CLIMATE

CLASS- IX    GEOGRAPHY: CONTEMPORARY INDIA

04.   CLIMATE

Climate 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 refers to the state of the atmosphere over an area at any point of time. The elements of weather and climate are the same, i.e. temperature, atmospheric pressure, wind, humidity and precipitation. The climate of India is described as the monsoon type.

CLIMATIC CONTROLS

There are six major controls of the climate of any place. They are: latitude, altitude, pressure and wind system, distance from the sea (continentality), ocean currents and relief features.

1.   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.   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.   The pressure and wind system of any area depend on the latitude and altitude of the place. Thus it influences the temperature and rainfall pattern.

4.   The sea exerts a moderating influence on 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 along with onshore winds affect the climate of the coastal areas, For example, any coastal area with warm or cold currents flowing past it, will be warmed or cooled if the winds are onshore.

FACTORS AFFECTING INDIA’S CLIMATE

1. Latitude

The Tropic of Cancer passes through the middle of the country from the Rann of Kuchchh in the west to Mizoram in the east. 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.

2. Altitude

India has mountains to the north, which have an average height of about 6,000 meters. 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 as compared to central Asia.

3. Pressure and Winds

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.

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 an arid land, but, it is not so. Let us see why?

The pressure and wind conditions over India are unique. During winter, there is a high-pressure area north of the Himalayas. Cold dry winds blow from this region to the low-pressure areas over the oceans to the south.

In summer, a low-pressure area develops over interior Asia as well as over northwestern India. This causes a complete reversal of the direction of winds during summer.

Air moves from the high-pressure area over the southern Indian Ocean, in a south-easterly direction, crosses the equator, and turns right towards the low-pressure areas over the Indian subcontinent. These are known as the Southwest Monsoon winds. These winds blow over the warm oceans, gather moisture and bring widespread rainfall over the mainland of India.

Coriolis force: An apparent force caused by the earth’s rotation. The Coriolis force is responsible for deflecting winds towards the right in the northern hemisphere and towards the left in the southern hemisphere. This is also known as ‘Ferrel’s Law’.

• The upper air circulation in this region is dominated by a westerly flow. An important component of this flow is the jet stream. Jet streams are located approximately over 27°-30° north latitude known as westerly jet stream. These jet streams blow south of the Himalayas, all through the year except in summer.
• In summer, the subtropical westerly jet stream moves north of the Himalayas with the apparent movement of the sun called the sub-tropical easterly jet stream(blows over peninsular India)

Jet stream

These are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere. Their speed varies from about 110 km/h in summer to about 184 km/h in winter. A number of separate jet streams have been identified. The most constant are the mid-latitude and the sub tropical jet stream.

 

Western Cyclonic Disturbances

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. Tropical cyclones occur during the monsoon as well as in October - November, and are part of the easterly flow. These distrurbances affect the coastal regions of the country. Have you read or heard about the disasters caused by them on Orissa and Andhra Pradesh coast?

THE INDIAN 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.

1. 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.
2. The shift of the position of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain.

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.

3. The presence of the high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean.
4. 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.
5. 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.
6. Changes in the pressure conditions over the southern oceans also affect the monsoons. Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. 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. 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. The presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region.

THE ONSET OF THE MONSOON AND WITHDRAWAL

1. The duration of the monsoon is between 100- 120 days from early June to mid-September. Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as the burst of the monsoon, and can be distinguished from the pre-monsoon showers.
2. The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June. Subsequently, it proceeds into two the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch reaches Mumbai about ten days later approximately the 10th of June.
3. The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June.
4. The lofty mountains cause the monsoon winds to deflect towards the west over the Ganga plains. By mid-June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country.
5. The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains.

Withdrawal or Retreat of Monsoon

(i) Withdrawal or the retreat of the monsoon is a more gradual process.

(ii) The north-western states of India by early September.

(iii) By mid October, it withdraws completely from the northern half of the peninsula.

(iv) The withdrawal from the southern half of the peninsula is fairly rapid. By early December, the monsoon has withdrawn from the rest of the country.

(v) The islands receive the very first monsoon showers, progressively from south to north from the last week of April to the first week of May.

(vi) The withdrawal takes place progressively from north to south from the 1st week of December to the first week of January. By this time, the rest of the country is already under the influence of winter monsoon.

THE SEASONS

The Cold Weather Season (Winter)

• The cold weather season begins from mid-November in northern India and stays till February.
• December and January are the coldest months in the northern part of India. The temperature decreases from south to the north.
• 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.
• A characteristic feature of the cold weather season over the northern plains is the inflow of cyclonic disturbances from the west and the northwest. These low-pressure systems, originate over the Mediterranean Sea and western Asia and move into India, along with the westerly flow.
• The weather is normally marked by clear sky, low temperatures and low humidity and feeble, variable winds.
• Total amount of winter rainfall locally known as mahawat is small, they are of immense importance for the cultivation of rabi crops.

The Hot Weather Season (Summer)

Due to the apparent northward movement of the sun, the global heat belt shifts northward.

• The summer months experience rising temperature and falling air pressure in the northern part of the country. Towards the end of May, an elongated low-pressure area develops in the region extending from the Thar Desert in the northwest to Patna and Chotanagpur plateau in the east and southeast. Circulation of air begins to set in around this trough.
• A striking feature of the hot weather season is the loo. These are strong, gusty, hot, dry winds blowing during the day over the north and northwestern India.
• 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.
• This is also the season for localized thunderstorms, associated with violent winds, torrential downpours, often accompanied by hail. In West Bengal, these storms are known as the Kaal Baisakhi.
• 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)

By early June, the low-pressure condition over the northern plains intensifies->attracts the trade winds of the southern hemisphere(originate over the warm subtropical areas of the southern oceans).They cross the equator and blow in a southwesterly direction entering the Indian peninsula as the south-west monsoon.

• As these winds blow over warm oceans, they bring abundant moisture to the subcontinent.
• These winds are strong and blow at an average velocity of 30 km per hour.
• As these winds blow over warm oceans, they bring abundant moisture to the subcontinent. These winds are strong and blow at an average velocity of 30 km per hour. Except for the extreme north-west, the monsoon winds cover the country in about a month.
• Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall in the world.
• Another phenomenon associated with the monsoon is its tendency to have breaks in rainfall. Thus, it has wet and dry spells.
• For various reasons, the trough and its axis keep on moving northward or southward, which determines the spatial distribution of rainfall. When the axis of the monsoon trough lies over the plains, rainfall is good in these parts. On the other hand, whenever the axis shifts closer to the Himalayas, there are longer dry spells in the plains, and widespread rain occur in the mountainous catchment areas of the Himalayan rivers.
• The frequency and intensity of tropical depressions too, determine the amount and duration of monsoon rains. These depressions form at the head of the Bay of Bengal and cross over to the mainland. The depressions follow the axis of the monsoon trough of low pressure.

Retreating Monsoon (The Transition Season) 

(a) During October and November the south west monsoon winds weaken and start withdrawing gradually. By the beginning of October, the monsoon withdraws from the northern plains.

(b) The months of October November form a period of transition from hot rainy season conditions. The retreat of the monsoon is marked by clear skies and rise in temperature.

(c) Day temperatures are high, nights are cool and pleasant. The land is moist.

(d) Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October Heat.’ In the second half of October, the mercury begins to fall rapidly in northern India.

(e) The low pressure conditions over north western India, get transferred to the Bay of Bengal by early November. This shift is associated with the occurrence of cyclonic depressions which originates over the Bay of Bengal. Deltas of Krishna and Kaveri are struck by cyclones, which are often very destructive and cause damage to life and property.

(f) Sometimes these cyclones arrive on the coasts of Odisha, West Bengal and Bangladesh.

(g) The maximum rainfall on the Coromandal coast is derived from depressions and cyclones.

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. A third area of low precipitation is around Leh in Jammu and Kashmir. The rest of the country receives moderate rainfall. Snowfall is restricted to the Himalayan region.

SOME IMPORTANT FACTS-

1. In Rajasthan, the weather is very hot and there is less rainfall. Some part of the state is covered with desert. The thick walls of the houses insulate the people against the heat in summer and extreme cold in winter due to the desert. Flat roofs are easier to construct and as there is not much rainfall, water will not collect on the rooftops.
2. The houses in the Tarai region and in Goa and Mangalore have sloping roofs because they get heavy rain during the monsoon season. When there are sloping roofs, the rain water can easily flow off towards the ground or to a receptive unit where water is collected instead of collecting on the rooftop.
3. Houses in Assam are built on stilts because the state receives abundant rainfall due to which there are chances of floods. In case of flood the water might get inside the houses, if the houses are built on ground level, so in order to avoid flooding of houses, houses are built on stilts and above the ground level.
4. Most of the worlds deserts are located in the western margins of continents in the subtropics because the prevailing winds in the tropics are tropical easterly winds. The tropical easterly winds become dry by the time they reach the western margins of the continents and, so they bring no rainfall. Thus, the region becomes devoid of moisture which causes dry conditions leading to formation of deserts.

MONSOON AS A UNIFYING BOND

The Himalayas protect the subcontinent from extremely cold winds from central Asia. This enables northern India to have uniformly higher temperatures when compared to other areas on the same latitudes.

Similarly, the peninsular plateau, under the influence of the sea from three sides, has moderate temperatures. Despite such moderating influences, there are great variations in the temperature conditions.

Nevertheless, the unifying influence of the monsoon on the Indian subcontinent is quite perceptible. The seasonal alteration of the wind systems and the associated weather conditions provide a rhythmic cycle of seasons. Even the uncertainties of rain and uneven distribution are very much typical of the monsoons.

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. Year after year, people of India from north to south and from east to west, eagerly await the arrival of the monsoon.

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.

 

 

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