NCERT Notes for Class 9 Social Science Geography Chapter 4 Climate
Chapter 4 Climate NCERT Notes
Chapter Name | Climate Notes |
Class | CBSE Class 9 |
Textbook Name | Contemporary India- I Class 9 |
Related Readings |
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Climate
Climate refers to the sum total of weather conditions and variations over a large area for a long period of time (more than thirty years).
Weather
Weather refers to the state of the atmosphere over an area at any point of time.The elements of weather and climate are the same, i.e. temperature, atmospheric pressure, wind, humidity and precipitation.
On the basis of the monthly atmospheric conditions, the year is divided into seasons such as :
- Winter
- Summer
- Rainy Seasons
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weather |
Climate |
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Weather is a day-to-day state of the atmosphere of an area at any given point of time. |
Climate is the total of weather conditions of a given place over a longer period of time. |
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Weather changes on a daily basis. |
Climate of a given place remains constant for over 30 to 40 years. |
Monsoon
- The word monsoon is derived from the Arabic word 'Mausim' which literally means season.
- Monsoon refers to the seasonal reversal in the wind direction during a year.
The Climate of India
- The climate of India is described as the 'monsoon' type. In Asia, this type of climate is found mainly in the south and the southeast. Despite an overall unity in the general pattern, there are perceptible regional variations in climatic conditions within the country.
Examples
- There is hardly 20 cm of rainfall annually in the western parts of Rajasthan and Gujarat, Assam and Arunachal Pradesh receive more than 250 cm of rainfall. During winters, the night temperature may fall below -45°C, while on the same night, Thiruvananthapuram may experience a temperature of 45°C. These variations have resulted in different lifestyles of the people.
Climatic Controls
There are six major controls of the climate of any place. They are:
- Latitude
- Altitude
- Pressure and wind system
- Distance from the sea (continentally)
- Ocean currents
- Relief features.
Factors affecting climate of India
- Latitude
- Altitude
- Distance from sea
- Ocean current
- Relief features or landscape or surface of earth.
- Pressure and Wind system
1. Latitude
- Tropic of Cancer, a latitude, passes through India at 23°26’N to the north of the equator. It passes through eight Indian states running from the Rann of Kutch of Gujarat to Mizoram in the east.
- The Tropic of Cancer divides the country into two halves. The areas in India which lie to the north of the Tropic of Cancer lie in the sub-tropical regions, while the areas to the south of the tropic lie in the tropical regions.
- The temperature remains high throughout the tropical regions.
2. Altitude
- The temperature declines as we travel to places above the sea level. Thus, the temperature decreases with an increase in altitude.
- There is a decrease of 1°C for every 166 m rise in height. This is the reason that hill stations in India are cooler than the plain regions.
3. Air Pressure and Surface Winds
- The climate and weather conditions in India are governed by the following atmospheric conditions:
- Pressure and surface winds
- Upper air circulation
- Western cyclonic disturbances and tropical cyclones
- Air moves from the high-pressure area over the southern Indian Ocean, in a south-easterly direction, crosses the equator and turns right towards the low-pressure areas over the Indian subcontinent. These are known as the Southwest Monsoon winds. These winds blow over the warm oceans, gather moisture and bring widespread rainfall over the territories of India.
4. Jet streams
- Jet streams are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere.
- Their speed varies from about 110 km/h in summer to about 184 km/h in winter.
- These jet streams are located approximately over 27°-30° north latitude, They are also known as subtropical westerly jet streams.
- Over India, jet streams blow south of the Himalayas, all through the year except in summer.
- In summer, the subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the sun.
- An easterly jet stream, called the sub- tropical easterly jet stream blows over peninsular India, approximately over 14°N during the summer months.
5. The western cyclonic
- The western cyclonic disturbances experienced in the north and north-western parts of the country are brought in by this westerly flow.
- The western cyclonic disturbances are weather phenomena of the winter months brought in by the westerly flow from the Mediterranean region.
- They usually influence the weather of the north and north-western regions of India.
The Indian Monsoon
- India has a monsoon type of climate as its climate is strongly influenced by the monsoon winds. It is important to keep certain facts in mind before studying the monsoon season in India. These are
- The differences in heating and cooling of land and water results in the creation of low pressure on the Indian mainland. The seas at the same time experience high pressure conditions.
- Inter Tropical Convergent Zone (ITCZ) is a broad trough of low pressure in the equatorial latitudes.
- The presence of high pressure area east of Madagascar over the Indian Ocean affects the Indian monsoon.
- The movement of the westerly jet stream to the north of the Himalayas and the tropical easterly jet stream over the Indian Peninsula also affect the rainfall in India.
- It has been seen that apart from other factors, the changes in the pressure conditions over the southern oceans also affect the monsoons. Generally, when the tropical eastern South Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. However, it has been noticed that there has been a reversal in the pressure conditions. While the eastern South Pacific Ocean has low pressure conditions, the eastern Indian Ocean has relatively high pressure conditions. This periodic change in pressure conditions is known as the Southern Oscillation or SO.
The Onset of the Monsoons and their Withdrawal
- In India, duration of the monsoon is generally from the months of June to mid-September. When the monsoon arrives, the intensity of the rainfall increases which continues for several days; this is known as the ‘burst’ of the monsoon.
- By the first week of June, the monsoon arrives in Kerala. It is then divided into two branches-the Bay of Bengal branch and the Arabian Sea branch.
- The Arabian Sea branch reaches Mumbai in the second week of June. The Bay of Bengal branch also arrives in Assam in the first week of June. The mountains in the region lead to the deflection of the monsoon winds over the North Indian Plains.
- By mid-June, the monsoon strikes the central parts of the country and the Saurashtra and Kutch regions.
- Uttar Pradesh, Punjab, Haryana and eastern Rajasthan receive rainfall by the first week of July.
- The monsoon winds begin to withdraw from the northwestern parts of India by early September. It withdraws from the peninsular region by mid-October and it completely withdraws from the country by the first week of December.
The Seasons
A country with a monsoon type of climate experiences distinct seasons. India experiences the following seasons :- The cold weather season
- The hot weather season
- The advancing monsoon
- The retreating monsoon (with some regional variations)
1. The cold weather season (Winter Season)
- It begins from mid November in northern India and stays till February.
- The temperature decreases from south to the north.
- Days are warm and nights are cold. Frost is common in the north and the higher slopes of the Himalayas experience snowfall.
- The northeast trade winds prevail over the country.
- In the northern India, a feeble high-pressure region develops, with light winds moving outwards from this area.
2. The Hot Weather Season (The Summer Season) :-
- It begins from March to May.
- The summer months experience rising temperature and falling air pressure in the northern part of the country.
- A striking feature of the hot weather season is the ‘Loo’. ‘Loo’ is strong, gusty, hot, dry winds blowing during the day over the north and northwestern India.
- Dust storms are common and sometime may bring light rain and cool breeze.
- During summer Sometimes localized thunderstorm along with violent wind, torrential downpours accompanied by hail occurs which is called ‘Kaal Baisakhi’ in west Bengal.
- Pre-monsoon showers are common especially, in Kerala and Karnataka. They help in the early ripening of mangoes, and are often referred to as ‘mango showers’.
3. Advancing Monsoon (The Rainy Season) :-
- By week of June, low pressure conditions get intensified over the North Indian Plains. These conditions attract the southeast trade winds which originate from the Southern oceans.
- These winds blow over the warm oceans and thus bring moisture and rainfall to the country.
- During the early monsoon season, the places located on the windward side of the Western Ghats receive heavy rainfall up to 250 cm.
- The northeastern parts of the country receive heavy rainfall. Mawsynram in Meghalaya receives the highest amount of rainfall in the world.
- As we go from the east to the west, the rainfall decreases. Rajasthan and Gujarat receive scanty rainfall.
- Rain does not fall continuously during the monsoon season. Thus, there are dry spells and wet spells. Monsoons are uncertain and irregular.
4. Retreating Monsoon
- During October and November, the southwest monsoon winds become weaker and start retreating from the Northern Plains by the beginning of October.
- The retreat is indicated by clear skies and an increase in the temperature in the northern plains. This increase in temperature is termed ‘October heat’.
- At this time, the low pressure conditions shift to the Bay of Bengal, giving rise to cyclonic depressions. These cyclonic depressions often result in the destruction of life and property, generally on the eastern coast and the southern coast.
- Most of the rainfall in the Coromandel Coast is derived from depressions and cyclones.
Distribution of Rainfall
- Rainfall distribution is not the same for every part of the country. While northeastern India and the parts of the western coast receive about 400 cm of rainfall annually, western Rajasthan and the western parts of Gujarat, Punjab and Haryana receive less than 60 cm of rainfall.
- The winds which rise from the Arabian Sea are full of moisture. These winds strike the mountains on the western side resulting in heavy rainfall. By the time these winds reach the Eastern Ghats, they are already dry as they already shed their moisture. Hence, the Western Ghats receive more rainfall than the Eastern Ghats.
- Rainfall is also low in the Eastern Ghats, interior parts of the Deccan Plateau and the Leh region of Jammu and Kashmir.
- The other parts of the country receive moderate rainfall.
- Such a distribution of rainfall causes great climatic variations. While the eastern parts of the country almost get flooded every year, the western parts of the country experience drought like situation.
Monsoon as a Unifying Bond
- Although the monsoon winds are irregular and uncertain, they unify the entire country. The farmers eagerly wait for the arrival of rainfall. Rainfall provides the water required to set agricultural activities in motion. Its arrival is welcomed with the celebration of festivals, singing and dancing.