CBSE Notes Class 9 Social Science Geography Chapter 4 Climate here in this page. We (Net Explanations Teacher) discussed here each and every keyword from this Chapter 4 Climate.
CBSE Notes Class 9 Geography Chapter 4 Climate
THINGS TO ANSWER AFTER READING THE CONTEXT
- What are the elements that affect the climate of a place?
- Mention the mechanisms responsible for monsoon in India.
- Explain the onset and withdrawal of Monsoon in India.
- Write a short note on the summer season of India.
Definitions:
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:
- Temperature
- Atmospheric pressure
- Wind
- Humidity
- Precipitation
The climate of India is said to be monsoon type. But, as mentioned earlier, climate is the average of all weather conditions observed over a large area of land. So, even within the country one can see variations in the climatic conditions.
- ❖ Monsoon is derived from Arabic word ‘mausim’ which means season.
For example, let us take temperature into consideration. One part of India, say Rajasthan will have temperature of around 50⁰C, while at the same time, Jammu may experience temperature of around 20⁰C. The same can be seen for precipitation. In Himalayas, precipitation is in the form of snowfall, while in other parts it is in the form of rain. Also, some parts will receive rainfall upto 400 cm (Meghalaya), while some will receive less than 10 cm (Ladakh and Western Rajasthan).
Also, location of a place in respect to sea is also important as it controls climatic conditions to a moderate extent. While coastal regions experience moderate climatic conditions, the interior landmass experiences the extremities of climatic and seasonal variations.
CLIMATIC CONDITIONS
There are six major controlling factors of a climate of a place:
- Latitude
- Altitude
- Pressure and wind system
- Distance from the sea
- Ocean currents
- Relief features
Latitude: distance of a place from the equator. As 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 so does the temperature.
Altitude: distance from the ground level. As one move above the ground, the thickness of air decreases, leading to decrease in temperature.
Pressure and wind system: depends on the latitude and altitude of a place and influences temperature and winds.
Distance from the sea: Sea exerts a moderating effect on the climate of a place, leading to warm days and cool nights. But as the distance from the sea increases, a place starts to experience extremities in the temperature.
Ocean currents: It mostly affects the coastal regions. The type of current flowing from the oceans (warm or cool) will accordingly affect the climate of a place.
Relief features: Relief features such as mountains act as barriers act very hot or very cold winds, thus maintain the temperatures moderate.
- Coriolis force: it is a force caused by the rotation of the earth; this is responsible for deflection of winds towards the right in the Northern hemisphere and towards the left in the Southern hemisphere. Also known as Ferrel’s law.
- Jet stream: high altitude westerly winds, usually above 12000 m; speed varies from 110 km/h in summer to 184 km/h in winter.
FACTORS AFFECTING INDIA’S CLIMATE
Latitude: Tropic of Cancer passes through the mid of India, from Rann of Kachchh in the west to Mizoram in the east. Places staying above the Tropic of Cancer experience tropical type of climate while the places staying below it experience subtropical type of climate.
Altitude: India has mountains in the north which prevents cold bearing winds entering the country. Because of this, India experiences milder winters.
Pressure and winds:
- Pressure and surface winds
- Upper air circulation
- Western cyclonic disturbances and tropical cyclones
During winter, high pressure prevails over the North of Himalayas. Winds blow from this high pressure region to low pressure regions over the oceans in the south. During summer, low pressure develops over western India and interior of Asia and high pressure develops over oceans in the south. This reversal of pressure regions causes winds to blow from oceans and seas to lands, carrying with them moisture resulting in heavy rainfall. The winds are south easterly, but while moving towards the equator gets deflected towards its right, entering India through the south west direction – hence known as South west monsoons.
Jet streams that affect the Indian monsoon are subtropical westerly jet streams, which flow south of the Himalayas. These jet streams cause the cyclonic disturbances experienced by the north and north western parts of the country.
THE INDIAN MONSOON
The term ‘monsoon’ was given by Arab traders who came to India and had benefited from this reversal of the wind system.
Some facts important to understand the mechanism of monsoon in India are:
- The different intensity with which land and water heats; causing low pressure on landmass and high pressure over seas
- Shifting over ITCZ (InterTropical Convergence Zone) over the 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
- Presence of tropical easterly jet stream over the Indian peninsula and westerly jet stream over the north of Himalayas
Apart from the above mentioned facts, some other events also influence the monsoons.
- Tropical eastern south Pacific Ocean experience high pressure, while tropical eastern Indian ocean experience low pressure region. But after certain intervals, there is a reversal in the pressure regions. This periodic change is called Southern Oscillations or SO.
- El-Nino is a phenomenon in which every 2 to 5 years a warm current flows past the Peruvian Coast. This is related to changes in pressure regions over the oceans. Hence, it is referred to 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.
Onset:
In India, monsoon arrives in early June and stays till mid September. The duration of monsoon is 100 – 120 days. Monsoon arrival causes heavy rainfall over a period of days – this phenomenon is called ‘burst’ of the monsoon.
- Monsoon enters India from the subcontinent’s southern tip by the first week of June. It gets divided in two branches – the Arabian Sea branch and the Bay of Bengal branch.
- The Arabian Sea branch first reaches Mumbai, approximately 10 days later. By mid June, it will arrive in Saurashtra – Kachchh and the centre of the country.
- The Bay of Bengal branch reaches Assam in the first week of June. Presence of mountains deflects the wind over the Ganga plains. Delhi receives rainfall through it by the end of June.
- Arabian Sea branch and Bay of Bengal branch merge over western part of Ganga plains.
- Western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan receive monsoon by the first week of July and till mid July Himachal Pradesh and the rest of the country is blessed by the monsoon winds.
Withdrawal:
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.
Islands:
- 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.
THE SEASONS
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.
- Temperature decreases as one moves from north to south, for example, average temperature in Northern India ranges between 10 and 15⁰C, while in south it ranges between 20 and 25⁰C.
- During the winter season, north east trade winds move from land to sea which does not cause any rainfall. Clear skies, low temperature, low humidity and feeble winds are characteristic features of this season.
- During this time of season, low-pressure winds blow from west and north-west. They originate over the Mediterranean Sea and enter India through the western region, bringing rainfall. Though the rainfall (mahawat in local language) is not very abundant, it is important for growing ‘rabi’ crops.
- The southern part of India does not experience much variation in the winter season; as it is located near the sea. Coast of Tamil Nadu experiences rainfall in the winter season as north east trade winds blow from sea (carrying moisture) into the land.
The Hot Weather Season:
- The hot weather season is experienced in India from March to May. This happens due to the shift of sun rays to the northern direction.
- The shifting of sun rays affects With time, as shifting intensifies, so does the temperature. In March, the average temperature recorded is around 38⁰C, while in April and June, it is on an average 42⁰C and 45⁰C respectively.
- As temperature rises, low-pressure areas become a common feature. With increasing temperature from March to May, an extended low-pressure area develops from regions of Thar Desert in north-west India to Patna and Chotanagpur area in east and south east.
- ‘Loos’, which are strong, gusty, hot and dry winds blow through north and north western India. Mostly, they blow during the days but sometimes, they may continue till the evening. They may be fatal.
Some places experience dust storms – which lower the temperature to some extent. Thunderstorms with violent winds and downpours are also experienced. In Bengal, such storms are called ‘Kaal Baisakhi’.
- 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):
- During the summer season, a low-pressure area prevails, which grows with time. This development of low-pressure area causes winds from south eastern oceans (southeast trade winds) to enter India as south west monsoons. As the winds blow over oceans, it carries moisture which causes heavy rainfall.
- The onset of monsoon brings a great change in the weather. The maximum rainfall is received by the north eastern part of the country. Mawsynram, situated in the southern ranges of Khasi Hills, receives the highest average rainfall in the world.
- India experiences ‘breaks’ during monsoon. This means there is no continuous downpour of rain for the entire season. It will rain heavily for a few days, stop for the next few and will start again for the next few days. This is due to a shift in the monsoon trough and its 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, too, alter the time and intensity of monsoons. The depressions follow the pattern of ‘monsoon trough of low pressure’.
- The Indian monsoon is very uncertain. Its time of arrival and retreat varies and also the intensity and duration of downpour also varies. This has a major role in impacting farming activities – which is the primary source of occupation of a major section of population of India.
Retreating/ Pot Monsoons (The Transition Season):
- The months of October and November experience the retreat of monsoon. This is due to the shift of sun-rays towards the south.
- Low pressure trough becomes weak, causing retreat of south west monsoon winds. This starts from early October.
- The weather is marked with clear skies. In October, days are hot while nights are cool and pleasant.
- As the rainy season does not retreat completely, humidity is prevalent – which makes the days unpleasant and oppressive. This is called ‘October heat’.
- Low pressure regions shift towards Bay of Bengal during early November, causing generation of cyclones – causing heavy rainfall. Such rainfall occurs in the deltas of the Godavari, the Krishna and the Kaveri – causing damage.
DISTRIBUTION OF RAINFALL
- Entire landmass of India does not receive the same amount of rainfall. While parts of western coast and north eastern India receive rainfall up to 400 cm or above; while Rajasthan, parts of Gujarat, Haryana and Punjab receive rainfall of less than 60 cm.
- Along with variations over place, India also witnesses variations in rainfall from year to year. Ths becomes more prominent in areas receiving less rainfall.
- Snowfall is restricted to the Himalayan region.
MONSOON AS A UNIFYING BOND
India has a diversifying landscape. 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.
Monsoons keep the cycle of seasons in proper rotation. India’s lifestyle, plants and wildlife are very much dependent on monsoon and the water it provides.