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[[File:Agasthiyamalai range and Tirunelveli rainshadow.jpg|thumb|right|alt=Daytime view of an almost lifeless expanse, dry rocks and sand marked only by the odd lone shrub. The dry terrain reaches to a chain of mountains in the far distance, near the horizon. A bank of clouds soars above the void, but it does not appear to hold the promise of rain. A far darker, larger, more turgid cloud bank sits above the distant mountains, above the horizon.|A semi-[[arid]] wasteland near [[Tirunelveli]], [[Tamil Nadu]]. Monsoon clouds dump torrents of rain on lush forests that are only kilometres away in [[windward and leeward|windward]]-facing [[Kerala]], but are [[rain shadow|prevented]] from reaching Tirunelveli by the [[Agastya Malai|Agasthyamalai Range]] of the Western Ghats (background).]]
Lacy!![[File:Agasthiyamalai range and Tirunelveli rainshadow.jpg|thumb|right|alt=Daytime view of an almost lifeless expanse, dry rocks and sand marked only by the odd lone shrub. The dry terrain reaches to a chain of mountains in the far distance, near the horizon. A bank of clouds soars above the void, but it does not appear to hold the promise of rain. A far darker, larger, more turgid cloud bank sits above the distant mountains, above the horizon.|A semi-[[arid]] wasteland near [[Tirunelveli]], [[Tamil Nadu]]. Monsoon clouds dump torrents of rain on lush forests that are only kilometres away in [[windward and leeward|windward]]-facing [[Kerala]], but are [[rain shadow|prevented]] from reaching Tirunelveli by the [[Agastya Malai|Agasthyamalai Range]] of the Western Ghats (background).]]
[[File:Valley of flowers uttaranchal full view.JPG|thumb|right|alt=Daytime view of a montaine valley set between two peaks at left and right, their peaks cloaked by cloud cover. All present an aspect of summer verdure: grassland meadows and wildflowers. Rocky snow-capped peaks are seen in the far distance, between the flanking peaks in the middle distance.|A scene in [[Uttarakhand]]'s [[Valley of Flowers National Park]]. In contrast to Tirunelveli, the park receives ample [[orographic lift|orographic precipitation]] due to its location in a mountainous [[windward and leeward|windward]]-facing region wedged between the [[Zanskar Range|Zanskars]] and the [[Great Himalayas|Greater Himalayas]].]]
[[File:Valley of flowers uttaranchal full view.JPG|thumb|right|alt=Daytime view of a montaine valley set between two peaks at left and right, their peaks cloaked by cloud cover. All present an aspect of summer verdure: grassland meadows and wildflowers. Rocky snow-capped peaks are seen in the far distance, between the flanking peaks in the middle distance.|A scene in [[Uttarakhand]]'s [[Valley of Flowers National Park]]. In contrast to Tirunelveli, the park receives ample [[orographic lift|orographic precipitation]] due to its location in a mountainous [[windward and leeward|windward]]-facing region wedged between the [[Zanskar Range|Zanskars]] and the [[Great Himalayas|Greater Himalayas]].]]


Revision as of 20:57, 18 March 2010

Lacy!!

Daytime view of an almost lifeless expanse, dry rocks and sand marked only by the odd lone shrub. The dry terrain reaches to a chain of mountains in the far distance, near the horizon. A bank of clouds soars above the void, but it does not appear to hold the promise of rain. A far darker, larger, more turgid cloud bank sits above the distant mountains, above the horizon.
A semi-arid wasteland near Tirunelveli, Tamil Nadu. Monsoon clouds dump torrents of rain on lush forests that are only kilometres away in windward-facing Kerala, but are prevented from reaching Tirunelveli by the Agasthyamalai Range of the Western Ghats (background).
Daytime view of a montaine valley set between two peaks at left and right, their peaks cloaked by cloud cover. All present an aspect of summer verdure: grassland meadows and wildflowers. Rocky snow-capped peaks are seen in the far distance, between the flanking peaks in the middle distance.
A scene in Uttarakhand's Valley of Flowers National Park. In contrast to Tirunelveli, the park receives ample orographic precipitation due to its location in a mountainous windward-facing region wedged between the Zanskars and the Greater Himalayas.

The climate of India defies easy generalisation, comprising a wide range of weather conditions across a large geographic scale and varied topography. 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; this is particularly true of 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 is kept 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, the whole country is considered to be tropical.

As in much of the tropics, monsoonal and other weather conditions in India are unstable: major droughts, floods, cyclones and other natural disasters are sporadic, but have killed or displaced millions. India's long-term climatic stability may be further threatened by global warming[citation needed].

History

Daytime view of a chain of snow-capped mountains. They advance diagonally back from the far-middle distance at left to the far distance at right. In the foreground are high montaine meadows and brushland.
The formation of the Himalayas (pictured) during the Early Eocene some 53 million years ago was a key factor in determining India's modern-day climate; global climate and ocean chemistry may have been impacted.[1]

During the Late Permian (some 260–251 Ma), the Indian subcontinent was part of 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 Greenland and parts of the Antarctic Peninsula, India likely experienced a humid temperate climate with warm, frost-free weather, though with well-defined seasons.[2] Later, India joined the southern supercontinent Gondwana, a process beginning some 550–500 Ma. 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) was positioned, resulting in a mild climate favourable to hosting high-biomass ecosystems. This is underscored by India's vast coal reserves—much of it from the late Paleozoic sedimentary sequence—the fourth-largest reserves in the world.[3] During the Mesozoic, the world, including India, was considerably warmer than today. With the coming of the Carboniferous, global cooling stoked extensive glaciation, which spread northwards from South Africa towards India; this cool period lasted well into the Permian.[4]

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. This resulted in a massive flood basalt event that laid down the Deccan Traps some 60–68 Ma,[5][6] at the end of the Cretaceous period. This 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 warmer weather 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, were severe enough to cause the extinction of many endemic Indian forms.[7] The formation of the Himalayas resulted in blockage of frigid Central Asian air, preventing it from reaching India; this made its climate significantly warmer and more tropical in character than it would otherwise have been.[8]

Regions

Main article: Climatic regions of India
"India Average Temperature Map": A map of India overlaid with five zones. A violet zone, with ambient temperatures averaging less than 20.0 degrees Celsius, envelops Himalayan and trans-Himalayan India, as well as the Khasi Hills on the Meghalaya Plateau. A transitionary blue zone of between 20.0 and 22.5 degrees, lies just south of the violet areas; another blue area lies in the extreme southwest of the country, focused on the high Western Ghats. Two contiguous green areas (averaging 22.5 to 25.0 degrees) envelop the blue regions, with the northern one snaking into the Vindhya Range of central India. The remaining yellow and red areas, designating average temperatures above 25.0 degrees Celsius, constitute by far the greater part of the country.
Average annual temperatures across India:

  Below 20.0 °C
 (< 68.0 °F)
  20.0–22.5 °C
 (68.0–72.5 °F)
  22.5–25.0 °C
 (72.5–77.0 °F)
  25.0–27.5 °C
 (77.0–81.5 °F)
  Above 27.5 °C
 (> 81.5 °F)
"India Climatic Zone Map".
Climatic zones in India, based on the Köppen classification system:

  Alpine
 E (ETh)
  Humid subtropical
 C (Cfa)
  Tropical wet and dry
 A (Aw)
  Tropical wet
 A (Am)
  Semi-arid
 B (BSh)
  Arid
 B (BWh)

India is 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 nation's climate is strongly influenced by the Himalayas and the Thar Desert.[9] 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.[10] 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.[9][11] Four major climatic groupings predominate, into which fall seven climatic zones that, as designated by experts, are defined on the basis of such traits as temperature and precipitation.[12] Groupings are assigned codes (see chart) according to the Köppen climate classification system.

Tropical wet

A tropical rainy climate covers regions experiencing persistent warm or high temperatures, which normally do not fall below 18 °C (64 °F). India hosts two climatic subtypes that fall under this group. The most humid is the tropical wet climate (also known as a tropical monsoon climate) that covers a strip of southwestern lowlands abutting the Malabar Coast, the Western Ghats, and southern Assam. India's two island territories, Lakshadweep and the Andaman and Nicobar Islands, are also subject to this climate. Characterised by moderate to high year-round temperatures, even in the foothills, its rainfall is seasonal but heavy—typically above 2,000 millimetres (79 in) per year.[13] Most rainfall occurs between May and November; this is adequate for the maintenance of lush forests and other vegetation throughout the remainder of the year. December to March are the driest months, when days with precipitation are rare. The heavy monsoon rains are responsible for the extremely biodiverse tropical wet forests of these regions.

In India, a tropical wet and dry climate is more common. Noticeably drier than areas with a tropical monsoon climate, it prevails over most of inland peninsular India except for a semi arid rain shadow east of the Western Ghats. Winter and early summer are long, dry periods with temperatures averaging above 18 °C (64 °F). Summer is exceptionally hot; temperatures in low-lying areas may exceed 50 °C (122 °F) during May, leading to heat waves that can each kill hundreds of Indians.[14] The rainy season lasts from June to September; annual rainfall averages between 750–1,500 millimetres (29.5–59.1 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.[15]

Tropical 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. The region, which includes Karnataka, inland Tamil Nadu, western Andhra Pradesh, and central Maharashtra, gets between 400–750 millimetres (15.7–29.5 in) annually. It is drought-prone, as it tends to have less reliable rainfall due to sporadic lateness or failure of the southwest monsoon.[16] North of the Krishna River, the summer monsoon is responsible for most 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). The months between March to May are hot and dry; mean monthly temperatures hover around 32 °C, with 320 millimetres (13 in) precipitation. Hence, without artificial irrigation, this region is not suitable for permanent agriculture.

View of dunes at sunset. The setting sun hangs low and dim in a sky graded from fiery orange at top to maroon near the horizon. Terrestrial features are difficult to discern in the crepuscular illumination.
The Thar Desert.

Most of western Rajasthan experiences an arid climatic regime. Cloudbursts are responsible for virtually all of the region's annual precipitation, which totals less than 300 millimetres (11.8 in). Such bursts happen when monsoon winds sweep into the region during July, August, and September. Such rainfall is highly erratic; regions experiencing rainfall one year may not see precipitation for the next couple of years or so. Atmospheric moisture is largely prevented from precipitating due to continuous downdrafts and other factors.[17] The summer months of May and June are 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. There is a large diurnal range of about 14 °C (25.2 °F) during summer; this 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 that are less extreme than those of the desert. Average annual rainfall is 300–650 millimetres (11.8–25.6 in), but is 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.

Subtropical humid

Most of Northeast India and much of North India are subject to a humid subtropical 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 this climate, Cfa (under the Köppen system).[18] In most of this region, there is very little precipitation during the winter, owing to powerful anticyclonic and katabatic (downward-flowing) winds from Central Asia.

Humid subtropical regions are subject to pronounced dry winters. Winter rainfall—and occasionally snowfall—is associated with large storm systems such as "Nor'westers" and "Western disturbances"; the latter are steered by westerlies towards the Himalayas.[19] 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 millimetres (39 in) in the west to over 2,500 millimetres (98 in) in parts of the northeast. As most of this region is far from the ocean, the wide temperature swings more characteristic of a continental climate predominate; the swings are wider 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.

Montane

Daytime view of a large body of water standing before a prominent peak, which communicates with several others partly out of view and behind. A gravel beach at the far end of the lake gives way to steep slopes leading up to the peaks; The mountains lack trees. Patchy snowcover defines their recesses, and whitish vein-like streaks extend up from the base of the largest.
Pangong Lake in Ladakh, an arid montane region lying deep within the Himalayas.

India's northernmost areas are subject to 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 dry adiabatic lapse rate) is 9.8 °C/km.[20] In terms of environmental lapse rate, ambient temperatures fall by 6.5 °C (11.7 °F) for every 1,000 metres (3,281 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 are also common. The northern side of the western Himalayas, also known as the trans-Himalayan belt, is 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 are largely protected 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 metres (7,513 ft). The Himalayas experience their heaviest snowfall between December and February and at elevations above 1,500 metres (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 metres (16,404 ft) never experience rain; all precipitation falls as snow.[21]

Seasons

The India Meteorological Department (IMD) designates four official seasons:[22]

Daytime view of a small marshy lake surrounded by dry brush in winter. Behind the lake in the far-middle distance, a large stand of trees interposes itself in front of a lone massif in the far distance. The sun is out of view to the left.
A winter scene in Bandhavgarh National Park, Madhya Pradesh.
  • Winter, occurring from December to early 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. South India typically receives more precipitation.
  • Post-monsoon season, lasting from October to December. In northwestern India, October and November are usually cloudless. Tamil Nadu receives most of its annual precipitation in the northeast monsoon season.

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 (grīṣma), monsoon season (varṣā), early autumn (śarada), late autumn (hemanta), and winter (śiśira). These are based on the astronomical division of the twelve months into six parts. The ancient Hindu calendar also reflects these seasons in its arrangement of months.

Winter

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 (1.08 °F) per degree of latitude. December and January are the coldest months, with mean temperatures of 10–15 °C (50–59 °F) in Indian Himalayas. Mean temperatures are higher in the east and south, where they reach 20–25 °C (68–77 °F).

Daytime view looking down from a snowy ridge onto a mountain valley far below, lost in mist. Continuing into the far distance at right, a series of high snow-covered mountains continue the ridge. The mountains are mostly covered in evergreen forest; dappled sunlight strikes the snow cover.
Inclement conditions in the Indian Himalayas: a view of Gulmarg, a popular tourist destination in Jammu and Kashmir in winter.

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 °C (28.8 °F) – 20 °C (36.0 °F). However, from March to May, "western disturbances" bring heavy bursts of rain and snow. These extra-tropical low-pressure systems originate in the eastern Mediterranean Sea.[23] They are carried towards India by the subtropical westerlies, which are the prevailing winds blowing at North India's range of latitude.[19] Once their passage is hindered by the Himalayas, they are unable to proceed further, and they release significant precipitation over the southern Himalayas.[24] 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. However, 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 hallmark of the season is the notorious fog, which frequently disrupts daily life; fog grows thick enough to hinder visibility and disrupt air travel 15–20 days annually.[25] Eastern India's climate is much milder, 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 can depress temperatures near the Brahmaputra River.[26] The two Himalayan states in the east, Sikkim and Arunachal Pradesh, receive substantial snowfall. The extreme north of West Bengal, centred 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 Template:C to F; in the southern Deccan Plateau, they reach Template:C to F. Coastal areas, especially those near the Coromandel Coast, and low-elevation interior tracts are warm, with daily high temperatures of Template:C to F and lows of around Template:C to F. The Western Ghats, including the Nilgiri Range, are exceptional; there, lows can fall below freezing.[27] This compares with a range of 12 °C (21.6 °F) – 14 °C (25.2 °F) on the Malabar Coast; there, as is the case for other coastal areas, the Indian Ocean exerts a strong moderating influence on weather.[10]

Summer

Daytime sunlit view of a verdant bowl-shaped meadow with a small pond in a valley rimmed by several prominent hills covered in evergreen forest. At center-left and looking out on the meadow, a small cluster of tourist lodges with pitched roofs, one a bright red. Around them and the pond are small, gaily coloured gazebos and shade-giving umbrellas. Several dozen tourists and sheep mill about the pond and in the meadow.
A summer view of Khajjiar, a hill station in Himachal Pradesh.

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. The hottest month for the western and southern regions of the country is April; for most of North India, it is May. Temperatures of 50 °C (122 °F) and higher have been recorded in parts of India during this season.[14] In cooler regions of North India, immense pre-monsoon squall-line thunderstorms, known locally as "Nor'westers", commonly drop large hailstones.[28] 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 temperatures are higher on the east coast by a few degrees compared to the west coast.

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.[29] 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.[19]

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 may also occur, concentrated in a corridor stretching from northeastern India towards Pakistan. They are rare, however; only several dozen have been reported since 1835.[30]

Monsoon

"India Monsoon Onset Map".
Onset dates and prevailing wind currents of the southwest summer and northeast winter monsoons.
"India Average Annual Rainfall Map": A map of India overlaid with various zones of differing shades of blue. Each shade represents a region receiving a similar annual precipitation total. The wettest region comprises the northeastern "Seven Sisters" states centered on Assam; the southwestern littoral in Kerala, Karnataka, Goa, and Maharashtra is another wet region of over 250 centimetres per annum, depicted in a dark shade of navy blue. The rest of the country in between them is shown in lighter shades; the driest region is seen in the northwest near the borders with Pakistan and China, parts of which, according to the legend, are seen to obtain less than 20 centimetres per annum.
Regional variation in rainfall across India. The monsoon season delivers four-fifths of the country's precipitation.

The southwest summer monsoon, a four-month period when massive convective thunderstorms dominate India's weather, is Earth's most productive wet season.[31] A product of southeast trade winds originating from a high-pressure mass centered over the southern Indian Ocean, the monsoonal torrents supply over 80% of India's annual rainfall.[32] Attracted by a low-pressure region centered over South Asia, the mass spawns surface winds that ferry humid air into India from the southwest.[33] These 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.[34]

The main factor behind this shift is the high summer temperature difference between Central Asia and the Indian Ocean.[35] This is accompanied by a seasonal excursion of the normally equatorial intertropical convergence zone (ITCZ), a low-pressure belt of highly unstable weather, northward towards India.[34] This system intensified to its present strength as a result of the Tibetan Plateau's uplift, which accompanied the Eocene–Oligocene transition event, a major episode of global cooling and aridification which occurred 34–49 Ma.[36]

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 and is roughly three times stronger than the Bay of Bengal branch. The monsoon typically breaks over Indian territory by around 25 May, when it lashes the Andaman and Nicobar Islands in the Bay of Bengal. It strikes the Indian mainland around 1 June near the Malabar Coast of Kerala.[37] By 9 June, it reaches Mumbai; it appears over Delhi by 29 June. The Bay of Bengal branch, which initially tracks the Coromandal Coast northeast from Cape Comorin to Orissa, swerves to the northwest towards the Indo-Gangetic Plain. 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. However, Northeast India receives the most precipitation. Monsoon clouds begin retreating from North India by the end of August; it withdraws from Mumbai by 5 October. As India further cools during September, the southwest monsoon weakens. By the end of November, it has left the country.[34]

Daytime view over a city: de-laminating concrete housing blocks surrounded by rough slum tenements. In the middle distance, an expanse of trees: perhaps a park. Near the horizon, the largely concrete structures that compose the city continue. The whole is enveloped by an ominous sky filled with storm clouds promising imminent heavy rain.
Pre-monsoon clouds, as they appear in Mumbai, western Maharashtra.

Monsoon rains impact the health of the Indian economy; as Indian agriculture employs 600 million people and composes 20% of the national GDP,[38] good monsoons correlate with a booming economy. Weak or failed monsoons (droughts) result in widespread agricultural losses and substantially hinder overall economic growth.[39][40][41] The rains reduce temperatures and replenish groundwater tables, rivers, and lakes.

Post-monsoon

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.[42] Though the India Meteorological Department (IMD) and other sources refers to this period as a fourth ("post-monsoon") season,[22][43][44] other sources designate only three seasons.[45] Depending on location, this 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, this period marks the transition from wet to dry seasonal conditions. Average daily maximum temperatures range between 28 and 34 °C (82 and 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. However, the large indentation made by the Bay of Bengal into India's eastern coast means that the flows are humidified 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.[15] However, parts of West Bengal, Orissa, Andhra Pradesh, Karnataka and North-East India also receive minor precipitation from the northeast monsoon.[46]

Statistics

Shown below are temperature and precipitation data for selected Indian cities; these represent the full variety of major Indian climate types. Figures have been grouped by the four-season classification scheme used by the IMD;[α] year-round averages and totals are also displayed.

Temperature

Average temperatures in various Indian cities (°C)[47][48][49][50]
— Winter
(Jan – Feb) 		Summer
(Mar – May) 		Monsoon
(Jun – Sep) 		Post-monsoon
(Oct – Dec) 		Year-round
City Min Avg Max Min Avg Max Min Avg Max Min Avg Max Avg
Port Blair 23 26 28 25 27 29 25 27 27 25 26 28 27
Thiruvananthapuram 23 26 29 24 27 30 28 26 24 29 26 23 26
Bangalore 7 12 18 13 18 23 15 19 23 8 13 18 17
Nagpur 14 21 28 24 32 40 24 27 30 16 22 28 26
Bhopal 13 18 24 23 30 36 23 26 28 16 22 26 25
Guwahati 11 17 24 19 25 31 25 28 32 17 22 27 24
Lucknow 10 15 21 23 30 35 24 29 33 15 20 25 25
Jaisalmer 7 14 23 24 33 40 23 29 35 12 19 27 22
Dehradun 4 12 20 14 23 32 22 26 30 7 15 23 18
Amritsar 4 10 18 13 25 34 25 28 32 10 16 24 21
Shimla 1 5 9 10 14 18 15 18 20 7 10 13 13
Srinagar −2 4 6 7 14 19 16 22 30 1 8 16 13
Leh −13 −6 0 −1 6 12 10 16 24 −7 0 7 6
Leh
Climate chart (explanation)
J
F
M
A
M
J
J
A
S
O
N
D
 
 
12
 
 
−1
−13
 
 
8.6
 
 
0
−12
 
 
12
 
 
6
−6
 
 
6.5
 
 
12
−1
 
 
6.5
 
 
17
2
 
 
4.3
 
 
21
6
 
 
16
 
 
24
10
 
 
20
 
 
24
10
 
 
12
 
 
21
5
 
 
7.1
 
 
14
0
 
 
2.9
 
 
8
−6
 
 
8
 
 
1
−10
█ Average max. and min. temperatures in °C
█ Precipitation totals in mm
Source: Weatherbase, Plan Holidays
Imperial conversion
JFMAMJJASOND
 
 
0.5
 
 
30
9
 
 
0.3
 
 
32
10
 
 
0.5
 
 
43
21
 
 
0.3
 
 
54
30
 
 
0.3
 
 
63
36
 
 
0.2
 
 
70
43
 
 
0.6
 
 
75
50
 
 
0.8
 
 
75
50
 
 
0.5
 
 
70
41
 
 
0.3
 
 
57
32
 
 
0.1
 
 
46
21
 
 
0.3
 
 
34
14
█ Average max. and min. temperatures in °F
█ Precipitation totals in inches

Precipitation

Average precipitation in various Indian cities (mm)[47][48][49]
— Winter
(Jan – Feb) 		Summer
(Mar – May) 		Monsoon
(Jun – Sep) 		Post-monsoon
(Oct – Dec) 		Year-round
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total
Port Blair 40 20 10 60 360 480 400 400 460 290 220 150 2,890
Thiruvananthapuram 26 21 33 125 202 306 175 152 179 223 206 65 1,713
Bangalore 31 20 61 110 150 212 249 279 315 291 210 140 1,962
Nagpur 16 22 15 8 18 168 290 291 157 73 17 19 1,094
Bhopal 4 3 1 3 11 136 279 360 185 52 21 7 1,043
Guwahati 8 21 47 181 226 309 377 227 199 92 25 10 1,722
Lucknow 20 18 8 8 20 114 305 292 188 33 5 8 1,019
Jaisalmer – – 3 – 7 10 90 88 15 – 6 – 219
Dehradun 47 55 52 21 54 230 631 627 261 32 11 3 2,024
Amritsar 24 33 48 30 45 27 231 187 79 18 6 18 746
Shimla 60 60 60 50 60 170 420 430 160 30 10 20 1,530
Srinagar 74 71 91 94 61 36 58 61 38 31 10 33 658
Leh 12 9 12 6 7 4 16 20 12 7 3 8 116
Bangalore
Climate chart (explanation)
J
F
M
A
M
J
J
A
S
O
N
D
 
 
3
 
 
27
15
 
 
7
 
 
30
17
 
 
4
 
 
32
19
 
 
46
 
 
34
22
 
 
120
 
 
33
21
 
 
81
 
 
29
20
 
 
110
 
 
28
20
 
 
137
 
 
27
19
 
 
195
 
 
28
19
 
 
180
 
 
28
19
 
 
65
 
 
27
17
 
 
21
 
 
26
16
█ Average max. and min. temperatures in °C
█ Precipitation totals in mm
Source: World Weather Information Service
Imperial conversion
JFMAMJJASOND
 
 
0.1
 
 
81
59
 
 
0.3
 
 
86
63
 
 
0.2
 
 
90
66
 
 
1.8
 
 
93
72
 
 
4.7
 
 
91
70
 
 
3.2
 
 
84
68
 
 
4.3
 
 
82
68
 
 
5.4
 
 
81
66
 
 
7.7
 
 
82
66
 
 
7.1
 
 
82
66
 
 
2.6
 
 
81
63
 
 
0.8
 
 
79
61
█ Average max. and min. temperatures in °F
█ Precipitation totals in inches

Disasters

Main article: Natural disasters in India
"India Climatic Disasters Risk Map".
Disaster-prone regions in India.
"India Wind Zone Map": A map of India overlaid with zones of various shades of blue, each representing a region that experiences a similar level of windiness. The entire eastern littoral and the northern half of the country are shaded in relatively dark blues, signifying relatively windy prevailing conditions of between 30 and 50 metres per second. The darkest blue region is in the extreme north, beyond the Himalayas in Ladakh, on the Tibetan Plateau; there, sustained winds average over 50 metres per second. The inland central, south, and especially the southwestern portions are shaded in light blues: they are relatively windless, averaging less than 30 metres per second.
Map showing winds zones, shaded by distribution of average speeds of prevailing winds.

Climate-related natural disasters cause massive losses of Indian life and property. Droughts, flash floods, cyclones, avalanches, landslides brought on by torrential rains, and snowstorms pose the greatest threats. Other dangers include frequent summer dust storms, which usually track from north to south; they cause extensive property damage in North India[51] and deposit large amounts of dust from arid regions. Hail is also common in parts of India, causing severe damage to standing crops such as rice and wheat.

Floods and landslides

See also: 2007 South Asian floods and 2008 Indian floods

In the Lower Himalaya, landslides are common. The young age of the region's hills result in labile rock formations, which are susceptible to slippages. Rising population and development pressures, particularly from logging and tourism, cause deforestation. The result, denuded hillsides, exacerbates the severity of landslides, since tree cover impedes the downhill flow of water.[52] Parts of the Western Ghats also suffer from low-intensity landslides. Avalanches occur in Kashmir, Himachal Pradesh, and Sikkim.

Floods are the most common natural disaster in India. The heavy southwest monsoon rains cause the Brahmaputra and other rivers to distend their banks, often flooding surrounding areas. Though they provide rice paddy farmers with a largely dependable source of natural irrigation and fertilisation, the floods can kill thousands and displace millions. Excess, erratic, or untimely monsoon rainfall may also wash away or otherwise ruin crops.[53][54] Almost all of India is flood-prone, and extreme precipitation events, such as flash floods and torrential rains, have become increasingly common in central India over the past several decades, coinciding with rising temperatures. Mean annual precipitation totals have remained steady due to the declining frequency of weather systems that generate moderate amounts of rain.[55]

Cyclones

Satellite view looking over the Bay of Bengal, deliminated by India to its west, Bangladesh to its north, and Indochina to its west. Various patches of substantial storm tracts define the southern margins of the shot, but a larger buzzsaw-shaped whorl of clouds sits above the northern Bay, just beginning to communicate with parts of Orissa, West Bengal, and Bangladesh.
Satellite imagery of Cyclone 05B in the Bay of Bengal.

Tropical cyclones, which are severe storms spun off from the Intertropical Convergence Zone, may affect thousands of Indians living in coastal regions. Tropical cyclogenesis is particularly common in the northern reaches of the Indian Ocean in and around the Bay of Bengal. Cyclones bring with them heavy rains, storm surges, and winds that often cut affected areas off from relief and supplies. In the North Indian Ocean Basin, the cyclone season runs from April to December, with peak activity between May and November.[56] Each year, an average of eight storms with sustained wind speeds greater than 63 km/h (39 mph) form; of these, two strengthen into true tropical cyclones, which have sustained gusts greater than 117 km/h (73 mph). On average, a major (Category 3 or higher) cyclone develops every other year.[57][58]

During summer, the Bay of Bengal is subject to intense heating, giving rise to humid and unstable air masses that morph into cyclones. The 1737 Calcutta cyclone, the 1970 Bhola cyclone, and the 1991 Bangladesh cyclone rank among the most powerful cyclones to strike India, devastating the coasts of eastern India and neighboring Bangladesh. Widespread death and property destruction are reported every year in the exposed coastal states of West Bengal, Orissa, Andhra Pradesh, and Tamil Nadu. India's western coast, bordering the more placid Arabian Sea, experiences cyclones only rarely; these mainly strike Gujarat and, less frequently, Kerala.

Cyclone 05B, a supercyclone that struck Orissa on 29 October 1999, was the deadliest in more than a quarter-century. With peak winds of 160 miles per hour (257 km/h), it was the equivalent of a Category 5 hurricane.[59] Almost two million people were left homeless;[60] another 20 million people lives were disrupted by the cyclone.[60] Officially, 9,803 people died from the storm;[59] unofficial estimates place the death toll at over 10,000.[60]

Droughts

Main article: Drought in India
A daytime view of a desiccated, scappled expanse that wholly fills the fore- and middle ground and arcs away to the horizon at left. Of clouds in the sky there are none whatever. At extreme right in the middle distance, a thin bank of trees sits on a slightly higher level, reaching to the middle and rightward horizon; people and livestock mill about before them. Several pink buildings, perhaps a village, sit similarly elevated in front of the trees at middle-right, beyond the dominating dusty expanse.
The dry bed of the Niranjana River, Bihar.

Indian agriculture is heavily dependent on the monsoon as a source of water. In some parts of India, the failure of the monsoons result in water shortages, resulting in below-average crop yields. This is particularly true of major drought-prone regions such as southern and eastern Maharashtra, northern Karnataka, Andhra Pradesh, Orissa, Gujarat, and Rajasthan. In the past, droughts have periodically led to major Indian famines. These include the Bengal famine of 1770, in which up to one third of the population in affected areas died; the 1876–1877 famine, in which over five million people died; the 1899 famine, in which over 4.5 million died; and the Bengal famine of 1943, in which over five million died from starvation and famine-related illnesses.[61][62]

All such episodes of severe drought correlate with El Niño-Southern Oscillation (ENSO) events.[63][64] El Niño-related droughts have also been implicated in periodic declines in Indian agricultural output.[65] Nevertheless, ENSO events that have coincided with abnormally high sea surfaces temperatures in the Indian Ocean—in one instance during 1997 and 1998 by up to 3 °C (5 °F)—have resulted in increased oceanic evaporation, resulting in unusually wet weather across India. Such anomalies have occurred during a sustained warm spell that began in the 1990s.[66] A contrasting phenomenon is that, instead of the usual high pressure air mass over the southern Indian Ocean, an ENSO-related oceanic low pressure convergence center forms; it then continually pulls dry air from Central Asia, desiccating India during what should have been the humid summer monsoon season. This reversed air flow causes India's droughts.[67] The extent that an ENSO event raises sea surface temperatures in the central Pacific Ocean influences the degree of drought.[63]

Extremes

Daytime view of a more-or-less cone-shaped peak, perhaps reaching two hundred feet above the surrounding terrain. It is sparsely covered in poor-looking shrubs and trees; it otherwise reveals only an aspect of heated naked rock. Assorted parti-coloured blocky concrete buildings reach down from its lower slopes to a temple tank in the near foreground, around which is arrayed the more rounded and ornate temple structures.
Alwar, on the fringes of the Thar Desert, registered a temperature of 50.6 °C (123.1 °F), India's highest.

India's lowest recorded temperature was −45 °C (−49 °F) in Dras, Ladakh, in eastern Jammu and Kashmir; however, the reading was taken with non-standard equipment. Readings as low as −30.6 °C (−23.1 °F) have been taken in Leh, further south in Ladakh. However, temperatures on the Indian-controlled Siachen Glacier near Bilafond La (5,450 metres (17,881 ft)*) and Sia La (5,589 metres (18,337 ft)*) have fallen below −55 °C (−67 °F),[68] while blizzards bring wind speeds in excess of 250 km/h (155 mph),[69] or hurricane-force winds ranking at 12 (the maximum) on the Beaufort scale. These conditions, not hostile actions, caused more than 97% of the roughly 15,000 casualties suffered by India and Pakistan during conflict in the region.[68][69][70] The highest reliable temperature reading was 50.6 °C (123.1 °F) in Alwar, Rajasthan in 1955. The India Meteorological Department (IMD) doubts the validity of 55 °C (131 °F) readings in Orissa from 2005.[71]

The average annual precipitation of 11,871 millimetres (467 in) in the village of Mawsynram, in the hilly northeastern state of Meghalaya, is the highest recorded in Asia, and possibly on Earth.[72] The village, which sits at an elevation of 1,401 metres (4,596 ft), benefits from its proximity to both the Himalayas and the Bay of Bengal. However, since the town of Cherrapunji, 5 kilometres (3.1 mi) to the east, is the nearest town to host a meteorological office (none has ever existed in Mawsynram), it is officially credited as being the world's wettest place.[73] In recent years, the Cherrapunji-Mawsynram region has averaged between 9,296 and 10,820 millimetres (366 and 426 in) [8] of rain annually, though Cherrapunji has had at least one period of daily rainfall that lasted almost two years.[74] India's highest recorded one-day rainfall total occurred on 26 July 2005, when Mumbai received more than 650 millimetres (25.6 in);[75] the massive flooding that resulted killed over 900 people.[76][77]

Remote regions of Jammu and Kashmir, such as Baramulla district in the east and the Pir Panjal Range in the southeast, experience exceptionally heavy snowfall. Kashmir's highest recorded monthly snowfall occurred in February 1967, when 8.4 metres (28 ft) fell in Gulmarg, though the IMD has recorded snowdrifts up to 12 metres (39.4 ft) in several Kashmiri districts. In February 2005, more than 200 people died when, in four days, a western disturbance brought up to 2 metres (7 ft)* of snowfall to parts of the state.[78]

Global warming

Main article: Effects of global warming on India
See also: 2007 South Asian heat wave
Daytime edge-on view of a small forested island amid a shallow turquoise lagoon. The dark sky blue above is streaked by thin, wispy clouds of various sizes. The lone island is evenly low-slung, no part of it reaching any substantial height whatever.
Lakshadweep, comprising tiny low-lying islands, are at risk of being inundated by sea level rises associated with global warming.

Current sea level rise, increased cyclonic activity, increased ambient temperatures, and increasingly fickle precipitation patterns are effects of global warming that have impacted or are projected to impact India. Thousands of people have been deplaced by ongoing sea level rises that have submerged low-lying islands in the Sundarbans.[79] Temperature rises on the Tibetan Plateau are causing Himalayan glaciers to retreat, threatening the flow rate of the Ganges, Brahmaputra, Yamuna, and other major rivers; the livelihoods of hundreds of thousands of farmers depend on these rivers.[80] A 2007 World Wide Fund for Nature (WWF) report states that the Indus River may run dry for the same reason.[81]

Severe landslides and floods are projected to become increasingly common in such states as Assam.[82] Ecological disasters, such as a 1998 coral bleaching event that killed off more than 70% of corals in the reef ecosystems off Lakshadweep and the Andamans, and was brought on by elevated ocean temperatures tied to global warming, are also projected to become increasingly common.[83][84][85]

The Indira Gandhi Institute of Development Research has reported that, if the predictions relating to global warming made by the Intergovernmental Panel on Climate Change come to fruition, climate-related factors could cause India's GDP to decline by up to 9%. Contributing to this would be shifting growing seasons for major crops such as rice, production of which could fall by 40%. Around seven million people are projected to be displaced due to, among other factors, submersion of parts of Mumbai and Chennai, if global temperatures were to rise by a mere 2 °C (3.6 °F).[86] Such shifts are not new. Earlier in the Holocene epoch (4,800–6,300 years ago), parts of what is now the Thar Desert were wet enough to support perennial lakes; researchers have proposed that this was due to much higher winter precipitation, which coincided with stronger monsoons.[87] Kashmir's erstwhile subtropical climate dramatically cooled 2.6–3.7 Ma and experienced prolonged cold spells starting 600,000 years ago.[88]

Atmospheric pollution

Satellite view of the northern portion of South Asia. The arc of a substantial range of mountains, the Himalayas, dips into the shot, then back up out of view. Immediately beneath, a large region of plains is hidden by a continuous cloud-like opaque mass that has collected along the southern margins of the mountains. It proceeds eastward, staying just south of the Himalayas, then bends due south to reach the Bay of Bengal. Two parts of the mass appear particularly dense, showing up as bright white blobs in the shot.
Clouds of thick haze and smoke form along the Ganges River Basin.

Thick haze and smoke, originating from burning biomass in northwestern India[89] and air pollution from large industrial cities in northern India,[90] often concentrate inside the Ganges Basin. Prevailing westerlies carry aerosols along the southern margins of the steep-faced Tibetan Plateau to eastern India and the Bay of Bengal. Dust and black carbon, which are blown towards higher altitudes by winds at the southern margins of the Himalayas, can absorb shortwave radiation and heat the air over the Tibetan Plateau. The net atmospheric heating due to aerosol absorption causes the air to warm and convect upwards, increasing the concentration of moisture in the mid-troposphere and providing positive feedback that stimulates further heating of aerosols.[90]

Notes

     α.   ^ The IMD-designated post-monsoon season coincides with the northeast monsoon, the effects of which are significant only in some parts of India.

References

Late daytime view looking far out over an ocean from a beach, which is out of view off the bottom margin. Three-fourths of the shot features a sky marked by heavy cloud cover, which is parting near the middle, revealing a dazzlingly bright cerulean blue sky that darkens near the margins. The ocean is striated with waves coming in parallel to the horizon.
Late-season monsoon clouds during a sunset over the Coromandel Coast.
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Further reading

  • Toman, MA (2003), India and Global Climate Change: Perspectives on Economics and Policy from a Developing Country, Resources for the Future Press, ISBN 1-8918-5361-9 {{citation}}: Unknown parameter |Given2= ignored (|given2= suggested) (help); Unknown parameter |Given3= ignored (|given3= suggested) (help); Unknown parameter |Last2= ignored (|last2= suggested) (help); Unknown parameter |Last3= ignored (|last3= suggested) (help).
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