Class 9 Geography Chapter 4 - Climate Full Notes

CBSE Notes Class 9 Geography Chapter 4 - Climate

1. Climate vs Weather 

2. Climatic Controls 

3. Factors affecting India's climate 

4. The Indian Monsoon

5. The Seasons

6. Distribution of rainfall. ☔ 

7. Monsoon as a unifying bond 

1. Climate vs weather

Climate :- It 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 :- It refers to the state of the atmosphere over an area at any point of time.  

The elements of weather and climate are the same .

➡️ On the basis of the generalised monthly atmospheric conditions, the year is divided into seasons such as Winter, Summer or Rainy seasons.

Note :- The climate of India is described as the ‘monsoon’ type. 

2. CLIMATIC CONTROLS

There are six major controls of the climate of any place. 

They are: 

 1. 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.

2. Altitude - As one goes from the surface of the earth to higher altitudes,the atmosphere becomes less dense and temperature decreases. The hills are therefore cooler during summers. 

3. Pressure and wind system --- This system of any area depend on the latitude and altitude of the place. Thus it influences the temperature and rainfall pattern.

4. Distance from the sea --  sea exerts a moderating influenceon climate: 

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). 

5. Ocean currents -- Any coastal area with warm or cold currents flowing past it, will be warmed or cooled respectively. 

6. Relief features --  

Relief too 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. 

The leeward side of mountains remains relatively dry.

3. FACTORS AFFECTING INDIA’S CLIMATE

Latitude :- Almost half of the country, lying south of the Tropic of Cancer, belongs to the tropical area.

All the remaining area, north of the Tropic, lies in the sub-tropics.

Therefore, India’s climate has characteristics of tropical as well as subtropical climates.

Altitude :- The Himalayas prevent the cold winds from Central Asia from entering the subcontinent. It is because of these mountains that this subcontinent experiences comparatively milder winters .

Himalaya 

Pressure and Winds

Pressure (Air Pressure) :- The pressure exerted on the earth by air is called air pressure or pressure. 

Low Pressure :-- When temperature increases, the air become hot enough to rise above and thus creates a zone of low pressure. 

High Pressure :-- When temperature decreases, the air become cold enough to subside below and thus creates a zone of high pressure. 

Wind::-- The horizontal movement of air from high pressure  to low pressure is called wind. 

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

• Pressure and surface winds;

• Upper air circulation; and

• Western cyclonic disturbances and tropical cyclones.

• Pressure and surface winds;

➡️ India lies in the region of north easterly winds. 

➡️These winds originate from the subtropical high-pressure belt of the northern hemisphere.

➡️ They blow south, get deflected to the right due to the Coriolis force, and move on towards the equatorial low-pressure area.

➡️ Generally, these winds carry very little moisture as they originate and blow over land.

➡️Therefore, they bring little or no rain. Hence, India should have been a complete desert. 

Why this does not happen? 

This does not happen because pressure and wind conditions over India are unique.

In Winter

➡️ During winter, there is a high-pressure area north of the Himalayas. in winter  Cold dry winds blow from this region to the low-pressure areas over the oceans to the south. 

In Summer 

➡️ Whereas In summer, a low-pressure area

develops over interior Asia as well as over

northwestern India and high pressure develops over oceanic areas.  This causes a complete reversal of the direction of winds during summer.

➡️ Air moves in south west direction over Indian Peninsula from the oceanic areas. These winds blow over the warm oceans, gather moisture and bring widespread rainfall over the mainland of India.

The Upper Air Circulation 

The upper air circulation in this region is dominated by a westerly flow. 

➡️  An important component of this flow is the jet stream.

➡️These jet streams are located approximately over 27°-30° north latitude, therefore, they are known as subtropical westerly jet streams.

➡️ Over India, these jet streams blow south of the Himalayas in winter and blow north of himalayas in summer. 

These also bring western cyclonic disturbances along with it. 

Western Cyclonic Disturbances

The western cyclonic disturbances are weather phenomena of the winter months.

They are brought in by the westerly flow from the Mediterranean region. 

They usually influence the weather of the north and north-western regions of India.

4. THE INDIAN MONSOON

➡️The climate of India is strongly influenced by monsoon winds. 

➡️The Arab sailors who had come to India as traders named this seasonal reversal of the wind system ‘monsoon'.

➡️The monsoons are experienced in the

tropical area roughly between 20° N and 20°S.

 To understand the mechanism of the monsoons, the following facts are important.

(a) 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.

(b) The shift of the position of Inter Tropical

Convergence Zone (ITCZ) in summer, over

the Ganga plain.

(c) The presence of the high-pressure area,

east of Madagascar, approximately at

20°S over the Indian Ocean. The intensity

and position of this high-pressure area

affects the Indian Monsoon.

(d) 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.

(e) 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.

Inter Tropical Convergence Zone

The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes. 

This is where the northeast and the southeast trade winds converge. 

This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.

Some other factors 

Apart from this, that changes in the pressure conditions over the southern oceans also affect the monsoons.

Normal situation 

 The tropical eastern south Pacific

Ocean experiences high pressure and the tropical eastern Indian Ocean experiences low pressure.

Southern Oscillation 

But in certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean. 

This periodic change in pressure conditions is known as the Southern Oscillation or SO.

El-Nino

A feature connected with the SO is the El Nino phenomenon in which a warm ocean current that flows past the Peruvian Coast, in place of the cold Peruvian current, every 2 to 5 years.

 The changes in pressure conditions are connected to the El Nino.

Hence, the phenomenon is referred to as ENSO

(El Nino Southern Oscillations).

El Nino: This is a name given to the periodic development of a warm ocean current along the coast of Peru as a temporary replacement of the cold Peruvian current. 

‘El Nino’ is a Spanish word meaning ‘the child’, and refers to the baby Christ,as this current starts flowing during Christmas.

Impact of El-Nino

The presence of the El Nino leads to :

a) an increase in sea-surface temperatures and 

b) weakening of the trade winds in the region.

THE ONSET OF THE MONSOON AND WITHDRAWAL

➡️ The duration of the monsoon :  between 100-120 days

➡️ Time Period : From early June to mid-September.

➡️ Burst : Around the time of its arrival, the sudden increase in the rainfall continuously for several days is known as the ‘burst’ of the monsoon.

Branch:

➡️The monsoon proceeds into two – the

Arabian Sea branch and the Bay of Bengal

branch.

➡️ The Arabian Sea branch reaches

Mumbai about ten days later on

approximately the 10th of June. 

➡️ By mid-June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country. 

➡️ The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June.

➡️ The lofty mountains causes the monsoon winds to deflect towards the the Ganga plains.

➡️The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains.

Note--Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June. 

➡️By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country.

Withdrawal of the Monsoon 

➡️The withdrawal or the retreat of the monsoon is a more gradual process .

 

➡️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. 

➡️ By early December, the monsoon has withdrawn from the rest of the country.

Note: The islands receive the very first monsoon showers.

Arrival :The monsoon arrives there from the last week of April to the first week of May. 

Withdrawal : The withdrawal, takes place from the first week of December to the first week of January. 

The seasons 

The monsoon type of climate has seasonal pattern.

 There are four main seasons in India 

1. The cold weather season

2.  The hot weather season

3. The advancing monsoon and

4.  The Retreating Monsoon

The Cold Weather Season (Winter)

 Begins --- from mid-November to February.

➡️ December and January are the coldest

months in the northern part of India. 

 Features 

➡️ Days are warm and nights are cold.

➡️Frost is common in the north and the higher slopes of the Himalayas experience snowfall.

➡️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.

More facts

➡️ The weather is normally marked by :   clear sky, low temperatures ,low humidity and

 

Winter Rainfall 

➡️ The ☔rainfall is also observed in winter season due to cyclonic disturbances from the west and the northwest. 

➡️ They cause the much-needed winter rains over the plains and snowfall in the mountains.

 ➡️Although the total amount of winter rainfall locally known as ‘mahawat’ is small, they help in the cultivation of ‘rabi’ crops.

Note --The peninsular region does not have a well-defined cold season.

➡️ There is hardly any noticeable seasonal change in temperature pattern during winters due to the moderating influence of the sea.

The Hot Weather Season (Summer)

duration--- March to May

Traits

Rise in temperature 

➡️ Temperatures increase in most of the places except the peninsular India, where temperatures remain lower due to the moderating influence of the oceans.

Loo

➡️A very strong, hot and dry wind called ‘loo’  blow during the day over the north and northwestern India. 

 ➡️Direct exposure to these winds may even prove to be fatal. 

Dust storm 

➡️ Dust storms are very common during the month of May in northern India.

 ➡️These storms bring temporary relief as they lower the temperature and may bring light rain and cool breeze. 

Storms

➡️This is also the season for localised thunderstorms,along with violent winds, heavy rains often accompanied by hail.

Note-- In West Bengal, these storms are known as the ‘Kaal Baisakhi’.

Premonsoon showers

Towards the close of the summer season,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’.

Advancing Monsoon (The Rainy Season)

Begin ---- By early June, the monsoon arrives in India. 

Spread -- With the exception of the extreme

north-west, the monsoon winds cover the

country in about a month.

Traits 

➡️It brings about a total change in the

weather. 

➡️Early in the season, the windward side of the Western Ghats receives very heavy rainfall, more than 250 cm.

 ➡️The maximum rainfall of this season is received in the north-eastern part of the country. 

Note--Mawsynram receives the highest average rainfall in the world.

➡️ Rainfall in the Ganga valley decreases from the east to the west. Rajasthan and parts of Gujarat get scanty rainfall.

Breaks in rainfall

 ➡️It has wet and dry spells. In

other words, the monsoon rains take place

only for a few days at a time.

➡️ They are interspersed with rainless intervals. These breaks in monsoon are due to the movement of the monsoon trough. 

Flood 

➡️Sometimes heavy rains bring in floods causing damage to life and property in the plains.

 ➡️The monsoon is known for its uncertainties. While it causes heavy floods in one part, it may be responsible for droughts in the other.

➡️It sometimes disturbs the farming schedule of millions of

farmers all over the country.

Retreating/Post Monsoons(The Transition Season)

Duration --  October-November

 ➡️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. 

Traits

➡️The retreat of the monsoon is marked by clear skies and rise in temperature.

 ➡️While day temperatures are high, nights are cool and pleasant. 

➡️Due to the conditions of high

temperature and humidity, the weather

becomes rather oppressive during the day.

This is commonly known as ‘October heat’.

DISTRIBUTION OF RAINFALL

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

➡️However, it is less than 60 cm in western Rajasthan and adjoining parts of Gujarat, Haryana and Punjab. 

Rainfall is equally low in the interior of the Deccan plateau, and east of the Sahyadris and Leh in Jammu and Kashmir.

➡️The rest of the country receives moderate rainfall.

➡️Snowfall is restricted to the Himalayan region.

MONSOON AS A UNIFYING BOND

This enables northern India to have uniformly higher temperatures .

The Indian landscape, its animal and plant life,its entire agricultural calendar and the life of the people, including their festivities, revolve around this phenomenon. 

These monsoon winds bind the whole country by providing water to set the agricultural activities in motion.

The river valleys which carry this water also unite as a single river valley unit.

~Seminary Classes