsius throughout the year, effectively eliminating any seasonality of temperature. The interior of the Deccan plateau (Maharashtra, Karnataka, Telangana) experiences moderate temperatures due to its elevation (300 to 900 m), while the great plains of north India have the most extreme range between summer maximum and winter minimum temperatures, reflecting continental interior conditions.

Climate Change Impacts on India's Climate

India is among the most vulnerable countries to anthropogenic climate change. Documented and projected impacts include: increasing frequency and intensity of heat waves in the Indo-Gangetic plain, with the 2022 heat wave being one of the most intense on record; increasing dry days during the southwest monsoon season from 2012 onward, reducing total seasonal rainfall even as individual rainfall events become more intense (cloud bursts); more frequent and intense tropical cyclones in the Arabian Sea (historically less active than the Bay of Bengal); accelerated melting of Himalayan glaciers, with the Gangotri, Siachen, and Zemu glaciers retreating significantly, increasing short-term river discharge but threatening long-term water security of Himalayan river systems including the Indus, Ganga, and Brahmaputra; decreasing snowfall in the Himalayas, threatening rabi crop irrigation in the Indo-Gangetic plains; rise in sea level threatening submergence of coastal settlements, the Sundarbans mangrove ecosystem, and low-lying islands; increasing inland flooding in Himalayan states (Uttarakhand cloudburst of 2013, Kashmir floods of 2014, frequent Brahmaputra floods); and large-scale coral bleaching in the Lakshadweep and Andaman reef systems. The Asian Brown Cloud, a 3-km-thick layer of black carbon and aerosol pollution over South Asia, suppresses solar radiation reaching the surface, reduces differential heating between land and sea, reduces evaporation and cloud formation, and increases dry days during the monsoon season, further stressing agriculture and water resources across the subcontinent.