Detailed note | CLIMATE | Chapter 4 | Class 9 | Geography

CLIMATE

Introduction 

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:-

(i) Temperature

(ii) Atmospheric pressure

(iii) Wind

(iv) Humidity

(v) Precipitation

Monsoon: 

Monsoon refers to the seasonal reversal in the

wind direction during a year. 

The climate of India is described as the monsoon.

Climatic controls

There are six major controls of the climate of any place They are:

Latitude:-

 Due to the curvature of the earth, the

amount of solar energy received varies

according to latitude. As a result, air

temperature generally decreases from the

equator towards the poles.

Altitude:- 

As one goes from the surface of the earth to higher altitudes, the atmosphere becomes less dense and temperature decreases.

Pressure and wind system:- 

The pressure and wind system of any area depend on the latitude and altitude of the place. Thus it  influences the temperature and rainfall pattern.

Distance from the sea:-

 As the distance from the sea

increases, its moderating influence decreases

and the people experience extreme weather

conditions. This condition is known as

continentality (i.e. very hot during summers

and very cold during winters).

Ocean currents:-

 Ocean currents along with onshore winds affect the climate of  the coastal areas.

Relief:-

 Relief is the difference between its highest and lowest elevations. 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. The leeward side of mountains remains relatively dry.

FACTORS AFFECTING INDIA’S CLIMATE

Latitude:- 

The Tropic of Cancer passes through the middle of the country from the Rann of Kutch in the west to Mizoram in the east. As tropic of cancer cuts india almost equally half, india experiences both tropical and sub-tropical climate.

Altitude:-

As India has a diversity of landscape from coastal area to huge mountains led to different climate pattern in different area of the country.

Pressure and Winds:-

The climate and associated weather conditions in India are governed by the following atmospheric conditions:

* Pressure and surface winds

* Upper air circulation

* Western cyclonic disturbances and tropical cyclones.

- North Easterly winds blow southward and get deflected due to coriolis force and move towards equatorial low-pressure area. These winds carry little moisture and bring little or no rain.

- During winter cold dry wind blows from Himalayan high pressure region to the low-pressure areas over the oceans to the south. In summer wind blows in a complete reversal direction Air moves from the high-pressure

area over the southern Indian Ocean towards the low-pressure areas (interior asia) 

over the Indian subcontinent. These are known

as the Southwest Monsoon winds. Which gather moisture and bring rainfall over the mainland of India.

NOTE

Coriolis force:- 

An apparent force that as a result of the earth's rotation deflects moving objects (such as air currents or projectiles) to the right in the northern hemisphere and to the left in the southern hemisphere.

Western Cyclonic Disturbances

- These are the weather phenomenon of winter.

- They are brought in by the westerly flow from the mediterranean region.

- They usually influence the weather of the 

north and north western part of the india.

Jet Stream

- 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.

Jet Stream

THE INDIAN MONSOON

- The name 'Monsoon' was given by arab traders.

- The monsoons are experienced in the

tropical area between 20° N and 20° S. 

- 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.

- In summer Inter Tropical Convergence Zone (ITCZ) normally positioned about 5°N of the

equator. It is also known as the monsoon-

trough during the monsoon season.

- The presence of the high-pressure area,

east of Madagascar, approximately at 20°S over the Indian Ocean affects the Indian Monsoon.

- 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.

Atmospheric Conditions over the Indian Subcontinent in the Month of January (Ref: NCERT)

Atmospheric Conditions over the Indian Subcontinent in the Month of June (Ref: NCERT)

NOTE

Inter Tropical Convergence Zone (ITCZ)

- ITCZ, is a belt of low pressure which circles the Earth generally near the equator where the trade winds of the Northern and Southern Hemispheres come together.

- It  moves north or south with the

apparent movement of the sun.

El Nino

- El Nino is a climate pattern that describes the unusual warming of surface waters in the eastern Pacific Ocean.

- El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.

The Onset Of The Monsoon And Withdrawal

- The duration of the monsoon is between 100-

120 days from early June to mid-September.

- Initially the normal

rainfall increases suddenly and continues

constantly for several days. This is known as

the ‘burst’ of the monsoon.

- The

withdrawal of the monsoon begins in

northwestern states of India by early

September and completes by mid-October. The withdrawal from the southern

half of the peninsula is by early

December.

The Season

Four main seasons can be identified in India.

These are as follows

1. The Cold Weather Season (Winter)

- The cold weather season begins from mid-

November in northern India and stays till

February.

- During this season, the northeast trade

winds prevail over the country. They blow from land to sea and hence, for most part of the country, it is a dry season.

- Some amount of rainfall occurs on the Tamil Nadu coast from these winds as, here they blow from sea to land.

- The inflow of  cyclonic disturbances from the west and the northwest are of immense importance for the cultivation of ‘rabi’ crops.

2. The Hot Weather Season (Summer)

- Due to the apparent northward movement of the sun, the global heat belt shifts northwards from March to May, it is hot weather season in India.

- A striking feature of the hot weather season

is the ‘loo’. These are strong, hot, dry winds blowing during the day over the north and

northwestern India.

- Dust storms are very common during the month of May in northern India. In West

Bengal, these storms are known as the ‘Kaal Baisakhi’.

- Towards the close of the summer season,

pre-monsoon showers are common especially,

in Kerala and Karnataka which refered to as 'Mango Showers'.

3. Advancing Monsoon (The Rainy Season)

- By early June, the low-pressure condition over the northern plains intensifies and attracts, the trade winds of the southern hemisphere. As these winds blow over warm oceans, they bring abundant moisture to the subcontinent. 

- The maximum rainfall of this season is received in the north-eastern part of the country. Mawsynram ( the wettest place on the earth ) in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.

- The monsoon is known for its uncertainties. i.e the monsoon rains take place only for a few days at a time and are interspersed with rainless intervals.

4. Retreating/Post Monsoons (The Transition Season)

- During October-November, with the apparent

movement of the sun towards the south, the south-west monsoon winds weaken and start withdrawing gradually. By the beginning of October, the monsoon withdraws from the Northern Plains.

- 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. 

- The low-pressure conditions, over north-

western India, get transferred to the Bay of

Bengal by early November. This shift is

associated with the occurrence of cyclonic

depressions.

DISTRIBUTION OF RAINFALL

- Western coast and northeastern India 

receive over about 400 cm of rainfall annually.

- Western Rajasthan and adjoining parts of Gujarat, Haryana and Punjab recive over about 60 cm of rainfall annually

- Rainfall is low in the interiors of the decan plateau and east of the Sahyadri.