CBSE Class 9 Geography Chapter 4 | Climate – NCERT NOTES And Explanation

What is 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).

What is 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:

·       temperature,

·       atmospheric pressure

·       wind

·       humidity

·       precipitation

On the basis of the monthly atmospheric conditions, the year is divided into seasons such as:

• Winter
• Summer
• Rainy Seasons

Climatic Controls

The climate of any place depends on the following factors:

1) Latitude

The amount of solar energy received varies according to latitude due to the curvature of the earth. That’s why air temperature generally decreases from the equator towards the poles.

2) Altitude

The atmosphere becomes less dense and temperature decreases when we go to the higher altitude from the earth surface. This is the reason why hills are cooler during summers.

3) Pressure and wind system

The pressure and wind system of any area depends on the latitude and altitude of the place. It influences the temperature and rainfall pattern.

4) Distance from the sea

The sea exerts a moderating influence on the climate. As the distance from the sea increases, its moderating influence decreases and people experience extreme weather conditions. This condition is known as continentality i.e. very hot during summers and very cold during winters.

5) Ocean currents

Ocean currents along with onshore winds affect the climate of the coastal areas. For example, any coastal area with warm or cold currents flowing past it, will be warmed or cooled if the winds are onshore.

6) Relief features

Relief plays a major role in determining the climate of a place. High mountains act as barriers for cold or hot winds. They may also cause precipitation if they are high enough and lie in the path of rain-bearing winds. Precipitation is any form of moisture which falls to the earth. It includes rain, snow, hail and sleet.

Factors Affecting India’s Climate

1)Latitude

The Tropic of Cancer passes through the middle of the country from the Rann of Kachchh in the west to Mizoram in the east. India’s climate has characteristics of tropical as well as subtropical climates.

2)Altitude

India has mountains to the north and also has a vast coastal area where the maximum elevation is about 30 metres. Owing to the mountains, subcontinent experiences comparatively milder winters as compared to central Asia.

3)Pressure and 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

 

1)Pressure and Surface Winds
India lies in the region of North-Easterly surface winds. These winds originate during winter from the sub-tropical high-pressure belt of the Northern hemisphere.

These winds blow South, get deflected to the right due to the Coriolis force and move towards the equatorial-low pressure region. These winds originate and blow over land and hence, carry very little moisture. Therefore, they bring no rain or very little rain. The unique feature of Indian pressure and wind conditions is its complete reversal. During winter, high-pressure areas develop over the areas North of Himalayas. This causes cold dry winds blow from the area towards low-pressure area over the oceans to the South.

In summer, due to high temperature, low-pressure area develops over interior Asia and over North-Western India. Air from high-pressure areas blow towards this region resulting in complete reversal of wind direction.

As these winds from high pressure area of Southern Indian ocean crosses the equator and turns right towards low pressure areas of Indian sub-continent. These winds gather large moisture while moving over the warm ocean and bring widespread rainfall over the mainland of India. These winds are known as the South-West Monsoon winds.

Coriolis force: An apparent force that as a result of the Earth’s rotation, deflects moving objects like air currents to the right in the Northern Hemisphere and to the left in the Southern hemisphere. This is known as Ferrel’s law. This law states that a wind in any direction tends to deflect towards right (West to East) in the Northern Hemisphere and to the left in the Southern Hemisphere with a force that is directly proportional to the mass of the wind in question, its velocity, the sine of the latitude and the angular velocity of the Earth’s rotation.

 

2)Upper Air Circulation and Western Cyclonic Disturbances
The upper air circulation of the region (Indian subcontinent) is dominated by a westerly flow which is governed by Jet stream. Due to their location over 27°-30° N latitude, these jet streams are known as sub-tropical westerly jet streams. They blow South of the Himalayas, throughout the year except in summer.

Jet Strem: These 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. A number of separate jet streams have been identified. The most constant are the mid-latitude and the subtropical jet stream.

 

3)Western Cyclonic Disturbances and Tropical Cyclones
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. Tropical cyclones occur during the monsoon as well as in October-November and are part of the easterly flow. These disturbances affectting  coastal regions of the country.

The westerly flow brings the Western cyclonic disturbances in the North and North-Western India. In summer, the sub-tropical westerly jet stream moves North of the Himalayas due to apparent shifting of the sun. An easterly jet stream, called sub-tropical easterly jet stream, blows over peninsular India approximately over 14° N during the summer months.

Atmospheric Condition over the Indian Subcontinent in the month of  January

Atmospheric Condition over the Indian Subcontinent in the month of  June

The Indian Monsoon:

The Indian Monsoon

What is Monsoon: The seasonal reversal of the wind system is known as “monsoon”.

The climate of India is strongly influenced by monsoon winds.

The monsoons are experienced in the tropical area roughly between 20° N and 20° S. Go through the following facts to understand the mechanism of the monsoons in a better way:

v The differential heating and cooling of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure.

v The shift of the position of Inter-Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain, is also known as the monsoon-trough during the monsoon season.

v The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean affects the Indian Monsoon.

v The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of low     pressure over the plateau at about 9 km above sea level.

v The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer also impact the Indian Monsoon.

Southern Oscillation or SO. The changes in the pressure conditions over the southern oceans also affect the monsoons. The irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean that affects the climate of the tropics and subtropics is known as Southern Oscillation or SO.

EL NINO: oceans also affect In the perodic Development of Warm Ocean Current along the coast of peru is called EL Nino

Due to EL Nino the Sea Surface temperature increases and the trade wind in this region become weak

The Onset of the Monsoon and Withdrawal

Monsoon Advancing

The duration of the monsoon is between 100-120 days from early June to mid-September. Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days, which is known as the “burst” of the monsoon.

Arrival of the Monsoon in different parts of India

The monsoon arrives at the southern tip of the Indian peninsula generally by the 1st week of June. Subsequently, it proceeds into two – the Arabian Sea branch and the Bay of Bengal branch.

• The Arabian Sea branch reaches Mumbai on approximately the 10th of June.
• The Bay of Bengal branch arrives in Assam in the 1st week of June.
• By mid-June, the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country.
• The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains.
• Delhi receives the monsoon showers from the Bay of Bengal branch by the end of June.
• By the first week of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoon.
• By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country.

Withdrawal of the Monsoon

The withdrawal of the monsoon begins in northwestern states of India by early September. By mid-October, it withdraws completely from the northern half of the peninsula. From north to the south, withdrawal of monsoon takes place from the first week of December to the first week of January. By early December, the monsoon has withdrawn from the rest of the country.

The Seasons

4 main seasons can be identified in India:

1.    The cold weather season (Winter)

2.    The hot weather season (Summer)

3.    The advancing monsoon (Rainy Season)

4.    The retreating monsoon with some regional variations (Transition Season)

Let’s discuss each of them in detail:

1)The Cold Weather Season (Winter)

Winter

Winter begins from mid-November in northern India and stays till February.

ü December and January are the coldest months in the northern part of India.         

ü The temperature decreases from south to the north.

ü Days are warm and nights are cold.

ü The weather is marked by clear sky, low temperatures and low humidity and feeble, variable winds.

ü This season is extremely important for the cultivation of ‘rabi’ crops.

The Hot Weather Season (Summer)

Summer

From March to May, hot weather season is observed in India. 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 the strong, gusty, hot, dry winds blowing during the day over the north and northwestern India.

Towards the close of the summer season, pre-monsoon showers come which help in the early ripening of mangoes, and are often referred to as ‘mango showers’.

Retreating Monsoon

Advancing Monsoon (The Rainy Season)

South-east trade winds originate over the warm subtropical areas of the southern oceans. They cross the equator and blow in a southwesterly direction entering the Indian peninsula as the south-west monsoon. The monsoon winds cover the country in about a month. Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.

A phenomenon associated with the monsoon is its tendency to have ‘breaks’ in rainfall. These breaks in monsoon are related to the movement of the monsoon trough. The trough and its axis keep on moving northward or southward and determine the spatial distribution of rainfall. The

frequency and intensity of tropical depressions also determine the amount and duration of monsoon rains. The monsoon is known for its uncertainties. The alternation of dry and wet spells vary in intensity, frequency and duration.

Retreating/Post Monsoons (The Transition Season)

The months of October-November form a period of transition from hot rainy season to dry winter conditions. The retreat of the monsoon is marked by clear skies and rise in temperature. While day temperatures are high, nights are cool and pleasant. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October heat’. In the second half of October, the mercury begins to fall rapidly in northern India.

Distribution of Rainfall

Seasonal Rainfall (June-September)

1.    Parts of the western coast and northeastern India receive over about 400 cm of rainfall annually.

2.    Rainfall is less than 60 cm in western Rajasthan and adjoining parts of Gujarat, Haryana and Punjab.

3.    Rainfall is low in the interior of the Deccan plateau, and east of the Sahyadris.

4.    Snowfall is restricted to the Himalayan region.

5.    The annual rainfall is highly variable from year to year.

Monsoon as a Unifying Bond

Annual Rainfall

The unifying influence of the monsoon on the Indian subcontinent is quite noticeable. The seasonal alteration of the wind systems and the associated weather conditions provide a rhythmic cycle of seasons. The uncertainties of rain and uneven distribution are very much typical of the monsoons. Year after year, people of India from north to south and from east to west, eagerly await the arrival of the monsoon. These monsoon winds bind the whole country by providing water to set the agricultural activities in motion.

      I.            Entire phenomenon such as landscape,animal and plant life revolves around the monsoon

  II.            Whole agriculture depends on monsoon

III.            The life of people including their festival revolves around the monsoon

IV.            It provides water to river and ponds which unites the whole country