Samacheer Class 10 Social Science - Climate and Natural Vegetation of India

Several important factors affect the climate of India. The first is latitude: the Tropic of Cancer passes through the middle of the country, so the south has a tropical climate while the north is more temperate. The second is altitude, because high mountains and plateaus are cooler than the plains. The third is the distance from the sea; coastal areas have an equable climate, while interior areas have extremes of heat and cold. The fourth is the Himalayan mountains, which block the cold winds from Central Asia and trap the monsoon winds, causing rainfall. The fifth is the monsoon winds themselves, which bring most of India's rain. Other factors include the direction of the mountain ranges, ocean currents, and upper-air movements such as jet streams. Together these factors give India its varied climate.

Jet streams are very strong, narrow belts of fast-moving winds that blow in the upper layers of the atmosphere, usually at heights of about nine to twelve kilometres. They move from west to east at very high speeds, sometimes more than 200 kilometres per hour. Jet streams have an important influence on the climate and weather of India. In winter, the westerly jet stream lies over northern India and helps to bring the western disturbances that cause winter rainfall in the north-west. In summer, this westerly jet stream withdraws northward, and an easterly jet stream develops over the southern part of the country, which helps in the sudden onset, or 'burst', of the south-west monsoon. Thus jet streams play a key role in the working of the Indian monsoon.

Monsoon winds are seasonal winds that exhibit a complete reversal in their direction between summer and winter, primarily due to the differential heating and cooling of large landmasses and adjacent oceans. During the summer, the land heats up much faster and to a higher temperature than the sea, creating a low-pressure area over the land. Conversely, the ocean remains relatively cooler, leading to a high-pressure area over the sea. This pressure gradient causes moisture-laden winds to blow from the sea towards the land, known as the Southwest Monsoon, bringing abundant rainfall to much of India from June to September. In winter, the land cools rapidly, developing a high-pressure system, while the sea remains warmer, creating a low-pressure area. This reversal causes winds to blow from the land to the sea, known as the Northeast Monsoon. This monsoon is generally dry over most of India, though it brings winter rainfall to the Coromandel Coast. These seasonal wind patterns are crucial for India's agriculture and economy.

Several parts of India receive very heavy rainfall, mainly during the south-west monsoon. The west coast, along the windward side of the Western Ghats, gets very heavy rain because the moist monsoon winds are forced to rise over the mountains; places along the Konkan and Malabar coasts receive abundant rainfall. The north-eastern region of India also gets very heavy rain; Mawsynram and Cherrapunji in Meghalaya are among the wettest places in the world, because the funnel-shaped hills trap the moist winds. The sub-Himalayan areas and the upper Ganga plains also receive good rainfall. In contrast, areas on the leeward side of the Western Ghats and the deserts of Rajasthan receive very little rain. Thus the rainfall in India is very unevenly distributed.

Mangrove forests are special tidal forests that grow in the salty, muddy water of coastal areas, deltas and river mouths, where the land meets the sea. The trees have special breathing roots that help them survive in waterlogged, salty soil. In India, the most important mangrove forest is the Sundarbans, in the delta of the Ganga and Brahmaputra in West Bengal, which is the largest mangrove forest in the world and the home of the Royal Bengal Tiger. Other mangrove forests are found in the deltas of the Mahanadi, the Godavari and the Krishna rivers on the eastern coast, in the Andaman and Nicobar Islands, and in the Gulf of Kutch along the western coast. Mangroves protect the coast from storms and erosion and support rich wildlife.

A biosphere reserve is a large protected area set up to conserve the variety of plants, animals and ecosystems of a region, along with the traditional life of the people living there. India has set up several biosphere reserves in different parts of the country. Five important ones are: the Nilgiri Biosphere Reserve, which was the first, in the Western Ghats of South India; the Gulf of Mannar Biosphere Reserve, off the coast of Tamil Nadu; the Sundarbans Biosphere Reserve, in the delta of West Bengal; the Nanda Devi Biosphere Reserve, in the Himalayas of Uttarakhand; and the Great Nicobar Biosphere Reserve, in the Andaman and Nicobar Islands. These reserves protect rare and endangered species and help to maintain the natural balance of the environment.

India is said to have a tropical monsoon type of climate, and there are good reasons for this. First, a large part of India lies within the tropics, as the Tropic of Cancer passes through the middle of the country, so most of India receives strong sunshine and high temperatures for much of the year. Second, and more importantly, the climate of India is completely dominated by the monsoon winds, which reverse their direction with the seasons. The moisture-bearing south-west monsoon winds bring almost all of India's rainfall during the summer months, while dry north-east winds blow in winter. The whole rhythm of the seasons, of agriculture and of daily life depends on the arrival and behaviour of these monsoon winds. Because both its tropical location and the monsoon winds shape its weather, India's climate is called tropical monsoon.

Mountains are cooler than the plains mainly because of the effect of altitude on temperature. As we go higher up in the atmosphere, the air becomes thinner and the temperature falls, normally at the rate of about 6.5 degrees Celsius for every 1,000 metres of rise; this is called the normal lapse rate. The thin air at high altitudes cannot hold and retain heat as well as the denser air of the plains. The air at great heights also receives less heat radiated back from the earth's surface. For these reasons, even places that lie in the same latitude can have very different temperatures: a hill station high up in the mountains will be cool and pleasant, while the plains below it at the same time may be very hot. This is why mountains are always cooler than the plains.

Southwest (summer) monsoon: arrives over India in early June and lasts till September. It originates over the warm tropical Indian Ocean and is driven by the low pressure over the Indian subcontinent and the shifting of the ITCZ; winds blow from the southwest carrying moisture. Two branches: Arabian Sea branch (gives heavy rainfall to west coast and western Ghats) and Bay of Bengal branch (moves northwards bringing heavy rains to northeast India and then into the Gangetic plains). The Himalayas and Western Ghats cause orographic rainfall. The monsoon is responsible for the majority of annual precipitation and is crucial for agriculture.

India has a great variety of forests because of its varied climate, rainfall and relief. The main types are as follows. Tropical evergreen forests grow in regions of very heavy rainfall, such as the Western Ghats and the north-east, and they remain green throughout the year with tall, dense trees like rosewood and ebony. Tropical deciduous, or monsoon, forests are the most widespread and grow in areas of moderate rainfall; their trees, such as teak, sal and sandalwood, shed their leaves in the dry season. Thorn forests and scrub grow in the dry regions of Rajasthan and the Deccan, with cacti and acacia. Tidal or mangrove forests grow in the deltas and coastal areas, such as the Sundarbans. Mountain forests change with altitude, from broad-leaved trees lower down to pines and, higher up, alpine vegetation. These forests are valuable for timber, wildlife and the environment.

The distinctions between the given pairs are as follows. First, Weather and Climate: Weather refers to the atmospheric conditions at a specific place and time, encompassing elements like temperature, humidity, precipitation, wind, and cloudiness, which can change hourly or daily. Climate, on the other hand, represents the average weather patterns of a region over a long period, typically 30 years or more, providing a generalized description of atmospheric conditions. Second, Tropical Evergreen Forest and Deciduous Forest: Tropical Evergreen Forests are found in regions receiving heavy rainfall (over 200 cm annually) and have high temperatures throughout the year. Trees in these forests do not shed their leaves simultaneously, remaining green all year round, hence the name "evergreen." They are characterized by dense, multi-layered vegetation. In contrast, Deciduous Forests, also known as Monsoon Forests, are found in areas receiving moderate rainfall (70-200 cm annually). Trees in these forests shed their leaves for about six to eight weeks in dry summer to conserve water, giving them a distinct seasonal appearance. Third, Northeast Monsoon and Southwest Monsoon: The Southwest Monsoon occurs during the summer months (June to September) and originates from the Indian Ocean, blowing from the southwest direction towards the Indian subcontinent. These winds are moisture-laden and are responsible for the bulk of India's annual rainfall. The Northeast Monsoon, occurring during the winter months (October to December), blows from the northeast direction, originating from the landmasses of Central Asia and the Himalayas. These winds are generally dry over most of India, but they pick up moisture over the Bay of Bengal and bring significant rainfall to the Coromandel Coast of Tamil Nadu.

Instructions for marking the map: 1) Draw arrows showing SW monsoon from Arabian Sea/Bay of Bengal toward the Indian landmass (southwest → northeast). 2) Draw arrows for NE monsoon from northeast (over Bay of Bengal) toward southeast India (northeast → southwest toward Tamil Nadu). 3) Shade/mark Western Ghats (west coast), northeastern hills (Meghalaya region), Andaman & Nicobar as heavy rainfall areas. 4) Mark mountain forests along the Himalaya and Western Ghats. 5) Mark Panna in central India (Panna district, Madhya Pradesh). 6) Mark Agasthiyamalai at the southern end of Western Ghats (Tamil Nadu–Kerala border).