The climate of India comprises a wide range of weather conditions across a vast geographic scale and varied topography. Based on the Köppen system, India hosts six major climatic subtypes, ranging from arid deserts 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, making it one of the most climatically diverse countries in the world.
The country's meteorological department follows the international standard of four climatological seasons with some local adjustments: winter (December, January and February), summer (March, April and May), a monsoon means rainy season (June to September), and a post-monsoon period (October to November).
India's geography and geology are climatically pivotal: the Thar Desert in the northwest and the Himalayas in the north work in tandem to effect a culturally and economically important monsoonal regime. As Earth's highest and most massive mountain range, the Himalayas bar the influx of frigid katabatic winds from the icy Tibetan Plateau and northerly Central Asia. Most of North India is thus kept warm or is only mildly chilly or cold during winter; the same thermal dam keeps most regions in India hot in summer.
Climatic Regions of India
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, keeping the bulk of the Indian subcontinent warmer than most locations at similar latitudes. Simultaneously, the Thar Desert plays a role in attracting moisture-laden southwest summer monsoon winds that, between June and October, provide the majority of India's rainfall.
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 (Humid)
A tropical rainy climate governs regions experiencing persistent warm or high temperatures, which normally do not fall below 18 °C. India hosts two climatic subtypes:
- tropical monsoon climate or the tropical wet climate: The Western Ghats, the Malabar Coast, southern Assam, Lakshadweep and Andaman and Nicobar Islands have the tropical monsoon climate. It experiences moderate to high temperature with seasonal but heavy rainfall. The months from May to November experience the most rainfall and the rain received during this period is sufficient for vegetation throughout the year.
- tropical wet and dry climate or savannah climate: Tropical wet and dry climate or the savannah climate is most common in the country and prevails mainly in the inland peninsular region of the country except for some portion of the Western Ghats. The summers are extremely hot and the rainy season extends from the month of June to September.
A tropical dry climate or 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:
- Tropical semi-arid (steppe) climate: Karnataka, central Maharashtra, some parts of Tamil Nadu and Andhra Pradesh experience the tropical semi-arid (steppe) climate. Rainfall is very unreliable in this type of climate and the hot and dry summers are experienced from March to May.
- Subtropical arid (desert) climate: With scanty and erratic rainfall and extreme summers, western Rajasthan witnesses the subtropical arid (desert) climate.
- Subtropical semi-arid (steppe) climate: The areas of the tropical desert that runs from the regions of Punjab and Haryana to Kathiawar witness the sub-tropical semi-arid (steppe) climate. The maximum temperature in summers goes up to 40°C and the rains are unreliable and generally take place during summer monsoon season in this climate.
Subtropical Humid Climate
This climate is witnessed by most of the North and Northeast India. Summers are very hot, while in winters, temperature can plunge to as low as 0°C. Rainfall mainly occurs in summers but snowfall or occasional rainfall in winters is also witnessed in some areas. The hottest months are May and June and frost also occurs for few months in winters.
Due to ample monsoon rains, India has only one subtype of this climate under the Köppen system: Cwa. 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. Most summer rainfall occurs during powerful thunderstorms associated with the southwest summer monsoon; occasional tropical cyclones also contribute.
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. In terms of environmental
lapse rate, ambient temperatures fall by 6.5 °C 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 hundred metres 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, has a cold desert climate. It is a region of barren, arid, frigid and wind-blown wastelands. 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.
The southern slopes of the western Himalayas, well-exposed to the monsoon, get heavy rainfall. Most precipitation occurs as snowfall during the late winter and spring months.
The temperature falls by 0.6°C for every 100 m rise in altitude in the Himalayas and results in a number of climates from tropical to tundra. The trans-Himalayan belt, which is the northern side of the western Himalayas, is cold, arid and windswept. There
is less rain in the leeward side of the mountains whereas heavy rainfall is received by the well exposed slopes. Heaviest snowfall occurs between the months of December to February.
Seasons in India
The country's meteorological department follows the international standard of four climatological seasons with some local adjustments. The India Meteorological Department (IMD) designates four climatological seasons as explained below.
Winter (December to February)
Generally the months of January and February are considered as the start of winter season in the country. However, in some parts of north-western India, the season begins from December. Generally, the average temperature during this season is about 10-15°C in northwest regions. In southeast region of mainland India, the average temperature is about 20-25°C. The western Himalayas, the extreme north-eastern parts and Kerala and Tamil Nadu, experience rains during this season.
Summer (March to Mid-June)
Also called as Pre-monsoon or Thunderstorm or Hot Weather, the country experiences this season from March till June. In the interior peninsular regions, the mean daily temperature is recorded at 30-35°C. The maximum temperature in daytime in Central India crosses 40°C in many areas. In some regions, temperature is high during daytime while in the nights, low temperature is recorded. The coastal areas of the country have mild temperature during this season due to the influence of land and sea breezes. Thunderstorms with rains and hail influence the weather in the land areas of the country. These thunderstorms are seen in the north-eastern and eastern parts of Bihar, Assam and West Bengal. In the plains of north-west India, hot and dry winds, along with dust winds, are frequently experienced.
South-west Monsoon (June to September)
The months from July to September are the most significant. It is also called as the Rainy season as about 75% of the total rainfall of the country is supplied during this season. The exact period of the South-west monsoon in a region depends upon the onset and withdrawal time of the season. For instance, it remains in west Rajasthan for about 75 days while for 120 days in the south-western regions of the country. The SW monsoon reaches in two branches: the Arabian Sea and the Bay of Bengal branch. The Arabian Sea branch extends towards the low-pressure area over Thar Desert and is about three times stronger than the Bay of Bengal branch. The northern hemispheric temperatures, El Nino, snow cover, sea surface temperature and many other are some of the local and global phenomenon which influences the monsoons in the country. The South-west monsoon starts to weaken by 1 September in Rajasthan and from 15 September in some north-western parts of the country. The monsoons in India are very important for the economy of the country as it affects the agriculture which is the mainstay of a huge workforce of the nation.
Retreating South-west Monsoon (October to November)
Also called as Autumn or Post Monsoon or Northeast Monsoon, it prevails in the country from the months of October to December. It is a transition season which is related to the establishment of the north-easterly wind regime over the subcontinent of the country. A large part of the country experiences cool, dry, and dense Central Asian air masses. Some parts of Tamil Nadu, Kerala, Karnataka and Andhra Pradesh receive about 35% of their total rainfall during this season. A decline in the mean temperature from 38°C in October to 28°C in November takes place during this season. The characteristic features of this season include decrease in humidity level and clear skies in most parts of the central and northern India.
The Himalayan states, being more temperate, experience an additional season, spring, which coincides with the first weeks of summer in southern India. Traditionally, North Indians note six seasons or Ritu, each about two months long. These are the spring season (Sanskrit: vasanta), summer (grīṣma), monsoon season (varṣā), autumn (śarada), winter (hemanta), and prevernal season (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.
Ritu (Sanskrit: ऋतु) defines "season" in different ancient Indian calendars used in countries of India, Bangladesh, Nepal and Sri Lanka, and there are six ritus. The word is derived from the Vedic Sanskrit word Ṛtú, a fixed or appointed time, especially the proper time for sacrifice (yajna) or ritual in Vedic religion; this in turn comes from the word Ṛta (ऋत), as used in Vedic Sanskrit literally means the "order or course of things".
India observes six ecological seasons. Southern parts of India experiences the seasons on a different schedule than the one depicted here.