Chapter - 4 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.

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 and they originate from the subtropical high-pressure belt of the northern hemisphere->and move on towards the equatorial low-pressure area in the south and get deflected to the right due to Coriolis force.

• During winter->high-pressure area in the 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 northwestern India->causes a complete reversal of the direction of winds ->air moves from the high-pressure area over the southern Indian Ocean in a south-easterly direction->crosses the equator->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.
• 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)

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.
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).

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.
6. Withdrawal or the retreat of the monsoon is a more gradual process. The withdrawal of the monsoon begins in northwestern states of India by early September. 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.

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 pressureâ€�.

Retreating/Post Monsoons (The Transition Season)

During October-November, with the apparent movement of the sun towards the south, the monsoon trough or the low-pressure trough over the northern plains becomes weaker. This is gradually replaced by a high-pressure system. The south-west monsoon winds weaken and start withdrawing gradually. By the beginning of October, the monsoon withdraws from the Northern Plains.

• The retreat of the monsoon is marked by clear skies and rise in temperature. While day temperatures are high, nights are cool and pleasant. The land is still moist. 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.
• The low-pressure conditions, over northwestern India, get transferred to the Bay of Bengal by early November. This shift is associated with the occurrence of cyclonic depressions, which originate over the Andaman Sea. These cyclones generally cross the eastern coasts of India cause heavy and widespread rain. These tropical cyclones are often very destructive.

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.