User:光合成/印度氣候

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).

The climate of India encompasses a wide range of weather conditions across a vast geographic scale and varied topography, making generalisations difficult. Based on the Köppen system, India hosts six major climatic subtypes, ranging from arid desert in the west, alpine tundra and glaciers in the north, and 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–September), and a post-monsoon period (October–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 cooled 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 is further threatened by global warming. Climatic diversity in India makes the analysis of these issues complex.

History

The formation of the Himalayas (pictured) during the Early Eocene some 52 mya was a key factor in determining India's modern-day climate; global climate and ocean chemistry were possibly impacted as well.

During the Late Permian, some 260–251 million years ago (mya), the Indian subcontinent, of which India composes the greater part, 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 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) 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. 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.

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 mya, 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 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, were 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; this made its climate significantly warmer and more tropical in character than it otherwise would have been.

Regions

主条目：Climatic regions of India

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)

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. The Himalayas, along with the Hindu Kush mountains in Pakistan, prevent cold Central Asian katabatic winds from blowing in. This keeps 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. 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. Groupings are assigned codes (see chart) according to the Köppen climate classification system.

Tropical wet

A tropical rainy climate covers regions experiencing persistent high temperatures, which normally do not fall below 18 °C（64 °F） even in the coolest months. India hosts two climatic subtypes that fall under this group. The most humid is the tropical wet 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毫米（79英寸） per year. 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.

However, a tropical wet and dry climate subtype defines most of India’s land falling into this group. 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. 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. The rainy season lasts from June to September; annual rainfall averages between 750–1500 millimetres (30–59 in) across the region. However, once the dry northeast monsoon begins in September, most precipitation falls in Tamil Nadu, leaving other states comparatively dry.

Tropical dry

A tropical arid and semi-arid climate dominates regions where the rate of water's evapotranspiration is higher than the rate of moisture received through precipitation; it is subdivided into three climate types. The first, a tropical semi-arid steppe climate, predominates over a long stretch of land situated to the 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 (16–30 in) annually. It is drought-prone, as it tends to have less reliable rainfall due to occasional late arrival of the southwest monsoon. North of the Krishna River, the summer monsoon is responsible for most rainfall; to the south, significant 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 320毫米（13英寸）. Hence, without artificial irrigation, this region is not suitable for permanent agriculture.

Most of western Rajasthan falls under an arid climatic regime. Cloudbursts are responsible for nearly the entire total of annual precipitation in this region, which totals less than 300毫米（12英寸）. Such bursts happen when monsoon winds sweep into the region during July, August, and September. The rainfall is very erratic; regions experiencing rainfall one year may not see precipitation for the next couple of years or so. In addition, whatever moisture existing in the region's atmosphere is largely prevented from precipitating due to continuous downdrafts and other factors. The summer months of May and June are exceptionally hot; mean monthly temperatures in the region hover around 35 °C（95 °F）, with daily maximums occasionally topping 50 °C（122 °F）. During winters, ambient temperatures in some areas can drop below the freezing point due to waves of cold air from Central Asia. There is a large diurnal range of about 14 °C（57 °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 30–65 cm, but is very unreliable; as in much of the rest of India, the southwest monsoon accounts for most precipitation. Daily maximum temperatures during summer can 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 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 this climate, Cfa (under the Köppen system). 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. Due to the region's proximity to the Himalayas, it experiences elevated prevailing wind speeds, again from the influence of Central Asian katabatic movements.

The humid subtropical climate is subject to pronounced and 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 east, 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毫米（39英寸） in the west to over 2,500毫米（98英寸） in parts of the east. As most of this climatic region is relatively distant from the ocean, temperatures swing more widely than in tropical wet regions; they typically range from 24 °C（75 °F） in north-central India to 27 °C（81 °F） in the east.

Montane

File:Pangong lake by martinl.jpg

Pangong Lake in Ladakh, an arid montane region lying deep within the Himalayas.

India's northernmost fringes are subject to a montane, or alpine, climate. In the Himalayas, the rate at which an air mass's temperature falls as the altitude rises (the adiabatic lapse rate) is 5.1 °C per km of elevation. In terms of environmental lapse rate, ambient temperatures fall by 0.6 °C (1.1 °F) for every 100米（328英尺） 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 prevalent. The northern side of the western Himalayas, also known as the trans-Himalayan belt, is a region of arid, frigid, and generally wind-blown wastelands. Most precipitation comes as snowfall during the late winter and spring months.

Areas to the south of the Himalayas are protected from cold winter winds coming in from interior of Asia. 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 altitudes of 1,070–2,290 m receive the heaviest rainfall, which decreases rapidly as one climbs above 2,290米（7,513英尺）. The Himalayas experience the heaviest snowfall between December to February and at altitudes above 1,500米（4,921英尺）. Snowfall increases with elevation by up to several dozen mm per 100 m increase; altitudes above 5,000米（16,404英尺） never experience rain, as all precipitation falls as snow.

季節

Clear skies prevail during the post-monsoon and winter seasons in most parts of India, including the coastal state of Goa

The India Meteorological Department (IMD) designates four distinct 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 (梵語：vasant), summer (greeshm), monsoon season (varsha), early autumn (sharad), late autumn (hemant), and winter (shishir). 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.

冬季

A winter scene in Bandhavgarh National Park, Madhya Pradesh.

Once the monsoons subside, average temperatures gradually fall across India after September. As the Sun's vertical rays move south of the equator, the most of the country experiences moderately cool weather; temperatures decrease by about 0.6 °C（33.1 °F） for every degree of latitude that one moves north. December and January are typically the coldest months, with mean temperatures of 10–15 °C (50–59 °F) in the northwest and the Himalayan region. Mean temperatures are higher in the east and south, where they reach 20–25 °C (68–77 °F).

October and November are virtually cloudless in northwestern India, resulting wide diurnal temperature swings; as in much of the Deccan Plateau, temperatures range between 16 to 20 °C. However, starting in March and lasting until May, bursts of heavy rain and snowfall result from "western disturbances", which are extra-tropical low-pressure systems originating in the eastern Mediterranean Sea. These are carried west-to-east by the subtropical westerlies, the prevailing winds blowing at North India's range of latitude. As they travel, their passage is hindered by the Himalayas; unable to proceed further, they release precipitation, including significant snowfall, over the southern margins of the Himalayas instead. In addition to rain in much of the north, 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.

Even in winter, temperatures in the Andaman and Nicobar Islands (pictured) and Lakshadweep remain remarkably equable; monthly averages do not vary by more than 2 °C（36 °F）.

The rest of India, including the Indo-Gangetic Plain, does not receive snow. However, in the plains, temperatures occasionally fall below freezing, though never for more than a couple of days during December or January. Highs in Delhi range between 16 °C（61 °F） to 21 °C（70 °F）. Nighttime temperatures average between 2–8 °C. Further north, lows in the plains of Punjab 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 substantially hinder visibility and disrupt air travel an average of 15–20 days annually. In contrast, 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. Further north, frigid winds from the Himalayas can 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 are warm, with daily high temperatures of 30 °C（86 °F） and lows of around 21 °C（70 °F）. The Western Ghats, including the Nilgiri Range, are exceptional; there, lows can fall below freezing. This compares with a range of 12–14 °C on the Malabar Coast; there, as is the case for other coastal areas, the Indian Ocean exerts a strong moderating influence on weather.

夏季

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. 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 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. 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 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.

季風季

Onset dates and prevailing wind currents of the southwest summer monsoon.

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 significant wet season. It results from the southeast trade winds originating from a high-pressure mass centered over the southern Indian Ocean; attracted by a low-pressure region centered over South Asia, it gives rise to surface winds that ferry humid air into India from the southwest. 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.

The main factor behind this shift is the high summer temperature difference between Central Asia and the Indian Ocean. 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. 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 mya.

There are two branches to the southwest monsoon: the Bay of Bengal branch; and the Arabian Sea branch, which extends to a low-pressure area over the Thar Desert. The Arabian Sea branch is roughly three times stronger than the Bay of Bengal branch. Taken as a whole, the monsoon usually breaks over Indian territory by around 25 May, hitting the Andaman and Nicobar Islands in the Bay of Bengal. It strikes the Indian mainland beginning around 1 June, supplies over 80% of India's annual rainfall. first striking near the Malabar Coast of Kerala. By 9 June, it reaches Mumbai; it appears over Delhi by 29 June. The Bay of Bengal branch, which initially closely hugs the Coromandal Coast, abruptly curves to the northwest after reaching Orissa; the Arabian Sea branch generally moves northeast towards the Himalayas. By the first week of July, the entire country experiences rain. During the season, South India usually receives more rainfall than North India. However, Northeast India receives the most precipitation. The monsoon clouds begin to retreat from North India by the last week of August. It withdraws from Mumbai by 5 October; after the air over India cools further during September, the monsoon gradually departs India altogether, usually by the end of November.

Late-season monsoon clouds during a sunset over the Coromandel Coast.

In addition to bringing down temperatures, the southwest monsoon rains are intricately linked to the health of the Indian economy; since Indian agriculture employs 600 million people and composes almost 20% of the national GDP, a good monsoon often correlates with a booming economy, while weak or failed monsoons (droughts) result in widespread agricultural losses, substantially slowing overall economic growth. The rains also recharge groundwater tables and reinvigorate rivers and lakes.

後季風季

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 this period as a fourth ("post-monsoon") season, other sources designate only three seasons. 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 °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. 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. However, parts of West Bengal, Orissa, Andhra Pradesh, Karnataka and North-East India also receive minor precipitation from the northeast monsoon.

統計數字

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; year-round averages and totals are also displayed.

溫度

Monthly temperature and rainfall averages in Chennai.

降水

Monthly temperature and rainfall averages in Mumbai.

災害

Disaster-prone regions in India.

India is prone to several types of climate-related natural disasters that are responsible for massive losses of life and property. These include droughts; flash floods, and widespread and destructive flooding from monsoon rains; severe cyclones; landslides and avalanches brought on by torrential rains; and snowstorms. Other events include frequent summer dust storms, which usually track from north to south and cause extensive property damage in northern India. These storms bring with them large amounts of dust from arid regions. Hail is also common in parts of India, and cause severe damage to standing crops like rice and wheat.

洪水與山崩

Landslides are common in the Lower Himalaya, owing to labile rock formations due to the young age of the hills. Deforestation, resulting from both rising population pressure and tourism-related and other development, has been implicated as a major factor in exacerbating the extent of landslides, since tree cover impedes the flow of monsoon rains down hillsides. Parts of the Western Ghats also suffer from low-intensity landslides. Avalanches occur in Kashmir, Himachal Pradesh, and Sikkim.

However, floods are the most common natural disaster in India. When the monsoon season arrives, the heavy rainfall that follows the dry winter months may cause rivers to distend their banks, often flooding the surrounding areas. One example is the Brahmaputra River, which is prone to perennial flooding during the monsoon season. But while the seasonal floods are often responsible for killing thousands and displacing millions, they also provide rice paddy farmers with a largely dependable source of natural irrigation. Nevertheless, excess, erratic, or untimely monsoon rainfall may still wash away or otherwise ruin crops. With the exception of a few states, almost all of India is prone to flooding. Extreme precipitation events, such as flash floods and torrential rains, have been increasingly common in central India over the past several decades, coinciding with rising temperatures; however, mean annual precipitation totals have remained steady due to the declining frequency of weather systems that generate more moderate amounts of rain.

熱帶氣旋

Map showing winds zones, shaded by distribution of average speeds of prevailing winds.

Tropical cyclones, severe storms spun off from the Intertropical Convergence Zone, affect thousands of Indians living in coastal regions annually. 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. The cyclone season in the North Indian Ocean Basin runs from April to December, with peak activity between May and November. Each year, an average of eight storms with sustained wind speeds greater than 63公里每小時（39英里每小時） form; of these, around two strengthen into true tropical cyclones, which have sustained gusts greater than 117公里每小時（73英里每小時）. On average, a major (Category 3 or higher) cyclone develops every other year.

NOAA satellite imagery of Cyclone 05B in the Bay of Bengal.

During summer, the Bay of Bengal is subject to intense heating, giving rise to humid and unstable air masses that birth cyclones. Many powerful cyclones, including the 1737 Calcutta cyclone, the 1970 Bhola cyclone, and the 1991 Bangladesh cyclone, have led to widespread devastation along parts of the eastern coast of India and neighboring Bangladesh. Many deaths and widespread destruction of property are reported every year in exposed coastal states like Andhra Pradesh, Orissa, Tamil Nadu, and West Bengal. On the more placid west coast and Arabian Sea, cyclones are rare, mainly affecting Gujarat and, to an even lesser extent, Kerala.

One notable example is Cyclone 05B, a supercyclone that struck Orissa on 29 October 1999; in terms of damage and loss of life, it was the worst in more than a quarter century. With peak winds of 160英里每小時（257公里每小時）, it was the equivalent of a Category 5 hurricane. Around 1.67 million people were left homeless; another 19.5 million people were affected by the cyclone to some degree. A total of 9,803 people officially died from the storm, though unofficial estimates place the death toll at over 10,000.

乾旱

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, such monsoon failures have periodically led to major Indian famines. Notable examples 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.

All such episodes of severe drought correlate with El Niño-Southern Oscillation (ENSO) events. El Niño-related droughts have also been implicated in periodic declines in Indian agricultural output. 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. 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 monsoon season. This reversed air flow thus results in India's sporadic droughts. The degree of such monsoon failure is determined by the extent that an ENSO event raises sea surface temperatures in the central Pacific Ocean.

極端氣候

Alwar, on the fringes of the Thar Desert, registered a temperature of 50.6 °C（123 °F）, India's highest.

The lowest recorded temperature reading in India was −45 °C (−49°F) in Dras, Ladakh, in eastern Jammu and Kashmir; however, the reading was taken with non-standard equipment. Further south, readings as low as −30.6 °C（−23 °F） have been taken in Leh, also in Ladakh. However, temperatures on the Indian-controlled Siachen Glacier near Bilafond La (5,450米（17,881英尺）) and Sia La (5,589米（18,337英尺）) have fallen below −55 °C（−67 °F）, while blizzards bring wind speeds in excess of 250公里每小時（155英里每小時）, or hurricane-force winds ranking at 12 (the maximum) on the Beaufort scale. It was those conditions, not actual military engagements, that were responsible for more than 97% of the roughly 15,000 casualties suffered by India and Pakistan over the course of conflict in the region. The highest reliable temperature reading was 50.6 °C（123 °F） in Alwar, Rajasthan in 1955. This mark was also reached at Pachpadra in Rajasthan. Recently, claims have been made of temperatures touching 55 °C（131 °F） in Orissa; these have been met with some skepticism by the India Meteorological Department (IMD), which has questioned the methods used in recording such data.

The average annual precipitation of 11,871毫米（467英寸） in the village of Mawsynram, in the hilly northeastern state of Meghalaya, is the highest recorded in Asia, and possibly on Earth. The village, which sits at an elevation of 1,401米（4,596英尺）, benefits from its proximity to both the Himalayas and the Indian Ocean. However, since the town of Cherrapunji, 5公里（3英里） 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. In recent years, the Cherrapunji-Mawsynram region has averaged 9,296毫米（366英寸） of rain annually, though Cherrapunji has had at least one period of daily rainfall that lasted almost two years. India's highest recorded one-day rainfall total occurred on 26 July 2005, when 650毫米（26英寸） fell in Mumbai during the monsoons; the massive flooding that resulted killed over 900 people.

In terms of snowfall, regions of Jammur and Kashmir, such as Baramulla district in the east and the Pir Panjal Range in the southeast, experience exceptionally heavy snowfall. For example, Kashmir's highest recorded monthly snowfall occurred in 1967, when 8.4米（331英寸） fell in Gulmarg during February. However, the IMD has recorded snowdrifts up to 12米（39英尺） in several Kashmiri districts. In another freak snowfall occurring in February of 2005, more than 200 people died as a result of a western disturbance, which, within a four-day span, delivered up to 2米（7英尺） of snowfall to various parts of the state.

全球暖化的影響

Lakshadweep, comprising tiny low-lying islands, are at risk of being inundated by sea level rises associated with global warming.

Thick haze and smoke along the Ganges Basin in northern India

Several aspects of climate change, including steady sea level rise, increased cyclonic activity, and changes in ambient temperature and precipitation patterns, have impacted or are projected to impact India. For example, ongoing sea level rises have submerged several low-lying islands in the Sundarbans, displacing thousands of people. Furthermore, as temperatures steadily rise on the Tibetan Plateau and Himalayan glaciers retreat, the long-term flow of major rivers such as Ganges, Brahmaputra, Yamuna, upon which hundreds of thousands of farmers depend, could be jeopardised. In the short term, increased landslides and flooding are projected to impact such states as Assam. In stark contrast, according to a 2007 World Wide Fund for Nature (WWF) report, the Indus River may run dry as a result of the ongoing melting of Himalayan glaciers. 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.

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（36 °F）. However, such shifts are not new: for example, earlier in the current 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. Similarly, Kashmir, which once had a warm subtropical climate, shifted to a substantially colder temperate climate 2.6–3.7 mya; it was then repeatedly subjected to extended cold spells starting 600,000 years ago.

Thick haze and smoke often concentrate along the Gangetic basin, originating from smoke from biomass burning in the northwest part of India, and air pollution from large cities in northern India. The average wind is directed to blow aerosols along the steep hillside of the Tibetan Plateau, to the eastern part of India and the Bay of Bengal. Dust and black carbon, which are blown towards higher altitudes by winds at the southern slope of the Himalayas, can absorb shortwave radiation and form elevated heat sources over the Tibetan Plateau. The net atmospheric heating due to aerosol absorption causes the air to warm and rise, increasing the concentration of moisture in the mid-troposphere, and providing a positive feedback to the aerosol heating.

注釋

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

參考資料

• Allaby, M, Floods, Facts on File, 1998, ISBN 0-8160-3520-2 .
• Allaby, M, Encyclopedia of Weather and Climate, Facts on File, 2002, ISBN 0-8160-4071-0 .
• Balfour, E, Encyclopaedia Asiatica: Comprising Indian Subcontinent, Eastern and Southern Asia, Cosmo Publications, 1976 .
• Burroughs, WJ, The Climate Revealed, Cambridge University Press, 1999, ISBN 0-521-77081-5 .
• Caviedes, C, El Niño in History: Storming Through the Ages, University Press of Florida, 2001, ISBN 0-8130-2099-9 .
• Chouhan, TS, Desertification in the World and Its Control, Scientific Publishers, 1992, ISBN 8-1723-3043-X .
• Collier, W; Webb, Floods, Droughts and Climate Change, University of Arizona Press, 2002, ISBN 0-8165-2250-2 .
• Heitzman, J; Worden, India: A Country Study, Library of Congress (Area Handbook Series), 1996, ISBN 0-8444-0833-6 .
• Nash, JM, El Niño: Unlocking the Secrets of the Master Weather Maker, Warner, 2002, ISBN 0-446-52481-6 .
• Posey, CA, The Living Earth Book of Wind and Weather, Reader's Digest Association, 1994, ISBN 0-8957-7625-1 .
• Singh, VP; Ojha; Sharma, The Brahmaputra Basin Water Resources, Springer, 2004, ISBN 1-4020-1737-5 .

延伸閱讀

• Toman, MA; Chakravorty; Gupta, India and Global Climate Change: Perspectives on Economics and Policy from a Developing Country, Resources for the Future Press, 2003, ISBN 1-8918-5361-9 .

外部連結

General overview

• Country Guide: India. BBC Weather. 
• India—Weather and Climate. High Commission of India, London. 

Maps, imagery, and statistics

• India Meteorological Department. Government of India. 
• Weather Resource System for India. National Informatics Centre. 

Forecasts

• India: Current Weather Conditions. National Oceanic and Atmospheric Administration (NOAA).