Factors determining India's climate

Also called: Climatic controls, Factors determining climate · Topic: Climate of India and the Monsoon · NCERT: Class 7, Ch 3 "Climates of India"; Class 11, Ch 4 "Climate"

Meaning

Factors determining India's climate (also called climatic controls) are the conditions that decide how hot, cold, wet or dry a place in India is. NCERT (Class 11) puts them in two groups: location and relief (latitude, the Himalayas, distribution of land and water, distance from the sea, altitude and relief) and air pressure and winds (surface winds, upper-air currents such as jet streams, western disturbances, tropical cyclones and links with the oceans such as El Niño).

These factors explain why India has such different climates, from warm Kanniyakumari to freezing Srinagar and from wet Konkan to the dry Deccan. They also explain why the whole country is still called a tropical monsoon land.

Explanation

Climate words you need first

• Climate is the average weather of a large area over a long period, about 30 years.
• Weather is the state of the air at one place and time.
• Daily (diurnal) range is the gap between the day's highest and lowest temperature.
• Annual range is the gap between the warmest month and the coldest month.

Group 1: location and relief

• Latitude (a place's distance north or south of the Equator, in degrees)
• The Tropic of Cancer (23°30′N) runs through the middle of India.
• South of it, the climate is tropical. The sun stays high all year, so it is hot, and the daily and yearly ranges are small.
• North of it, the climate is sub-tropical and temperate. The sun's angle changes a lot between summer and winter, so the ranges are large.

• Why this happens: near the Equator, the sun's rays fall almost straight down. The heat is packed into a small area. At high latitudes, the rays come in at a slant. They pass through more air and spread over a bigger area, so each spot gets less heat.

• The Himalayas as a climatic divide

• The range runs about 2,400 km from the Indus to the Brahmaputra. Many peaks are over 6,000 m high.
• Winter: the range acts as a wall. It stops most of the very cold Arctic and Central Asian air from flowing south, so north India has mild winters for its latitude.
• Summer: it stops the south-west monsoon winds and pushes them up, so they drop their rain inside the subcontinent.

• Modern research: model studies show that monsoon strength depends more on heating of the low plains of north India than on heating of the high mountain slopes. So the mountains work mainly as a thermal insulator: a wall that keeps cold, dry air away from warm, moist monsoon air [3].

• Distribution of land and water

• The sea surrounds the peninsula on three sides, and a mountain wall lies to the north.
• Differential heating means land heats up and cools down faster than water. Sunlight goes deep into water, water mixes and moves, and it needs more heat to warm up. Land heats only in a thin top layer and loses the heat quickly.
• Summer chain: the land gets hot → a low (an area of low air pressure) forms over north-west India → a high (an area of high air pressure) sits over the cooler ocean → moist south-west winds blow in from the sea and bring rain.
• Winter chain: the land cools faster → a high forms over the land, especially north of the Himalayas → dry, cool winds blow from land to sea, mostly from the north-east.

• Result: the winds reverse with the season. This is what monsoon means (from the Arabic mausim, "season"). This is the classical thermal theory of Edmond Halley (1686).

• Distance from the sea

• Equable (maritime) climate: mild temperatures with a small gap between summer and winter. The sea calms temperature swings, and land and sea breezes give daily relief.

• Extreme (continental) climate: hot summers, cold winters and a large range, because the sea's influence does not reach far inland.

• Altitude (height above sea level)

• Temperature falls by about 6.5°C for every 1,000 m you go up. This is called the normal lapse rate.

• Air higher up is thinner. It holds less heat and is farther from the warm ground.

• Relief (the shape of the land: mountains, plateaus, plains, valleys)

• Orographic (relief) rain: moist wind hits a mountain and is pushed up. The rising air cools, its water vapour condenses, and it rains on the windward side (the side facing the wind).
• The air sinks on the leeward side (the far side), where it warms and dries. This dry belt is the rain-shadow area.
• Relief also guides winds. The Arakan Yoma and the Meghalaya hills turn the Bay of Bengal branch of the monsoon westward up the Ganga valley.

Group 2: air pressure and winds

• These include surface winds, jet streams (narrow belts of very fast wind high in the atmosphere), western disturbances, tropical cyclones, and links with the oceans such as the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the Equator where the trade winds of the two hemispheres meet), ENSO (El Niño–Southern Oscillation) and the IOD (Indian Ocean Dipole).
• Winds (Class 7):
• Hot, dry westerlies bring heat waves (spells of unusually high temperature that harm people and animals) in May and June.

• Cold winds from beyond the Himalayas bring cold waves to the northern plains in winter.

• IMD heat wave rule: a station's maximum temperature must reach at least 40°C in the plains, 37°C at coastal stations or 30°C in hilly regions [2]. This must happen at 2 or more stations in a meteorological sub-division for at least 2 days in a row [2]. A station whose normal maximum is 45°C or more can be declared under a heat wave on the actual temperature alone [2].

• IMD cold wave scale: for stations with a normal minimum of 10°C or more, a fall of 5–6°C below normal is a moderate cold wave. A fall of 7°C or more is a severe cold wave [2].
• IMD records daily maximum and minimum temperatures at about 550 surface observatories [2].

Where it occurs

• Latitude: the Tropic of Cancer crosses 8 states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram. Kanniyakumari (about 8°N, Tamil Nadu) and the Nicobar Islands are warm all year. Srinagar (about 34°N, Jammu and Kashmir) has cool summers and freezing winters.
• Himalayas: Delhi (about 28°N) is far warmer in January than places at the same latitude in Central Asia or interior China. The Ganga plains and the north-east get heavy rain, while the Tibetan Plateau stays dry.
• Distance from the sea: Mumbai (west coast, about 19°N) has an annual range of about 14°C. Nagpur (centre of the Deccan, about 21°N) has about 34°C. Both are in Maharashtra. The Konkan coast has an equable climate. Delhi, Kanpur and Amritsar have extreme seasonal contrasts.
• Altitude:
• Agra (Uttar Pradesh) and Darjiling (West Bengal, about 2,000 m) are both at about 27°N. In January, Agra is at 16°C and Darjiling at 4°C.
• Coimbatore (25–38°C) and Ooty (Nilgiri Hills, about 2,240 m; 10–25°C) are both in Tamil Nadu.

• Hill stations range from Munnar (Kerala, Western Ghats) to Tawang (Arunachal Pradesh, Eastern Himalaya). Shimla, Mussoorie and Ooty grew as summer retreats in colonial times.

• Relief: the windward and wet areas are the western slopes of the Western Ghats (Konkan and Malabar coast) and Assam and the north-east hills. The leeward and dry area is the Deccan plateau. Mumbai gets far more rain than Pune, which lies just across the Ghats.

• Winds: heat waves hit Punjab, Haryana, Rajasthan and Madhya Pradesh in May and June. Cold waves hit the northern plains in winter.

Don't confuse with

• Climate vs weather: climate is the average over about 30 years for a large area. Weather is the state of the air at one place and time.
• Latitude vs distance from the sea: Mumbai and Nagpur are at almost the same latitude. Their very different annual ranges come from distance from the sea, not latitude.
• Altitude vs latitude: Agra and Darjiling are both at about 27°N. Darjiling is colder because of altitude.
• Microclimate vs regional factors: a microclimate is the climate of a small area that is different from its surroundings, such as an urban heat island, an enclosed valley or a forest. A heat wave is a regional wind effect, not a microclimate.

Prelims Hooks

• The Tropic of Cancer passes through 8 states, from Gujarat to Mizoram. It does not pass through Odisha, Bihar or Uttar Pradesh.
• Mumbai (about 14°C) vs Nagpur (about 34°C) annual range: the factor is distance from the sea.
• Agra 16°C vs Darjiling 4°C in January, both at about 27°N: the factor is altitude. Ooty (Nilgiri Hills) vs Coimbatore: both are in Tamil Nadu.
• The Himalayas keep out Arctic and Central Asian cold air and trap the south-west monsoon. They act mainly as a thermal insulator, not a heat source [3].
• The thermal (differential heating) theory of the monsoon: Edmond Halley, 1686.
• Rain shadow: the Deccan is leeward of the Western Ghats. IMD heat wave thresholds are 40°C (plains), 37°C (coast) and 30°C (hills), at 2 or more stations for 2 days in a row [2].

Mains Points

• Why India is a "tropical monsoon" country even though half of it lies north of the Tropic:
• The Himalayan wall blocks cold Central Asian air.
• Land–sea heating differences control the winds over the whole country.

• Latitude alone would make the north temperate (GS-I).

• Regional contrasts from relief and distance from the sea:

• The wet windward Ghats and the north-east stand against the dry Deccan rain shadow and the extreme interior plains.

• These contrasts help explain drought-prone Marathwada and Rayalaseema, cropping patterns, and the growth of hill stations and coastal cities (GS-I).

• Climatic factors and disasters (GS-III):

• Hot westerlies over Punjab, Rajasthan and Madhya Pradesh cause heat waves. Cold Himalayan air over the northern plains causes cold waves.
• IMD's fixed thresholds (40/37/30°C) trigger warnings and feed into state heat action plans [2].
• Urban heat islands make city heat worse, so green cover and cool roofs are used as responses.
• A complete answer joins Halley's model with the Himalaya-as-insulator idea [3], the ITCZ shift, jet streams and ENSO/IOD. It shows that India's climate comes from both location and relief and circulation.

Related concepts

• Himalayas as a climatic divide
• Differential heating of land and water

Read more

• Factors of India's climate: location and relief