Distribution of Rainfall in India: Temperature, Pressure, Winds and Local Winds

Climatic Elements of India: How They Fit Together

Weather, Climate and the Four Climatic Elements

Weather is the momentary state of the atmosphere, and climate is the average of weather over a longer period. India's climate is monsoonal in rhythm and character, but its regional diversity can be described as sub-types of that monsoon climate. Rainfall, temperature, pressure and wind, and local weather are best read together, because each one explains the others.

Temperature sets pressure, pressure sets wind, and wind decides where moisture goes and where it falls as rain. In summer the northern landmass heats up, a low-pressure trough forms over it, and moist winds are drawn in from the ocean. In winter the same landmass cools, pressure rises, and dry winds blow outward. Local winds and storms are the short-lived outcomes of the same contrast, seen when hot dry air meets moist air on a given afternoon.

Distribution of Rainfall in India

Rainfall Belts by Annual Total

The average annual rainfall in India is about 125 cm, but it varies greatly from place to place. More than 75 per cent of it arrives in the four rainy months of June to September. The country divides into four belts by annual total.

In some parts of the Khasi and Jaintia hills the rainfall exceeds 1,000 cm, and Mawsynram, on the crest of the Khasi Hills, receives the highest average annual rainfall in the world. The north-western Himalayas and the western deserts get less than 10 cm a year. Within the north-east, the Brahmaputra valley and its adjoining hills get less, under 200 cm, while the hills of Meghalaya get far more. Snowfall is restricted to the Himalayan region.

Rainfall in north India decreases westwards, and in peninsular India, except Tamil Nadu, it decreases eastwards. By area, about 30 per cent of the country is dry, with under 75 cm a year, about 20 per cent receives 115 to 200 cm, and about 8 per cent receives more than 200 cm.

Types of Rainfall in India

Rainfall is classified by origin into convectional, orographic and cyclonic or frontal. Each type has a different trigger and a different place on the Indian map.

• Orographic rain: A saturated air mass forced up a mountain expands, cools and loses its moisture on the windward slope. The windward side of the Western Ghats and of the Meghalaya hills are the clearest Indian cases. On the leeward slope the descending winds warm up, can hold more moisture, and leave a dry rain-shadow area.
• Convectional rain: Heated air rises in convection currents, expands, cools and forms cumulus clouds, giving heavy rain with thunder and lightning that does not last long. It is common in summer and in the hotter part of the day, and the pre-monsoon thunderstorms of the plains and the north-east belong here.
• Cyclonic rain: It comes from cyclones and their fronts. In India it has two sources, the tropical cyclones of the Bay of Bengal in the retreating monsoon season and the extra-tropical western disturbances of winter.

Seasonal Pattern of Rainfall

Rain reaches India in four phases. Each has its own source and its own region, and together they explain why a place can be wet in one season and dry in another.

1. Winter, December to February: Western disturbances bring light rain to Punjab, Haryana, Delhi and western Uttar Pradesh, and snow to the lower Himalayas. Arunachal Pradesh and Assam also get 25 to 50 mm in these months.
2. Pre-monsoon, March to May: Thunderstorms and showers occur at the edge of the heated interior. They include the mango showers of Kerala and coastal Karnataka and the nor'westers of Bengal and Assam.
3. South-west monsoon, June to September: The monsoon sets in over Kerala by 1 June, reaches Mumbai and Kolkata between 10 and 13 June, and covers the whole subcontinent by mid-July. The Arabian Sea branch drenches the Western Ghats, while the Bay of Bengal branch brings rain to the Ganga plains and the Brahmaputra valley.
4. Retreating and north-east monsoon, October and November: The trough of low pressure moves south, and the eastern Peninsula, which gets its rain from cyclonic depressions, receives its rainiest months.

This is why most parts of the country get rain between June and September, while on the coastal areas of Tamil Nadu it rains at the beginning of the winter season.

Rain Shadow and Regional Contrasts

The Western Ghats show the effect of relief most clearly. The south-west monsoon winds of the Arabian Sea climb the slopes from 900 to 1,200 m, cool, and give the windward Sahyadris and the western coastal plain very heavy rainfall of 250 to 400 cm. After crossing the crest the winds descend and heat up, humidity falls, and the region east of the Ghats gets little rain. That is the rain-shadow area of the Deccan, and it is why Mangaluru, on the windward coast, is far wetter than Bengaluru, on the leeward plateau.

The Tamil Nadu coast is dry in the south-west monsoon for two reasons. It runs parallel to the Bay of Bengal branch of the monsoon, and it lies in the rain shadow of the Arabian Sea branch. It makes up for this later, because the north-east monsoon crosses the Bay of Bengal in October and November, picks up moisture and brings torrential rain to the Tamil Nadu coast, southern Andhra Pradesh, south-east Karnataka and south-east Kerala.

Bengaluru gets rain from both the south-west and the north-east monsoons, but its total is still smaller than that of the windward coast, because the Ghats have removed much of the moisture before the south-west winds reach it. Mangaluru gets about 95 per cent of its rain between May and September and is extremely dry from December to March.

Rainfall Variability and IMD Monsoon Categories

Averages hide how unevenly rain arrives. The all-India annual rainfall changes little from year to year, with a coefficient of variation of about 9 per cent for the year and 10 per cent for the monsoon season. Individual months vary far more. January rainfall has a coefficient of variation of 71 per cent and December 80 per cent, against 14 per cent for July. In other words the wet months are dependable and the dry months are not.

The India Meteorological Department judges the monsoon against a long period average (LPA). For June to September, the LPA over the country for 1971 to 2020 is 87 cm. For the country as a whole the season falls into one of five categories. It is deficient below 90 per cent of the LPA, below normal at 90 to 95 per cent, normal at 96 to 104 per cent, above normal at 105 to 110 per cent, and excess above 110 per cent. Smaller areas are classified by departure from normal. Excess is 20 per cent above normal or more, normal is plus 19 to minus 19 per cent, deficient is minus 20 to minus 59 per cent, and scanty is minus 60 per cent or less.

Variability brings droughts or floods every year in some part of the country. In the Ganga delta and the coastal plains of Odisha, strong rain-bearing storms strike almost every third or fifth day in July and August, while the Coromandel coast, a thousand km to the south, goes generally dry in those months.

Why Rainfall Distribution Matters for Economic Life

Farming across India turns on whether the rain is timely and well spread. Regional variations in the monsoon let different crops grow in different places, and temperatures everywhere except the Himalayas stay above the threshold for crops through the year. If rain fails, farming suffers most in regions without irrigation. A sudden monsoon burst causes soil erosion over large areas. Winter rain from temperate cyclones is highly beneficial for rabi crops, and the snow that falls in the Himalayas sustains the flow of Himalayan rivers in summer.

Temperature Distribution Across India

Seasonal and Regional Temperature Patterns

Temperature in India varies by place, by season and by hour. On the same June day Churu in Rajasthan may record 50 °C or more while the mercury hardly touches 19 °C at Tawang in Arunachal Pradesh. On a December night Drass in Ladakh may fall to minus 45 °C while Thiruvananthapuram or Chennai records 20 °C or 22 °C.

The daily range also differs. In Kerala and the Andaman Islands the difference between day and night temperatures may be hardly 7 or 8 °C, but in the Thar desert a day temperature of 50 °C may fall to 15 to 20 °C at night.

Through the year the temperature follows the seasons:

• Cold weather season: It sets in by mid-November in northern India, and December and January are the coldest months. The mean daily temperature remains below 21 °C over most of the north, and nights may go below freezing in Punjab and Rajasthan.
• Hot weather season: In March the highest day temperature, about 38 °C, occurs on the Deccan Plateau. In April, 38 to 43 °C is found in Gujarat and Madhya Pradesh. In May the heat belt moves further north and temperatures around 48 °C are not uncommon in the north-west.
• South-west monsoon season: The first rain brings the temperature down, with a fall of 5 to 8 °C in day temperature between mid-June and mid-July.

The peninsula has no well-defined cold season. On the coast the sea and the nearness of the equator keep temperatures almost the same through the year, and only the hills of the Western Ghats stay comparatively cool.

Controls on Temperature: Latitude, Altitude and Sea

Five controls explain the map of temperature.

On the coast, isotherms run parallel to the shore, so temperature does not decrease from north to south but rises from the coast to the interior. The mean daily minimum in the hot season rarely falls below 26 °C on the coastal south.

January Isotherms, Land and Sea and the Tropical Boundary

An isotherm is a line joining places with equal temperature. Over the Northern Hemisphere the January pattern is shaped by land and sea. The isotherms bend north over the oceans and south over the continents, because the warm Gulf Stream and North Atlantic Drift make the North Atlantic warmer and the land cools sharply. The effect is pronounced in January and weaker in July.

Under the Köppen system a climate is tropical when the average temperature stays at 18 °C or higher in every month, and temperate when the coldest month falls between 0 and 18 °C. In January the mean daily temperature stays below 21 °C over most of the northern plain, and nights in Punjab and Rajasthan may fall below freezing, while the peninsula stays warm. That is why the north has a cold season and the south has none.

Heat Waves and Cold Waves

A heat wave is declared at a station when the maximum temperature reaches at least 40 °C in the plains or at least 30 °C in hilly regions. The India Meteorological Department then uses two tests. By departure from normal, a heat wave is 4.5 to 6.4 °C above normal and a severe heat wave is more than 6.4 °C above. By actual maximum temperature, a heat wave is 45 °C or more and a severe heat wave is 47 °C or more.

The criteria must be met at two or more stations in a meteorological sub-division on at least two consecutive days, and the wave is declared on the second day. For coastal stations a departure of 4.5 °C or more counts, provided the actual maximum is 37 °C or more.

Heat waves in India are lethal. The May 2015 heat wave caused the deaths of at least 2,500 people, with Andhra Pradesh and Telangana the most affected, and in May 2016 Phalodi registered an unprecedented 51 °C. The Loo, the hot wind of the northern plains, blows at 45 to 50 °C.

In winter, the cold waves of the north-west arise from three causes. Punjab, Haryana and Rajasthan are far from the moderating sea and have a continental climate. Snowfall in the nearby Himalayan ranges creates cold wave conditions. Around February, cold winds from the Caspian Sea and Turkmenistan bring cold waves with frost and fog.

Pressure and Wind Systems over India

Global Pressure Belts and Horse Latitudes

The pressure belts of the globe set the background for India's winds. Near the equator the sea-level pressure is low and the area is the equatorial low. Along 30° N and S lie the subtropical highs, further poleward along 60° N and S are the sub-polar lows, and near the poles the pressure is high again. These belts oscillate with the apparent movement of the sun, moving south in winter and north in summer in the Northern Hemisphere.

The subtropical highs form because air that rises at the Inter Tropical Convergence Zone reaches the top of the troposphere, moves towards the poles and accumulates about 30° N and S, where part of it sinks and forms the high. The latitudes about 30° north and south are the Horse Latitudes, characterised by sunny skies, calm winds and very little precipitation.

They sit at the divergence of the trade winds and the westerlies. Sinking air is dry, so the consistently warm, dry conditions of these belts are the main cause of the world's major hot deserts, including the Sahara, the Arabian desert, the Atacama and the Great Australian Desert. The Thar desert of India and Pakistan likewise has a hot desert climate under the subtropical ridge.

Winter Pressure and Winds

On 22 December the sun shines vertically over the Tropic of Capricorn. Feeble high pressure covers the northern plain, and in south India the air pressure is slightly lower. The isobars of 1019 mb and 1013 mb pass through north-west India and the far south, respectively.

Winds therefore start blowing from the north-western high-pressure zone towards the low pressure over the Indian Ocean. Because the pressure gradient is low, the winds are light, with a velocity of about 3 to 5 km per hour, and the topography of the region influences their direction. They are westerly or north-westerly down the Ganga valley, northerly in the Ganga-Brahmaputra delta, and clearly north-easterly over the Bay of Bengal, free from the influence of topography.

These winter monsoons do not cause rain as they move from land to sea. They have little humidity, and the anticyclonic circulation on land reduces the possibility of rainfall. They pick up moisture only when they cross the Bay of Bengal in October and November, which gives rain to the Tamil Nadu coast.

Summer Pressure, ITCZ and Monsoon Trough

In the summer months pressure falls in the northern half of the country. The Inter Tropical Convergence Zone (ITCZ), a low-pressure zone at the equator where trade winds converge and air tends to ascend, moves north. In July it is centred about 25° N, sometimes said to lie between 20° N and 25° N over the Gangetic plain, and it is then called the monsoon trough. This elongated trough extends from the Thar desert in the north-west to Patna and the Chotanagpur plateau in the east-south-east.

The trough draws a surface circulation. Winds are south-westerly on the west coast and along the coast of West Bengal and Bangladesh, and easterly or south-easterly over north Bengal and Bihar. The south-west monsoon is in reality a continuation of the south-east trade winds of the Southern Hemisphere, which cross the equator between 40° E and 60° E and turn south-westerly because of the Coriolis force. The influx of these winds by mid-June changes the weather to the rainy season. In winter the ITCZ moves south, and the winds reverse to the north-east monsoon.

This reversal of pressure and wind between January and July is the central fact of Indian climate.

Jet Streams over India

Jet streams are narrow belts of fast-moving air near the tropopause. The polar jet, most commonly found between latitudes 30° and 60°, and the subtropical jet, close to 30°, blow from the west. Over India a subtropical westerly jet in winter gives way in summer to a tropical easterly jet.

In winter the westerly jet flows south of the Himalayas over the north Indian plain. It steers the shallow cyclonic depressions from the east Mediterranean across West Asia, Iran, Afghanistan and Pakistan towards north-west India. In summer, intense heating of the Tibetan Plateau and Central Asia is believed to be the critical factor in forming the tropical easterly jet over India.

The shift in the ITCZ is related to the withdrawal of the westerly jet from its position over the north Indian plain. The easterly jet sets in along 15° N only after the westerly jet has withdrawn, and this easterly jet is held responsible for the burst of the monsoon in India. A timely northward shift of the westerly jet means a timely monsoon, and a delayed shift delays it.

Western Disturbances and Winter Rain

A western disturbance is an extra-tropical storm that originates in the Mediterranean region and brings sudden winter rain to the north-western parts of the Indian subcontinent. It is a non-monsoonal pattern driven by the westerlies, and the disturbances are embedded in the subtropical westerly jet stream. An average of four to five form in a winter season.

1. A high-pressure area over Ukraine pushes cold air towards warmer, moist air and starts an eastward-moving depression in the Mediterranean.
2. The depression travels across West Asia, Iran, Afghanistan and Pakistan, and its moisture increases from the Caspian Sea in the north and the Persian Gulf in the south.
3. It reaches north-west India and weakens as the Himalayas stop its progress.
4. Rain falls over Punjab, Haryana, Delhi and western Uttar Pradesh, and snow falls over the western Himalayas.

The amount is meagre but very useful. The winter rain averages about 53 mm in Delhi, about 25 mm in Punjab and about 18 mm in Bihar. Precipitation decreases from west to east in the plains and from north to south in the mountains. The rain supports rabi crops, especially wheat, and the Himalayan snow sustains river flow in summer. In some years strong disturbances bring cold waves and dense fog to the Indo-Gangetic plain, and their interaction with the monsoon trough can cause heavy rain, as in the 2013 North India floods.

Local Winds and Storms of India

Loo: The Hot Wind of the Northern Plains

The Loo is a strong, dusty, gusty, hot and dry summer wind from the west that blows over the Indo-Gangetic plain of north India and Pakistan, especially in May and June. In the heart of the ITCZ in the north-west it blows in the afternoon and very often continues well into midnight. It blows in the northern plains from Punjab to Bihar, with higher intensity between Delhi and Patna. Its temperatures of 45 to 50 °C make exposure to it a cause of fatal heatstroke.

The Loo originates mainly in the desert regions of north-western South Asia, including the Great Indian Desert, and it ends with the arrival of the monsoon. Its drying effect leaves vegetation brown, and homes in the region have long used damp khas (vetiver) screens to cool the air. Before the monsoon, brief violent dust storms called kali andhi strike some areas, and evening dust storms are common in May in Punjab, Haryana, eastern Rajasthan and Uttar Pradesh; they bring light rain and a cool breeze that give temporary respite.

Loo, Chinook and Foehn Compared

The Loo belongs to a group of winds that become hot and dry in a particular place and season. The Foehn and the Chinook are the best-known comparisons.

The key contrast is the cause. The Loo is a hot dry wind from a heated desert source, whereas the Foehn and the Chinook are warm dry winds produced by descent on the lee side of a mountain.

Pre-Monsoon Showers and Nor'westers

Before the monsoon, the heated interior and the moist air at its edge produce local storms. When dry and moist air masses meet suddenly at the periphery of the trough, they give local storms of great intensity with violent winds, torrential rain and even hailstorms.

The name Kalbaisakhi means a calamity of the month of Baisakh. In Assam the storms are known as Bardoisila. They commonly begin over the Chotanagpur plateau between Bihar and Jharkhand and gather strength as they move south-east into Odisha, West Bengal and Assam, with winds that can exceed 100 km per hour. The term nor'wester is used in English because the storms travel from the north-west to the south-east.

Sources

• NCERT: India, Physical Environment (Class XI), Chapter 4, Climate
• NCERT: Fundamentals of Physical Geography (Class XI), Chapters 8 to 10
• IMD: Frequently asked questions on heat waves
• IMD: Long range forecast for the 2026 south-west monsoon
• IMD Pune: Frequently asked questions on the monsoon
• MoSPI: Statistical Year Book India, Chapter 34, Rainfall
• Wikipedia: Horse latitudes
• Wikipedia: Subtropical ridge
• Wikipedia: Desert climate
• Wikipedia: Western Disturbance
• Wikipedia: Jet stream
• Wikipedia: Monsoon of India
• Wikipedia: Loo (wind)
• Wikipedia: Foehn wind
• Wikipedia: Chinook wind
• Wikipedia: Kalbaisakhi
• Wikipedia: Mango showers
• Wikipedia: Mawsynram
• Wikipedia: Mangalore
• Wikipedia: Bangalore
• Wikipedia: 2015 Indian heat wave
• Wikipedia: Köppen climate classification
• UPSC: Civil Services Examination question papers