Climate of India
The climate of India comprises a wide range of weather conditions across a large geographic scale and varied topography, making generalisations difficult. Analysed according to the Köppen system, India hosts six major climatic subtypes, ranging from desert in the west, to alpine tundra and glaciers in the north, to humid tropical regions supporting rainforests in the southwest and the island territories. Many regions have starkly different microclimates. The nation has four seasons: winter (January and February), summer (March to May), a monsoon (rainy) season (June to September), and a post-monsoon period (October to December).
India's unique geography and geology strongly influence its climate, particularly in the Himalayas in the north and the Thar Desert in the northwest. The Himalayas act as a barrier to the frigid katabatic winds flowing down from Central Asia. Thus, North India stays warm or only mildly cold during winter; in summer, the same phenomenon makes India relatively hot. Although the Tropic of Cancer—the boundary between the tropics and subtropics—passes through the middle of India, scientists consider the whole country tropical.
As in much of the tropics, India experiences unstable monsoonal and other weather conditions: major droughts, floods, cyclones and other natural disasters occur sporadically, killing or displacing millions. Global warming further threatens India's long-term climatic stability. Climatic diversity in India makes the analysis of those issues complex.
During the Late Permian (some 260–251 million years ago (mya)), the Indian subcontinent belonged to the vast supercontinent Pangaea. Despite its position within a high-latitude belt at 55–75° S (as opposed to its current position between 5 and 35° N), latitudes now occupied by such places as Greenland and parts of the Antarctic Peninsula, India likely experienced a humid temperate climate with warm, frost-free weather, though with well-defined seasons. Later, India joined the southern supercontinent Gondwana, a process beginning some 550–500 mya. During the Late Paleozoic, Gondwana extended from a point at or near the South Pole to near the equator, where the Indian craton (stable continental crust) positioned, resulting in a mild climate favorable to hosting high-biomass ecosystems. India's vast coal reserves (much of it from the late Paleozoic sedimentary sequence) the fourth-largest reserves in the world underscores that. During the Mesozoic, the world, including India, experienced considerably warmer weather than today. With the coming of the Carboniferous, global cooling stoked extensive glaciation, which spread northwards from South Africa towards India; that cool period lasted well into the Permian.
Tectonic movement by the Indian Plate caused it to pass over a geologic hotspot—the Réunion hotspot—now occupied by the volcanic island of Réunion. That resulted in a massive flood basalt event that laid down the Deccan Traps some 60–68 mya, at the end of the Cretaceous period. That may have contributed to the global Cretaceous-Tertiary (K-T) extinction event, which caused India to experience significantly reduced insolation. Elevated atmospheric levels of sulphur gases formed aerosols such as sulfur dioxide and sulfuric acid, similar to those found in the atmosphere of Venus; these precipitated as acid rain. Elevated carbon dioxide emissions also contributed to the greenhouse effect, causing global warming that lasted long after the atmospheric shroud of dust and aerosols had cleared. Further climatic changes 20 million years ago, long after India had crashed into the Laurasian landmass, proved severe enough to cause the extinction of many endemic Indian forms. Meanwhile, the formation of the Himalayas resulted in blockage of frigid Central Asian air, preventing it from reaching India; that made its climate significantly warmer and more tropical in character.
India serves as home to an extraordinary variety of climatic regions, ranging from tropical in the south to temperate and alpine in the Himalayan north, where elevated regions receive sustained winter snowfall. The Himalayas and the Thar Desert strongly influence the nation's climate. The Himalayas, along with the Hindu Kush mountains in Pakistan, prevent cold Central Asian katabatic winds from blowing in, keeping the bulk of the Indian subcontinent warmer than most locations at similar latitudes. Simultaneously, the Thar Desert plays a role in attracting moisture-laden southwest summer monsoon winds that, between June and October, provide the majority of India's rainfall. Experts have designated seven climatic zones falling into four major climatic groupings, defined according to traits such as temperature and precipitation. Meteorologists assigned codes groupings (see chart) according to the Köppen climate classification system.
A tropical rainy climate covers regions experiencing persistent warm or high temperatures, which normally stay above 18 °C (64 °F). India hosts two climatic subtypes that fall under that group. The most humid, the tropical wet monsoon climate, covers a strip of southwestern lowlands abutting the Malabar Coast, the Western Ghats, and southern Assam. That climate prevails in India's two island territories, Lakshadweep and the Andaman and Nicobar Islands. Characterized by moderate to high year-round temperatures, even in the foothills, rain falls seasonally but heavy—typically above 2,000 millimeters (79 in) per year. Most rainfall occurs between May and November, adequate for the maintenance of lush forests and other vegetation throughout the remainder of the year. December to March represent the driest months, when days with precipitation are rare. The heavy monsoon rains create the extremely biodiverse tropical wet forests of those regions.
India commonly experiences a tropical wet and dry climate. Significantly drier than tropical wet zones, it prevails over most of inland peninsular India except for a semi-arid rain shadow east of the Western Ghats. Long Winter and early summers typically bring dry periods with temperatures averaging above 18 °C (64 °F). Indians experience exceptionally hot Summers; temperatures in low-lying areas may exceed 50 °C (122 °F) during May, leading to heat waves that occasionally kill hundreds of Indians. The rainy season lasts from June to September; annual rainfall averages between 750–1500 millimetres (30–59 in) across the region. Once the dry northeast monsoon begins in September, most precipitation in India falls on Tamil Nadu, leaving other states comparatively dry.
A tropical arid and semi-arid climate dominates regions where the rate of moisture loss through evapotranspiration exceeds that from precipitation; it is subdivided into three climatic subtypes. The first, a tropical semi-arid steppe climate, predominates over a long stretch of land south of Tropic of Cancer and east of the Western Ghats and the Cardamom Hills. That region, which includes Karnataka, inland Tamil Nadu, western Andhra Pradesh, and central Maharashtra, gets between 400–750 millimetres (16–30 in) annually. Drought-prone, the region tends to have less reliable rainfall due to sporadic lateness or failure of the southwest monsoon. North of the Krishna River, the summer monsoon brings most the rainfall; to the south, significant post-monsoon rainfall also occurs in October and November. In December, the coldest month, temperatures still average around 20–24°C (68–75 °F). March to May experience hot and dry weather; mean monthly temperatures hover around 32°C, with 320 millimeters (13 in) precipitation. Without artificial irrigation, that region proves unsuitable for agriculture.
Most of western Rajasthan experiences an arid climatic regime. Cloudbursts bring virtually all of the region's annual precipitation, which totals less than 300 millimeters (12 in). Such bursts happen when monsoon winds sweep into the region during July, August, and September. Such rainfall proves highly erratic; regions experiencing rainfall one year may wait a couple of years or so before the next rain fall. Continuous downdrafts, for the most part, prevent precipitation. The summer months of May and June prove exceptionally hot; mean monthly temperatures in the region hover around 35 °C (95 °F), with daily maxima occasionally topping 50 °C (122 °F). During winters, temperatures in some areas can drop below freezing due to waves of cold air from Central Asia. A large diurnal range of about 14 °C (57 °F) appears during summer; that widens by several degrees during winter.
East of the Thar Desert, the region running from Punjab and Haryana to Kathiawar experiences a tropical and sub-tropical steppe climate. The zone, a transitional climatic region separating tropical desert from humid sub-tropical savanna and forests, experiences temperatures less extreme than those of the desert. Average annual rainfall measures 30–65 centimetres (12-26 in), although very unreliable; as in much of the rest of India, the southwest monsoon accounts for most precipitation. Daily summer temperature maxima rise to around 40 °C (104 °F). The resulting natural vegetation typically comprises short, coarse grasses.
Most of Northeast India and much of North India experience a humid sub-tropical climate. Though they experience hot summers, temperatures during the coldest months may fall as low as 0 °C (32 °F). Due to ample monsoon rains, India has only one subtype of that climate, Cfa (under the Köppen system). In most of that region, little measureable precipitation falls during the winter, owing to powerful anticyclonic and katabatic (downward-flowing) winds from Central Asia. Due to the region's proximity to the Himalayas, it experiences elevated prevailing wind speeds, again from the influence of Central Asian katabatic movements.
Humid subtropical regions experience pronounced dry winters. Winter rainfall, and occasionally snowfall, associates with large storm systems such as "Nor'westers" and "Western disturbances"; westerlies steer the latter towards the Himalayas. Most summer rainfall occurs during powerful thunderstorms associated with the southwest summer monsoon; occasional tropical cyclones also contribute. Annual rainfall ranges from less than 1,000 millimeters (39 in) in the west to over 2,500 millimeters (98 in) in parts of the northeast. As most of that region lay far from the ocean, the wide temperature swings more characteristic of a continental climate predominate, with wider swings than in those in tropical wet regions, ranging from 24 °C (75 °F) in north-central India to 27 °C (81 °F) in the east.
India's northernmost fringes experience a montane, or alpine, climate. In the Himalayas, the rate at which an air mass's temperature falls per kilometre (3,281 ft) of altitude gained (the adiabatic lapse rate) is 5.1 °C/km. In terms of environmental lapse rate, ambient temperatures fall by 0.6 °C (1.1 °F) for every 100 meters (328 ft) rise in altitude. Thus, climates ranging from nearly tropical in the foothills to tundra above the snow line can coexist within several dozen miles of each other. Sharp temperature contrasts between sunny and shady slopes, high diurnal temperature variability, temperature inversions, and altitude-dependent variability in rainfall commonly occur. The northern side of the western Himalayas, also known as the trans-Himalayan belt, constitutes a region of barren, arid, frigid, and wind-blown wastelands. Most precipitation occurs as snowfall during the late winter and spring months.
Areas south of the Himalayas enjoy protection much of the time from cold winter winds coming in from the Asian interior. The leeward side (northern face) of the mountains receives less rain while the southern slopes, well-exposed to the monsoon, get heavy rainfall. Areas situated at elevations of 1,070-2,290 metres (3,510-7,510 ft) receive the heaviest rainfall, which decreases rapidly at elevations above 2,290 meters (7,513 ft). The Himalayas experience their heaviest snowfall between December and February and at elevations above 1,500 meters (4,921 ft). Snowfall increases with elevation by up to several dozen millimetres per 100 metre (~2 in/330 ft) increase. Elevations above 5,000 meters (16,404 ft) never experience rain; all precipitation falls as snow.
The India Meteorological Department (IMD) designates four official seasons:
- Winter, occurring between January and March. The year's coldest months are December and January, when temperatures average around 10–15 °C (50–59 °F) in the northwest; temperatures rise as one proceeds towards the equator, peaking around 20–25 °C (68–77 °F) in mainland India's southeast.
- Summer or pre-monsoon season, lasting from March to June (April to July in northwestern India). In western and southern regions, the hottest month is April; for northern regions, May is the hottest month. Temperatures average around 32–40 °C (90–104 °F) in most of the interior.
- Monsoon or rainy season, lasting from June to September. The season is dominated by the humid southwest summer monsoon, which slowly sweeps across the country beginning in late May or early June. Monsoon rains begin to recede from North India at the beginning of October.
- Post-monsoon season, lasting from October to December. South India typically receives more precipitation. Monsoon rains begin to recede from North India at the beginning of October. In northwestern India, October and November are usually cloudless. Parts of the country experience the dry northeast monsoon.
The Himalayan states, being more temperate, experience an additional two seasons: autumn and spring. Traditionally, Indians note six seasons, each about two months long. These are the spring (Sanskrit: vasanta), summer (gri?ma), monsoon season (var?a), early autumn (sarada), late autumn (hemanta), and winter (sisira). These are based on the astronomical division of the 12 months into six parts. The ancient Hindu calendar also reflects these seasons in its arrangement of months.
Once the monsoons subside, average temperatures gradually fall across India. As the Sun's vertical rays move south of the equator, most of the country experiences moderately cool weather; temperatures change by about 0.6 °C (1.35 °F) per degree of latitude. December and January constitute the coldest months, with mean temperatures of 10–15 °C (50–59 °F) in Indian Himalayas. The east and south experience higher mean temperatures, where they reach 20–25 °C (68–77 °F).
In northwestern India, virtually cloudless conditions prevail in October and November, resulting in wide diurnal temperature swings; as in much of the Deccan Plateau, they range between 16–20 °C (61–68 °F). From March to May, "western disturbances" bring heavy bursts of rain and snow. Those extra-tropical low-pressure systems originate in the eastern Mediterranean Sea, carried towards India by the subtropical westerlies, the prevailing winds blowing at North India's range of latitude. Once the Himalayas hinder their passage, they stop, releasing heavy precipitation over the southern Himalayas. The three Himalayan states (Jammu and Kashmir in the extreme north, Himachal Pradesh, and Uttarakhand) experience heavy snowfall; in Jammu and Kashmir, blizzards occur regularly, disrupting travel and other activities.
The rest of North India, including the Indo-Gangetic Plain, almost never receives snow. In the plains, temperatures occasionally fall below freezing, though never for more one or two days. Winter highs in Delhi range from 16 °C (61 °F) to 21 °C (70 °F). Nighttime temperatures average 2–8 °C (36–46 °F). In the Punjab plains, lows can fall below freezing, dropping to around −6 °C (21 °F) in Amritsar. Frost sometimes occurs, but the notorious fog marks the season, frequently disrupting daily life; fog grows thick enough to hinder visibility and disrupt air travel 15–20 days annually. Eastern India enjoys a much milder climate, experiencing moderately warm days and cool nights. Highs range from 23 °C (73 °F) in Patna to 26 °C (79 °F) in Kolkata (Calcutta); lows average from 8 °C (46 °F) in Patna to 14 °C (57 °F) in Kolkata. Frigid winds from the Himalayas on occasion depress temperatures near the Brahmaputra River. The two Himalayan states in the east, Sikkim and Arunachal Pradesh, receive substantial snowfall. The extreme north of West Bengal, centered around Darjeeling, also experiences snowfall, but only rarely.
In South India, particularly the hinterland of Maharashtra, Madhya Pradesh, parts of Karnataka, and Andhra Pradesh, somewhat cooler weather prevails. Minimum temperatures in western Maharashtra, Madhya Pradesh and Chhattisgarh hover around 10 °C (50 °F); in the southern Deccan Plateau, they reach 16 °C (61 °F). Coastal areas, especially those near the Coromandel Coast, and low-elevation interior tracts experience warm temperatures, with daily highs of 30 °C (86 °F) and lows of around 21 °C (70 °F). The Western Ghats, including the Nilgiri Range, experience exceptional temperatures with lows sometimes fall below freezing. That compares with a range of 12–14 °C (54–57 °F) on the Malabar Coast where, as with other coastal areas, the Indian Ocean exerts a strong moderating influence on weather.
Summer in northwestern India lasts from April to July, and in the rest of the country from March to June. The temperatures in the north rise as the vertical rays of the Sun reach the Tropic of Cancer. April has the hottest temperatures for the western and southern regions of the country, while May has the hottest temperatures for most of North India. Temperatures of 50 °C (122 °F) and higher have been recorded in parts of India during that season. In cooler regions of North India, immense pre-monsoon squall-line thunderstorms, known locally as "Nor'westers," commonly drop large hailstones. Near the coast the temperature hovers around 36 °C (97 °F), and the proximity of the sea increases the level of humidity. In southern India, the east coast experiences higher temperatures than the west coast by a few degrees.
By May, most of the Indian interior experiences mean temperatures over 32 °C (90 °F), while maximum temperatures often exceed 40 °C (104 °F). In the hot months of April and May, western disturbances, with their cooling influence, may still arrive, but rapidly diminish in frequency as summer progresses. Notably, a higher frequency of such disturbances in April correlates with a delayed monsoon onset (thus extending summer) in northwest India. In eastern India, monsoon onset dates have been steadily advancing over the past several decades, resulting in shorter summers there.
Altitude affects the temperature to a large extent, with higher parts of the Deccan Plateau and other areas being relatively cooler. Hill stations, such as Ootacamund ("Ooty") in the Western Ghats and Kalimpong in the eastern Himalayas, with average maximum temperatures of around 25 °C (77 °F), offer some respite from the heat. At lower elevations, in parts of northern and western India, a strong, hot, and dry wind known as the Loo blows in from the west during the daytime; with very high temperatures, in some cases up to around 45 °C (113 °F); it can cause fatal cases of sunstroke. Tornadoes occur rarely, concentrated in a corridor stretching from northeastern India towards Pakistan; only several dozen have been reported since 1835.
The southwest summer monsoon, a four-month period when massive convective thunderstorms dominate India's weather, constitutes the Earth's most valuable wet season. Resulting from the southeast trade winds originating from a high-pressure mass centered over the southern Indian Ocean, a low-pressure region centered over South Asia attracts monsoons. They give rise to surface winds that ferry humid air into India from the southwest. Those inflows ultimately result from a northward shift of the local jet stream, which itself results from rising summer temperatures over Tibet and the Indian subcontinent. The void left by the jet stream, which switches from a route just south of the Himalayas to one tracking north of Tibet, then attracts warm, humid air.
The high summer temperature difference between Central Asia and the Indian Ocean embodies the main factor behind that shift. Accompanied by a seasonal excursion of the normally equatorial intertropical convergence zone (ITCZ), a low-pressure belt of highly unstable weather moves northward towards India. That system intensified to its present strength as a result of the Tibetan Plateau's uplift, accompanying the Eocene–Oligocene transition event, a major episode of global cooling and aridification occurring 34–49 mya.
The southwest monsoon arrives in two branches: the Bay of Bengal branch and the Arabian Sea branch. The latter extends toward a low-pressure area over the Thar Desert, measuring roughly three times stronger than the Bay of Bengal branch. The monsoon usually breaks over Indian territory by around May 25, when it lashes the Andaman and Nicobar Islands in the Bay of Bengal. It strikes the Indian mainland around June 1, supplies over 80 percent of India's annual rainfall. first appearing near the Malabar Coast of Kerala. By June 9, it reaches Mumbai; it appears over Delhi by June 29. The Bay of Bengal branch, which hugs the Coromandal Coast between Cape Comorin and Orissa, swerves to the northwest. The Arabian Sea branch moves northeast towards the Himalayas. By the first week of July, the entire country experiences monsoon rain; on average, South India receives more rainfall than North India. Yet Northeast India receives the most precipitation. Monsoon clouds begin retreating from North India by the end of August; it withdraws from Mumbai by October 5. As India further cools during September, the southwest monsoon weakens. By the end of November, it has left the country.
Monsoon rains impact the health of the Indian economy; as Indian agriculture employs 600 million people and comprises 20 percent of the national GDP, good monsoons correlate with a booming economy. Weak or failed monsoons (droughts) result in widespread agricultural losses and substantially hinder overall economic growth. The rains reduce temperatures and replenish groundwater tables, rivers, and lakes.
During the post-monsoon months of October to December, a different monsoon cycle, the northeast (or "retreating") monsoon, brings dry, cool, and dense Central Asian air masses to large parts of India. Winds spill across the Himalayas and flow to the southwest across the country, resulting in clear, sunny skies. Though the India Meteorological Department (IMD) and other sources refers to that period as a fourth ("post-monsoon") season, other sources designate only three seasons. Depending on location, that period lasts from October to November, after the southwest monsoon has peaked. Less and less precipitation falls, and vegetation begins to dry out. In most parts of India, that period marks the transition from wet to dry seasonal conditions. Average daily maximum temperatures range between 28 °C and 34 °C (82–93 °F).
The northeast monsoon, which begins in September, lasts through the post-monsoon seasons, and only ends in March, carries winds that have already lost their moisture while crossing central Asia and the vast rain shadow region lying north of the Himalayas. They cross India diagonally from northeast to southwest. The large indentation made by the Bay of Bengal into India's eastern coast means that the flows humidify before reaching Cape Comorin and rest of Tamil Nadu, meaning that the state, and also some parts of Kerala, experience significant precipitation in the post-monsoon and winter periods. Parts of West Bengal, Orissa, Andhra Pradesh, Karnataka and North-East India also receive minor precipitation from the northeast monsoon.
Temperature and precipitation data for selected Indian cities, representing the full variety of major Indian climate types, follow. Figures have been grouped by the four-season classification scheme used by the IMD,[a] along with year-round averages and totals.
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