25 August 2015


Climate: The sum total of weather conditions and variations over a large area for a long period of time (more than thirty years) is called climate.
Weather: The state of the atmosphere over an area at any point of time is called weather.
Monsoon: This type of climate is mainly found in the south and Southeast Asia. The climate of India is of monsoon type. The seasonal reversal in wind direction during a year is called monsoon.
Coriolis Force: An apparent force caused by the earth’s rotation is called Coriolis Force. The winds are deflected towards right in the northern hemisphere and towards the left in the southern hemisphere due to this force. This is also known as ‘Ferrel’s Law’.
Jet Stream: Fast flowing and narrow air currents are called jet streams. The streams flow at high altitudes (about 12,000 feet) in the troposphere. Their speed varies from about 110 km/h in summer to 184 km/h in winter.

Climate of India Climatic Controls The Mechanism of Monsoons The Seasons of India, NCERT Revision Notes, NCERT-CBSE Solution, Geography : Class 9 NCERT / CBSE Revision Notes
(Ref: Fig: Jet Streams-Wikipedia.org)

Western Cyclonic Disturbance: The western cyclonic disturbances are weather phenomena of the winter months brought in by the westerly flow from the Mediterranean region. They usually influence the weather of the north and north-western regions of India.
Inter Tropical Convergence Zone (ITCZ): The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes. The northeast and the southeast trade winds converge in this zone. This zone lies more or less parallel to the equator. It moves north or south with the apparent movement of the sun.


There are six major controls of the climate of any place. They are:
Latitude, Altitude, Pressure and wind system, Distance from the sea, Ocean currents and Relief features

Factors Affecting India’s Climate

Latitude: The Indian landmass is equally divided by The Tropic of Cancer. Hence, half of India has tropical climate and another half has subtropical climate.
Altitude: While the average elevation in the coastal areas is about 30 metre, the average elevation in the north is about 6,000 metre. The Himalayas prevent the cold winds from Central Asia from entering the Indian subcontinent. Due to this, the subcontinent gets comparatively milder winters as compared to Central Asia.
Pressure and Winds: The Indian subcontinent lies in the region of north-easterly winds. These winds originate from the subtropical high-pressure belt of the northern hemisphere. After that, these winds blow towards south. They get deflected to the right due to the Coriolis force and then move towards the low pressure area near the equator.
  • The north-easterly winds originate and blow over the land and hence they carry very little moisture. India should have been an arid land because of these winds but that is not the case.
  • There is high-pressure area towards the north of the Himalayas. Cold winds from this region blow to the low pressure areas over the oceans in the south.
  • During summer, low-pressure area develops over interior Asia and also over northwestern India. This results in a complete reversal of the direction of winds during summer.
  • Air; from the high-pressure area moves over the southern Indian Ocean in a south-easterly direction. It crosses the equator and turns right towards the low-pressure areas over the Indian subcontinent.
  • These winds are known as the Southwest Monsoon wind. They collect moisture from the warm oceans and bring widespread rainfall over the mainland of India.
  • The upper air circulation in this region is dominated by a westerly flow. Jet stream is an important component of this flow. These jet streams are called subtropical westerly jet streams because they are located approximately over 27°-30° north latitude.
  • The westerly jet streams are responsible for western cyclonic disturbances in the north and north-western parts of India. The subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the sun.
  • The tropical jet stream (an easterly jet stream) blows over the Indian Peninsula; approximately over 14° north during the summer months.
9 Geography - Atmospheric condition-Indian Subcontinent - January
Fig: Atmospheric condition over the Indian subcontinent in January (ref: NCERT Text Book)
9 Geography - Atmospheric condition-Indian Subcontinent - June
Fig: Atmospheric condition over Indian subcontinent in June (Ref: NCERT Textbook)

The Mechanism of Monsoons

The climate of India is strongly influenced by the monsoon winds. It refers to a season in which the wind system reverses completely. The monsoons are experienced in the tropical area roughly between 20° N and 20° S.

Various atmospheric conditions influence the monsoon winds. The first condition is the differential heating and cooling of land and water. This creates low pressure on the landmass, while high pressure is created over the seas around during day time, but is reversed during the night time.

The second condition is the shift in the position of Inter-Tropical Convergence Zone (ITCZ). In summer, the equatorial trough normally positioned about 5°N of the equator moves over the Ganga plain creating a monsoon trough during the monsoon season.

The third condition is the presence of the high-pressure area that develops east of Madagascar. It is approximately at 20°S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.

The fourth condition develops during the summer. The Tibetan Plateau gets intensely heated resulting in strong vertical air currents and high pressure over the plateau about 9 km above sea level. The fifth condition develops during the summer due to the movement of the westerly jet streams to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian Peninsula.

Changes in pressure over the southern oceans also affect the monsoons. In certain years, there is a reversal in the pressure conditions. This periodic change in pressure conditions is known as the Southern Oscillation, or SO.

The Southern Oscillation is connected to El Nino, which is a warm ocean current that flows past the Peruvian Coast. It flows every two to five years in place of the cold Peruvian current. The phenomenon is, referred to as ENSO (El Nino Southern Oscillations). In India, the monsoon lasts for 100 to 120 days from early June and to mid-September. The monsoon winds encounter various atmospheric conditions on their way and hence are pulsating in nature, and not steady.

The monsoon arrives with a sudden downpour of rainfall that continues for several days. This is known as the ‘burst’ of the monsoon.

The monsoon arrives at the southern tip of the Indian Peninsula generally by the first week of June. By early September, the monsoon starts to withdraw or retreat and is a more gradual process. By mid-October, it withdraws completely from the northern half of the peninsula. The withdrawal takes place progressively from north to south from the first week of December to the first week of January. This is the start of the winter season.

The retreating monsoon winds move over the Arabian Sea and the Bay of Bengal, and collect moisture on the way. These monsoon winds reach the southern states of India by October, and are responsible for a second round of rainfall. These are called the winter monsoons. The winter monsoon is experienced in the states of Tamil Nadu, Kerala and Andhra Pradesh in the first week of January.

The Seasons of India

The changes in weather conditions of India can be broadly categorised into four distinct patterns. These are the cold weather season, the hot weather season, the advancing monsoon and the retreating monsoon.

India has cold weather in the winter that begins in mid-November and lasts till February. The northern parts of India are the first to experience the cold. December and January are the coldest months in northern India. The southern parts of India do not experience very low temperatures.

Frost is a common phenomenon in the north plains, while the higher slopes of the Himalayas experience snowfall.

During winter, the north-east trade winds blow over the country.  In most parts of India, the weather during the winter is dry. In some parts like the coast of Tamil Nadu, the winds blow from the sea to the land. Hence, these places receive some amount of rainfall during the winter.

Other characteristic feature of the winter season experienced over the northern plains is the inflow of cyclonic disturbances from the west and the north-west. The winter rainfall is locally known as “mahawat”.

The sea has a moderating influence over the peninsular region of India. Hence, there are no drastic seasonal variations in temperature pattern, and winters are not as distinct as in the north. The hot weather season in India refers to the summers that begin in March and lasts till mid-June.

In India, summer is caused due to the apparent northward movement of the sun, which shifts the global heat belt northward. ‘Loo’ are strong, gusty, hot, dry winds that blow during the day over north and north-western India.

In northern India, dust storms are also very common during the month of May. This is also the season for localised thunderstorms, associated with violent winds, torrential downpours, often accompanied by hail. Pre-monsoon showers are common, especially in Kerala and Karnataka and are often referred to as ‘mango showers,’ as they help in the early ripening of mangoes.

The advancing monsoon in India refers to the rainy season that lasts for a period of four months from June to September. The trade winds from the southern hemisphere, which originate over the warm subtropical areas of the southern oceans, bring the south-west monsoon winds into India.

Another characteristic feature of the monsoon is the ‘breaks’ in rainfall and refers to the alternating wet and dry spells of rains. The Indian monsoon is very unpredictable. These breaks occur due to the movement of the monsoon trough.

The farming schedule of millions of farmers all over the country, which is dependent on the monsoons, can sometimes get disturbed. The retreating monsoon refers to the transition season that lasts from October to December.

The months of October-November form a period of transition from the hot rainy season to dry winter conditions. The main cause of this transition is the movement of the sun towards the south. By the beginning of October, the monsoon withdraws from the Northern Plains. The retreating monsoon season is marked by clear skies and a rise in the temperatures. The land is still moist and the weather becomes hot and humid. The days can become quite oppressive. This is commonly known as ‘October heat’.

Due to the unpredictable nature of the monsoon, the distribution of rainfall varies drastically from region to region. Major variations can also be seen in the annual precipitation from year to year. The phenomenon of monsoon is an integral part of the Indian landscape, animal and plant life, agricultural calendar, and the life of the people and their festivities.

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