# Monsoon, rainfall, weather and climate

> These study notes explain Monsoon, rainfall, weather and climate for the Group 4 / VAO (Combined Civil Services Examination-IV) exam in plain language: the same reviewed notes the TNPSC Guide app shows, published by our editors with their source. Read the key points first, then practise the unit's questions. For exam facts such as dates and vacancies, always check tnpsc.gov.in.

Last updated: 2026-11-02 · By TNPSC Guide editorial team
URL: https://www.tnpscguide.com/exams/group-4/notes/gs-geography-climate

## Why the exam asks this

The 2022, 2024 and 2025 Group 4 papers we read had no question on this topic. But the geography unit of the syllabus names monsoon, rainfall, weather and climate. The layers of the atmosphere and what marks each, the measuring instruments, the local winds with their places, and India's four seasons suit matching questions. Recorded temperatures, rainfall figures and the date the monsoon covers all of India suit choose-the-correct-statement questions. Do not take as an answer any number on which two books differ; those are in "Easy to confuse".

## Weather and climate

Weather is the short-term state of the atmosphere at a particular place. Climate is the long-term average weather. The Std 8 book gives that period as 35 years; the Std 10 book says about 30-35 years. So remember the definition "long-term average weather"; do not take the number of years as a single answer. The English word climate comes from the ancient Greek word "klima"; "klimo" means inclination. The science of weather is meteorology; the science of climate is climatology. Example: "harsh sun in Madurai today" is a weather report; Madurai's average over many years is its climate.

## The gases of the atmosphere

By the Std 8 book the atmosphere holds 78% nitrogen, 21% oxygen, 0.97% argon, 0.03% carbon dioxide and 0.04% other gases. The Std 9 book calls nitrogen (78%) and oxygen (21%) the permanent gases; the remaining one per cent holds argon (0.93%), carbon dioxide (0.03%) and others; water vapour is 0-0.4%. Nitrogen 78% and oxygen 21% are the same in both books; the argon figure differs between them, so do not hold it as a single answer. Daniel Rutherford found nitrogen in the atmosphere in 1772 and Joseph Priestley found oxygen in 1774.

## The five layers of the atmosphere

The five layers from the bottom up: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The troposphere reaches 8 km over the poles and 18 km over the equator; the Greek word "tropos" means change. All weather happens here, so it is "the weather-making layer"; its upper limit is the tropopause. The stratosphere reaches 50 km; it has many ozone molecules, so it is the "ozonosphere", and it suits jet aircraft. The mesosphere lies from 50 km to 80 km; meteors coming towards the Earth burn up here. The ionosphere in the thermosphere sends radio waves back to the Earth. The aurora australis and aurora borealis occur in the exosphere. The magnetosphere reaches about 64,000 km from the Earth's surface.

## The lapse rate and the day's temperature

In the troposphere the temperature falls by 6.5°C for every 1000 metres of height; this is the lapse rate. A day's maximum temperature is recorded between 2.00 and 4.00 in the afternoon, and the minimum from 4.00 in the early morning until just before sunrise. Example: because it gets cooler with height, hill towns such as Udhagamandalam are cooler than the plains at their foot.

## Heat zones and recorded temperatures

The area between the Tropic of Cancer and the Tropic of Capricorn is the torrid zone. The areas from the Tropic of Cancer to the Arctic Circle and from the Tropic of Capricorn to the Antarctic Circle are the temperate zones. Beyond the Arctic and Antarctic Circles lie the frigid zones. The highest temperature recorded on Earth is 56.7°C (134°F), in Death Valley, California, in the United States, on 10 July 1913. The lowest is -89.2°C, at the Soviet station Vostok in Antarctica, on 21 July 1983.

## Instruments and isolines

Rainfall is measured with a rain gauge; in general rain is heavy near the equator and decreases towards the poles. Humidity is measured with a hygrometer; when relative humidity reaches 100% the air is saturated. Wind speed is measured with an anemometer and its direction with a wind vane. The Beaufort scale was made in 1805 by Francis Beaufort, an officer of the Royal Navy. The isolines the book gives: isotherm, isohel, isobar, isohyets and isallobar.

## Air pressure

The standard air pressure at sea level is 1013.25 millibars. The highest pressure recorded is 1083 millibars (Agata, Russia, 31 December 1968); the lowest is 870 millibars (the eye of a typhoon near Guam). On a weather map a low-pressure area is marked "L" and a high-pressure area "H". A low is also called a cyclone and a high an anticyclone.

## Winds

The Std 9 book gives four groups of wind: planetary winds, periodic winds, variable winds and local winds. The Std 8 book says three; so remember the names, not the number of groups. The trade winds, westerlies and polar easterlies are planetary winds. In the southern hemisphere the westerlies are called the "Roaring Forties" at 40° latitude, the "Furious Fifties" at 50° and the "Shrieking Sixties" at 60°. Because the Earth rotates, winds turn to the right in the northern hemisphere and to the left in the southern; this is the "Coriolis effect", the same as "Ferrel's law" proposed by William Ferrel.

## Local winds

The local winds the book gives, with their places: Foehn (the Alps), Sirocco (the north coast of Africa), Chinook (the Rockies), Loo (the Thar desert, India), Mistral (France) and Bora (Italy). Of these, the one that blows in India is the Loo, the wind of the Thar desert. In a matching question, hold each wind and its place as a pair.

## Cyclones

"Cyclone" is a Greek word meaning "coiled snake". A tropical cyclone has different names in different places: cyclone in the Indian Ocean, typhoon in the western Pacific and Japan, hurricane in the eastern Pacific and the Atlantic, baguio in the Philippines and willy-willy in Australia. In the super cyclone that hit the Odisha coast on 29 October 1999 the wind blew at 260 km an hour and about 10,000 people died. To name Indian Ocean cyclones, Bangladesh, India, the Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand met in 2000; in 2004 each country gave a list of names. The temperate cyclonic wind that forms in the Mediterranean region and reaches India is called the "Western Disturbance".

## Clouds and the kinds of rain

All clouds are found only in the troposphere. Cumulonimbus clouds bring heavy rain with thunder. Cirrus clouds bring no rain; they are also called "mares' tails". Rainfall is of three kinds: convectional rain, cyclonic or frontal rain, and orographic rain. Convectional rain falls in the evening in equatorial areas, so it is also called "4 o'clock rain". In orographic rain the windward side gets heavy rain; the leeward side is the "rain-shadow region". Mawsynram is on the windward side and Shillong on the leeward side. In the south-west monsoon season Mangalore on the west coast gets 280 cm of rain, while Bengaluru, in the rain shadow of the Western Ghats, gets only 50 cm.

## India's location and climate

India lies from 8°4' N to 37°6' N latitude. The Tropic of Cancer, at 23°30' N, divides the country into two equal halves. India's climate is defined as a tropical monsoon climate. The word "monsoon" comes from the Arabic word "mausim", meaning season.

## India's four seasons

Indian meteorologists recognise four seasons: winter (January-February), summer (March-May), the south-west monsoon season (June-September) and the north-east monsoon season (October-December). In winter the Western Disturbance from the Mediterranean region brings rain to Punjab, Haryana and Himachal Pradesh and snow to Jammu and Kashmir; this rain helps the winter wheat. The Western Disturbance is not a monsoon. In May the "mango showers", rain with thunder on the coasts of Kerala and Karnataka, help mangoes ripen early. In April and May the Nor'westers (Kalbaisakhi) from the north-west bring rain to Bihar, West Bengal and Assam.

## The south-west and north-east monsoons

The south-west monsoon usually starts in south India in the first week of June; it reaches the Konkan coast in the second week of June and all of India by 15 July. Its start in south India with thunder and lightning is the "burst of the monsoon". The Aravalli range lies parallel to the Arabian Sea branch, so Rajasthan gets no rain from that branch. 75 per cent of India's total rainfall comes in the south-west monsoon season. Through the north-east monsoon, Kerala, Andhra, Tamil Nadu and the interior of south Karnataka get about 35 per cent of their total rain. Mawsynram in Meghalaya gets the most rain in the world (1141 cm); India's annual average rainfall is 118 cm. Example: the monsoon rain that falls in Tamil Nadu from October to December is the rain of the north-east monsoon.

**Key numbers**

| | |
| --- | --- |
| Nitrogen and oxygen in the atmosphere | 78%, 21% (class 8, page 111; class 9, page 193) |
| Height of the troposphere: poles / equator | 8 km / 18 km (class 9, page 194) |
| Stratosphere; mesosphere | up to 50 km; 50-80 km (class 9, page 194) |
| Lapse rate | 6.5°C per 1000 metres (class 8, page 113) |
| Highest temperature, Death Valley | 56.7°C, 10 July 1913 (class 8, page 115) |
| Lowest temperature, Vostok | -89.2°C, 21 July 1983 (class 8, page 115) |
| Standard air pressure at sea level | 1013.25 millibars (class 8, page 116) |
| Beaufort scale | 1805 (class 8, page 118-119) |
| Odisha super cyclone | 29 October 1999; 260 km an hour (class 9, page 198) |
| Cyclone names: meeting; lists | 2000; 2004 (class 9, page 199) |
| South-west monsoon covers all of India | 15 July (class 10, page 155) |
| Share of the south-west monsoon | 75% of the total rain (class 10, page 156) |
| Mawsynram; India's annual average | 1141 cm; 118 cm (class 10, page 157) |

> **Exam tip:** Memory hooks
Say the layers from the bottom: "tropo, strato, meso, thermo, exo". Each layer has one mark: troposphere = weather and clouds; stratosphere = ozone and jets; mesosphere = burning meteors; thermosphere = the ionosphere that returns radio waves; exosphere = the aurora. Hold the seasons as "two, three, four, three months": winter (January-February), summer (March-May), south-west (June-September), north-east (October-December). "Forty roars, fifty is furious, sixty shrieks".

**Easy to confuse**

| | |
| --- | --- |
| Weather / climate | short-term state / long-term average; the books give the period as 35 and about 30-35 years, so remember only the definition |
| Argon 0.97% / 0.93% | Std 8 / Std 9; do not hold either as the single answer |
| Ozone / burning meteors | stratosphere / mesosphere |
| Ionosphere / aurora | thermosphere (radio waves) / exosphere |
| Cyclone / anticyclone | low pressure "L" / high pressure "H" |
| Cumulonimbus / cirrus clouds | heavy rain with thunder / no rain, "mares' tails" |
| Mawsynram / Shillong | windward side / leeward side (rain shadow) |
| Mango showers / Nor'westers / Loo | Kerala and Karnataka coasts / Bihar, West Bengal, Assam / the Thar desert |
| Western Disturbance / monsoon | winter rain for Punjab, Haryana and Himachal; it is not a monsoon |
| Typhoon / hurricane / willy-willy | western Pacific and Japan / eastern Pacific and Atlantic / Australia |

> **Exam tip:** Quick check
1. In which layer does all weather happen? Answer: the troposphere.
2. Which layer has the most ozone molecules? Answer: the stratosphere.
3. What share of India's total rain comes in the south-west monsoon season? Answer: 75 per cent.
4. Which local wind blows in the Thar desert? Answer: the Loo.
5. Which instrument measures humidity? Answer: the hygrometer.

Source: சமூக அறிவியல், பத்தாம் வகுப்பு, தமிழ் வழி (Social Science, Standard X, Tamil medium; Samacheer Kalvi) (10ஆம் வகுப்புச் சமூக அறிவியல், புவியியல் அலகு 2 (இந்தியா – காலநிலை மற்றும் இயற்கைத் தாவரங்கள்), பக்கம் 153-157, 161; 8ஆம் வகுப்புச் சமூக அறிவியல், புவியியல் அலகு 2 (வானிலை மற்றும் காலநிலை), பக்கம் 111-119; 9ஆம் வகுப்புச் சமூக அறிவியல், புவியியல் அலகு 3 (வளிமண்டலம்), பக்கம் 193-202)

## Sources

- [Official syllabus page](https://www.tnpsc.gov.in/english/syllabus.html)
- [Notifications](https://www.tnpsc.gov.in/English/Notification.aspx)