Notes for CBSE Class 9 Social Science Geography Chapter 4 Climate can be found on this page. During this discussion (Net Explanations Teacher), we discussed each and every word from Chapter 4.
CBSE Notes Class 9 Geography Chapter 4 Climate
- Climate is affected by a number of factors.
- Indicate the mechanisms that cause India’s monsoons.
- Explain the onset and withdrawal of Monsoon in India.
- Write a short note on the summer season of India.
Weather: state of atmosphere over an area at any point of time
Climate: average of all weather conditions over a large area of land for a long period of time (more than thirty years).
The governing elements of weather and climate are:
- Atmospheric pressure
In India, it is said that the climate is monsoon-like. A climate is, however, an average of all weather conditions observed over a large area of land. There are, therefore, climatic variations within each country.
- Monsoon is derived from the Arabic word ‘mausim’ which means season.
Let’s consider the temperature as an example. In one part of India, say Rajasthan, the temperature will reach 500C, while in Jammu, it may reach 200C. Likewise, precipitation will vary between the two regions. It snows in the Himalayas, while it rains in other parts. Additionally, some parts will receive rain up to 400 cm (Meghalaya), while others will receive less than 10 cm (Ladakh and Western Rajasthan).
Additionally, the location of a place in relation to the sea is important because it influences climatic conditions in a moderate way. The coastal regions experience moderate climate conditions; however, the interior landmass experiences extreme climatic and seasonal changes.
There are six major controlling factors of a climate of a place:
- Pressure and wind system
- Distance from the sea
- Ocean currents
- Relief features
Latitude: distance of a place from the equator. As the equator receives direct sunlight, the places located near it are very hot. But as one moves away from it towards the poles, the amount of sunlight received decreases, as does the temperature.
Altitude: distance from the ground level. As one moves above the ground, the thickness of air decreases, leading to decrease in temperature.
Temperature and wind are influenced by pressure and wind systems, which are dependent on latitude and altitude.
The distance from the sea influences the climate of a place, making it warmer during the day and cooler at night. The temperature in a place becomes more extreme as the distance from the sea increases.
Coastal regions are mostly affected by ocean currents. Temperatures will be influenced by the type of ocean currents (warm or cool).
Relief features: Relief features such as mountains act as barriers act very hot or very cold winds, thus maintaining the temperatures moderate.
- Winds in the Northern hemisphere are deflected right by the Coriolis force; in the Southern hemisphere, they are deflected left by the Coriolis force. Ferrel’s law is also known as the Coriolis effect.
- Jet stream: high altitude westerly winds, normally above 12000 meters; speed varies from 110 kilometers per hour in summer to 184 kilometers per hour in winter.
FACTORS AFFECTING INDIA’S CLIMATE
From Rann of Kachchh in the west to Mizoram in the east, the Tropic of Cancer passes through middle India. Tropical climate occurs above the Tropic of Cancer, while subtropical climate occurs below it.
A mountain range in the north of India prevents cold winds from entering the country. This results in mild winters in India.
Pressure and winds:
- Pressure and surface winds
- Upper air circulation
- Western cyclonic disturbances and tropical cyclones
North of the Himalayas experiences high pressure during winter. Over the southern oceans, winds blow from this high pressure region to low pressure regions. In summer, low pressure develops over western India and the interior of Asia, and high pressure develops over the oceans in the south. Winds move from oceans and seas to lands as a result of this reversal of pressure, which results in heavy rain. As the winds move towards the equator they deflect to the right, entering India through the south west – thus the name South West monsoons.
The Indian monsoon is affected by subtropical westerly jet streams that flow south of the Himalayas. North and north western parts of the country experience cyclonic disturbances due to these jet streams.
THE INDIAN MONSOON
Arab traders who came to India and benefited from this reversal of the winds gave the term monsoon.
Some facts important to understand the mechanism of monsoon in India are:
- A difference in the intensity of heat between land and water; causing low pressure over land and high pressure over sea
- ITCZ (InterTropical Convergence Zone) shifting over Ganga plains in summer – also known as monsoon trough
- High pressure area over east of Madagascar
- Extreme heating of Tibetan plateau, resulting formation of low pressure area, causing strong vertical air currents to flow
- Over the Indian peninsula, there is a tropical easterly jet stream, and over the Himalayas to the west is a westerly jet stream
Apart from the above mentioned facts, some other events also influence the monsoons.
- In the tropical eastern Pacific Ocean, high pressure prevails, while in the tropical eastern Indian Ocean, low pressure prevails. The pressure regions revert after a certain interval. This periodic shift is known as a Southern Oscillation.
- Every 2-5 years, the Peruvian coast is hit by a warm current. The cause is due to changes in pressure areas over the oceans. The El-Nino Southern Oscillations are therefore known as ENSO (El-Nino Southern Oscillations).
El Nino is a Spanish word meaning ‘the child’- referring to the time of birth of Jesus Christ.
THE ONSET OF MONSOON AND WITHDRAWAL
Monsoon winds are not steady, but pulsating in nature. Their course and intensity are affected by the different atmospheric conditions they encounter on their way to India.
Early June marks the beginning of the monsoon season in India, which lasts until mid September. There are 100 – 120 days of monsoon in India. The arrival of the monsoon is accompanied by heavy rainfall over a period of days – this is called ‘burst’ of the monsoon.
- In the first week of June, the monsoon reaches India from the southern tip of the subcontinent. There are two branches – one on the Arabian Sea and one on the Bay of Bengal.
- In approximately 10 days, the branch reaches Mumbai. The train will reach Saurashtra – Kachchh and the country’s center by mid-June.
- Assam is reached by the Bay of Bengal branch in the first week of June. Over the Ganga plains, the presence of mountains deflects the wind. Rainfall falls on Delhi through the mountains by June 30.
- Arabian Sea branch and Bay of Bengal branch merge over western part of Ganga plains.
- Himachal Pradesh and the rest of the country are blessed with monsoon winds until mid-July, except for western Uttar Pradesh, Punjab, Haryana, and eastern Rajasthan.
Withdrawal is gradual, starting from western parts of India by early September. By mid October, it retreats from the northern plains and by early December, it withdraws from the remaining part of the country.
- Receive monsoon showers from south to north; from last week of April to first week of May.
- Withdrawal of monsoon id from north to south; from first week of December to first week of January.
India experiences a monsoon type of climate. This includes seasonal variation which is mostly felt by the interior part of the country.
There are mainly four types of seasons experienced in India:
- the cold weather season
- the hot weather season
- the advancing monsoon
- the retreating monsoon
The Cold Weather Season:
- The cold weather season begins from mid November and stays till February. December and January are the coldest months in Northern India.
- As one moves from north to south, the temperature decreases, for example, the average temperature in northern India ranges between 10 and 150C, while it ranges between 20 and 250C in the south.
- As the north east trade winds move from land to sea, there is no rain during the winter season. During this time of year, clear skies, low temperatures, low humidity, and weak winds are typical.
- During this time of year, low-pressure winds blow from the west and north-west. They enter India through the western region, bringing rainfall. They originate over the Mediterranean Sea. Although rainfall (mahawat in local language) is not very abundant, it is crucial for growing ‘rabi’ crops.
- Since it is situated near the sea, the southern part of India does not experience much variation in the winter season. Tamil Nadu’s coast experiences rain in winter as north east trade winds carry moisture from the sea to the land.
The Hot Weather Season:
- Between March and May, India experiences a hot weather season. The reason behind this is the shift of sun rays to the northern direction.
- The shifting of sun rays affects the temperature over time as the shifting intensifies. The average temperature recorded in March is about 380C, while the average in April and June is 420C and 450C, respectively.
- High temperatures cause low-pressure areas to occur more frequently. A prolonged low-pressure area develops between the Thar Desert in the north-west of India and Patna and Chotanagpur in the east and southeast of India as the temperatures increase from March to May.
- The Loos wind, which is a strong, gusty, hot and dry wind, blows through northern and northwestern India. The wind blows mostly during the daytime, but sometimes it can last until evening. The winds may be fatal.
Dust storms, for example, can lower the temperature a bit. Downpours and violent winds accompany thunderstorms. Such storms are referred to as ‘Kaal Baisakhi’ in Bengal.
- Mango showers’ are pre-monsoon rain, common in Kerala and Karnataka. They are called so as they help in early ripening of mangoes.
Advancing Monsoon (Rainy Season):
- There is a low-pressure area that prevails during the summer season and grows over time. With the development of this low-pressure area, winds from south-eastern oceans (southeast trade winds) enter India as south-west monsoons. As the wind blows over an ocean, it carries moisture, causing heavy rainfall.
- Monsoon season brings a great deal of change to the weather. Northern parts of the country receive the most rain. In Mawsynram, located in the southern range of the Khasi Hills, the average rainfall is the highest in the world.
- During monsoon season, India suffers ‘breaks’. That is, rain does not continuously fall throughout the entire season. There will be heavy rain for a few days, then it will stop and start again for a few days. This is due to a shift of the monsoon trough’s axis.
When the axis lies over plains – rainfall is good in these parts. When the axis lies closer to the Himalayas – the region receives abundant rainfall, leading to floods, damage to life and property.
- Tropical depressions also affect the monsoons’ timing and intensity. Low-pressure troughs occur during the monsoon season.
- There is great uncertainty surrounding the Indian monsoon. In addition to the time at which rain arrives and retreats, the intensity and duration of downpours also vary. As a result, agricultural activities – the primary source of occupation for a significant section of the Indian population – are significantly impacted.
Retreating/ Pot Monsoons (The Transition Season):
- During October and November, the monsoon retreats. The sun’s rays shift to the south during this time.
- Low pressure trough weakens, causing the south west monsoon winds to retreat. In October, this starts to happen.
- It’s a clear day with no clouds in the sky. It’s hot during the day and cool at night in October.
- As the rainy season doesn’t end completely, humidity prevails, which makes the days uncomfortable and oppressive. It is called ‘October heat’.
- During early November, low pressure systems shift towards the Bay of Bengal, leading to cyclone formation and heavy rainfall. There is damage caused by this kind of rainfall in the deltas of the Godavari, the Krishna, and the Kaveri rivers.
DISTRIBUTION OF RAINFALL
- In India, not all of the landmass receives similar amounts of rainfall. While Rajasthan, parts of Gujarat, Haryana, and Punjab receive rainfall of less than 60 cm, parts of the western coast and north eastern parts of India receive rainfall up to 400 cm or more.
- India also experiences seasonal variations in rainfall along with variations in place. Less rainy areas tend to have more of these variations.
- Snowfall is restricted to the Himalayan region.
MONSOON AS A UNIFYING BOND
Landscapes in India are becoming more diverse. Along with the landscape, diversity can be seen in climatic conditions of places from east to west or from north to south. Despite these diversities, monsoon’s arrival is awaited with much eagerness and expectations to set the farming activities in motion.
The monsoons maintain the rotation of the seasons. In India, life, plants and wildlife are heavily dependent on the monsoon and its water supply.