Climate (Class 9 Geography Chapter 4 Notes)

Climate (Class 9 Geography Chapter 4 Notes)
Annual Rainfall of India/Source: NROER
Weather refers to the state of atmosphere at any given point of time. It can change at any moment and varies throughout day. On the other hand, climate refers to the state of atmosphere for a longer period of time. It is the average of weather conditions of last 30 years. It can take tens to millions for the climate of a certain place to change.

The elements of weather and climate are same. These are
1. temperature
2. atmospheric  pressure
3. wind
4. humidity
5. precipitation

On the basis of monthly atmospheric activies, a year can be divided into seasons, namely, Summer, Winter, Spring and Autumn.

On the basis of climatic conditions, the world is divided into several zone. Most of the South and Southeast Asia falls under the Monsoon climate type. India as part of the Southeast Asia also falls under Monsoon climate type. Having said that the climate of India can be sub divided into seven different zones. (See Map)

Climatic Controls (Climatic Factors of A Region)
There are 6 major factors which controls the climate of any region. These are
1. latitude - insolation (amount of solar energy) varies from place to place due to earth's curvature - insolation is highest at equator and lowers down as we move toward poles
2. altitude - as one moves from lower to higher altitude, density of atmosphere lessens and temperature decreases
3. pressure and wind system - pressure and wind system of an area depend upon latitude and altitude of the area - they change according to them
4. distance from sea (continentality) - as distance from the sea increase, the moderating effect of sea decreases - people observe more variance in day to day atmospheric conditions
5. ocean currents - they majorly affect coastal areas - area where warm and cold current meets have increased fog
6. relief features - relief features such as mountains helps in causing precipitation
Climate of India
The average temperature of India varies from place to place. It has extreme temperature variance in places like Rajasthan and no temperature variance in places like West Bengal and Andaman and Nicobar Islands.
The precipitation ranges from less than 100 mm in Ladakh and parts of Rajasthan and more than 4000 mm in Meghalaya. There is decrease in rainfall from east to west.

Climatic Factors of India
The Tropic of Cancer divides India in two identical parts. The southern part have tropical climate whereas northern part have sub-tropical climate.

The average altitude of India ranges from 30 m (coastal area) and 6000 m (himalayan region). The Himalayan region prevents the cold winds from Central Asia and checks the temperature to not fall below certain range.

Pressure and Winds
There are 3 major factors related to atmospheric pressure and wind system which control the climate of India. These are

1. Pressure  and  surface  winds
2. Upper air circulation
3. Western cyclonic disturbances and tropical cyclones.

North Easterly Winds
India lies in the region of north easterly winds. These winds rises subtropical high-pressure belt of the northern hemisphere. They blow southwards and get deflected to the right because of Coriolis effect and moves toward equatorial low-pressure zone. Usually, these winds carry very little moisture, but not in the case of India due to its unique geographical position.

Coriolis Effect
An apparent force generated by the earth's rotation. It is responsible for deflecting winds toward right in the northern hemisphere and toward left in the southern hemisphere.

Southwest Monsoon Winds
In winters, a high pressure area in the north of Himalayas. Cold dry winds blow from this region toward low pressure area in southern zones. In summer, a low pressure zone is created over Central Asia as well as North India. This causes complete reversal in wind direction during summers. Air moves from high pressure area over the Indian Ocean in south-easterly direction. It crosses equator and moves toward the low-pressure areas over India. These winds are known as Southwest Monsoon Winds. These winds blow over the warm ocean, gather moisture and bring rainfall to India.

Jet Streams
The upper air circulation is controlled by westerly winds. Jet stream is one of the major components of this flow. These jet strems exist over 27°-30°N, therefore, known as subtropical westerly jet streams. They blow throughout the year in the Himalayan region except summers. The western cyclonic disturbances experienced by the North India are brought by westerly flow.

In summer, the westerly jet streams move northward of the Himalayan region with the movement of the sun and easterly jetstreams, known as the sub-tropical easterly jet streams start blowing over peninsular India, approximately over 14°N latitude.

The jet stream speed varies from 110 kmph (summers) to 187 kmph (winters). Most prominent jet streams are the mid-latitude and the sub tropical jet stream.

Cyclonic Disturbances
There are two of cylonic disturbance observed in India. These are western cyclonic disturbances and tropical cyclones.

Western cyclonic disturbances
It is a winter weather phenomena brought in by westerly flow from the Mediterranean region and known influencing the weather of North India.

Tropical cyclones
Tropical cyclones are visible during advancing and retreating monsoon seasons; and part of easterly flow. They mostly affect the coastal region of India.

The Indian Monsoon
The climate of India is influenced by the Monsoon winds. Arabs who called these winds, mausim, used these winds to mark their trading route. These winds are found in the tropical region between 20° N and 20° S. However, to understand mechanism of monsoon, one need to first understand few related facts

1. The differential heating and cooling of land and sea creates a low pressure zone on the land whereas a high pressure zone of sea.
2. Inter Tropical Convergence Zone (ITCZ) shifts over the Northern Plains.
3. A high pressure area develops over east of Madgascar which influence the intensity of monsoon winds.
4. The Tibetan plateaus heats up in summer which causes formation of a low pressure area.
5. The northward movement of westerly jet streams and presence of easterly jet streams in summer.

In addition, phenomena like El Nino also affect the intensity of the monsoon. It occurs in every two to five years when a warm current appears on the coast of Peru instead of a cold current.

Monsoon burst
Monsoon winds are not a steady wind but pulsating in nature. The appear for 100 to 120 days from June to mid-September. In this period, there is a sudden increase in the rainfall which is known as the Monsoon burst.

Onset of Monsoon
Monsoon reaches the southern tip by the first week of June. From here, it branches into two parts, one of the Bay of Bengal and second over the Arabian Sea. The Arabian branch reach Mumbai in next 10 days whereas the Bay of Bengal branch reach Assam in a week time. These winds are deflected toward Purvanchal hills of the Northeast toward Ganga (Northern) Plains. On another hand, by mid-June, monsoon arrives in Kutch Gujarat region. By 29 June, Monsoon reaches Delhi and Uttar Pradesh, Punjab, Haryana and eastern Rajasthan by the first week of July; and by mid-July, it reaches Himachal Pradesh and rest of India.

Withdrawl of Monsoon
The withdrawl of monsoon begins in the northwestern region of India by early September. By mid-October, it is withdrwan from the northern peninsular region and by early December, it has withdrawn completely from the mainland of India.

Seasons of India
There are four major seasons in India. These are Winter, Summer, Advancing Monsoon and Retreating Monsoon.

Winter (November-February)
  1. The average temperature in northern plains range from 10-15°C and in peninsular region, 20-25°C.
  2. Frost is common in the northern plains and the himalayan region experience snowfall.
  3. A small amount of rainfall, locally known as mahawat, also happen in this season. It is useful for rabi crops.

Summer (March-May)
  1. The average temperature can range from 35-45°C in different parts of the country. It can reach upto 50°C in desert region of Rajasthan.
  2. Loo, a hot, dry and strong wind blows in North and Northwestern India.
  3. Storms are common in the month of may. These storms lower the temperature and sometimes, heavy rains. These storms are known as Kaal Baisakhi in West Bengal.
  4. Toward the close summer seaon, there are pre-monsoon rains happen in Kerala and Karnataka. These rains are known as mango showers.

Rainy Season - Advancing Monsoon (June-September)
  1. Most of the rainfall happen during this region. Western Ghats and Northeast India recieves most rainfall while Rajasthan, Gujarat and Ladakh recieves scanty rainfall.
  2. Monsoon tends to take some "breaks" which means it has wet and dry spells.
  3. Monsoon is also irregular in arrival and withdrawl, affecting the lives of millions of farmers.

The Transition Season - Retreating Monsoon (October-November)
  1. Monsoon starts to retreat in this season. It is marked by warm and humid climate.
  2. Due to both high temperature and high humidity, the climatic conditions in first two weeks of October become unbearable. The phenomena is known as October heat. The temperature starts to fall from mid-October again.
  3. The tropical cyclones are common in peninsular India, especially, on the eastern coast. The bulk of rainfall in this region is brought down by these cyclones.
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