The monsoon winds are caused by the differential heating and cooling of the land and the sea. In India, they are divided into two systems—summer monsoon and winter monsoon.
Phenomena and Mechanism of the Monsoon in India
Summer Monsoon in India
- The land heats quickly during summers, and hence, low-pressure conditions develop over the land. As the sea is relatively cool, a high-pressure area develops over the sea.
- Because winds move from a high-pressure area to a low-pressure area, winds move from the sea to the land. This is called the summer monsoon.
- In May, June, and July, the plains of the Indian subcontinent receive vertical rays of the Sun. While a low-pressure area develops over the land, a high-pressure area develops over the sea.
- Thus, winds blow from the Indian Ocean northwards and northwestwards into Asia. Because they blow from the southwest, they are known as the southwest summer monsoon.
Winter Monsoon in India
- During the winter, oceans tend to become warm and the land cools down. A high-pressure area develops over the land and a low-pressure area develops over the Indian Ocean.
- The winds move from a high-pressure area to a low-pressure area. Thus, the winds which blow from the land to the sea are dry and are devoid of any moisture. They bring cold weather and do not produce any rainfall.
- When these winds blow over adjoining oceans, they pick up moisture and bring rainfall over the southern Coromandel Coast (coastal Tamil Nadu) and over the southern tip of Andhra Pradesh.
- Because these winds blow from the northeastern parts of the country, they are also known as northeast monsoon winds.
Features of Rainfall in India
- Rainfall in India is mainly concentrated to the months of July, August and September.
- In India, the rainfall is mainly orographic in nature. The places located on the windward side receive more rainfall than the places located on the leeward side.
- Only a small amount of rainfall is received from cyclones and convectional rainfall. 4. Monsoon in India is erratic, uneven and unpredictable. While there are floods in some regions, the other regions experience drought.
Seasons in India
In India, the southern monsoon winds have distinct seasonal patterns. In the monsoon type of climate, a year is divided into four seasons.
Hot Dry Summer
- Summer in India begins from March and continues till May.
- The highest temperature is recorded in the northwestern parts where the highest temperature may reach up to 48°C. South India does not experience intense heat because it experiences moderating influence of the sea.
- During the summer, a local hot wind called loo blows in the northern parts of the country. Direct exposure to the wind may prove fatal.
- During this season, thunderstorms known as kalbaisakhi hit West Bengal. These are also known as Bardoli Chheerha. These thunderstorms are accompanied by heavy rainfall. It also rains in coastal Kerala and Karnataka. These showers are known as mango showers as they help in the early ripening of mangoes. They are also known as cherry blossoms in Karnataka.
Southwest Monsoon in India
- The southwest monsoon in India begins in June and lasts till September. During summers, a low-pressure area is created over the northwestern part of the Indian Ocean. This attracts the southeast trade winds. These winds get deflected to their right because of the Coriolis force after crossing the equator.
- They reach the west coast as the southwest monsoon. They bring heavy rainfall accompanied by violent thunder and lightning. This violent onset of the monsoon is termed the burst of the monsoon.
- The monsoon winds get divided into the Arabian Sea branch and the Bay of Bengal branch.
Arabian Sea Branch
- One branch of these winds causes heavy rainfall on the areas which lie on the windward side of the Western Ghats and the Western Coastal Plains. The areas located on the leeward side of the Western Ghats hardly receive any rainfall.
- The second branch of the Arabian Sea monsoon winds strikes the northern coast of Mumbai. It further moves along the river valleys of Narmada and Tapti and causes rainfall in central India. It then mingles with the Bay of Bengal branch after entering the Ganga plains.
- The third branch strikes the Saurashtra Peninsula and the Kachchh. It then travels to western Rajasthan and runs parallel to the Aravalli Range. Hence, western Rajasthan gets scanty rainfall.
- The Arabian Sea branch joins the Bay of Bengal branch causing rainfall in the western Himalayas.
Bay of Bengal Branch
- The Bay of Bengal branch approaches Myanmar and southeast Bangladesh. Because of the presence of the Arakan Hills, they are deflected towards the Indian subcontinent. The monsoon thus enters West Bengal from the south and southeast instead of the southwesterly direction.
- This branch then divides into two—one running along the Ganga plains and the other along the Brahmaputra valley.
- These branches cause heavy rainfall in the Ganga plains, Brahmaputra valley, and Garo and Khasi Hills of Meghalaya. Mawsynram and Cherrapunji, which are located in the southern part of the Khasi Range, receive the highest average rainfall in the world.
This can be borne by the following examples:
- Mahabaleshwar located on the windward side of the Western Ghats receives heavy rainfall (250 cm). Pune, on the other hand, located on the leeward side gets less than 70 cm of rainfall annually.
- It rains heavily in the northeastern parts of the country because of the presence of the mountain ranges.
- There is a decrease in rainfall as one goes from the east to the west because the winds become dry as they shed their moisture in the course of their journey. Thus, Kolkata receives 120 cm of rainfall, Patna receives 102 cm, Allahabad receives 91 cm and Delhi gets 56 cm of rainfall.
- The coast of Tamil Nadu does not get rainfall during this season. It is because this coast is parallel to the Bay of Bengal branch of the southwest monsoon winds. It also lies in the rain shadow area of the Arabian Sea branch of the southwest monsoon.
- During October and November, the southwest monsoon winds become weaker and start retreating.
- This season is marked by clear skies and an increased temperature in the northern plains. The retreat of the monsoon is at a marked level. This increase in temperature is termed October heat.
- At this time, the low-pressure conditions shift to the Bay of Bengal giving rise to cyclonic depressions. These cyclonic depressions often result in the destruction of life and property generally on the eastern coast and the southern coast. Most of the rainfall of the Coromandel Coast is derived from depressions and cyclones.
- Tropical depressions originating in the Bay of Bengal are caused by local variation of heat and moisture. They result in tropical cyclones in November and December.
- They generally originate in the neighborhood of the Andaman and Nicobar Islands between 12°N and 17°N and travel eastwards towards the Bay of Bengal. They result in a heavy downpour and cause great loss of life and property.
- The month of October is marked by a clear cloudless sky, high temperature, and high humidity. This is called October heat.
- The month is also a period of transition between the hot rainy season and the cold dry season.
- The cold weather season in India begins during mid-November in northern India and stays till February. December and January are the coldest months.
- The temperature decreases from the south to the north. The places in the North Indian Plains experience cold climates. Thus, while the mean temperature during January at Thiruvananthapuram is as high as 31°C, it is only 16°C in Varanasi.
The excessive cold in north India during the winter is due to the following reasons:
- Cold winds blow from the Caspian Sea and Turkmenistan over the northwestern parts of India during February.
- Places such as Punjab, Haryana, and Rajasthan are far away from the moderating influence of the sea and hence experience the continental types of climate.
- The snowfall in the nearby Himalayan ranges creates a cold wave situation.
- There are high-pressure conditions over the northwestern parts of the country.
- The northeast trade winds blow from the land to the sea during this season, and hence, they are dry. They pick up moisture when they blow over the adjoining oceans and thus bring rainfall over the Coromandel Coast.
- Western disturbances are experienced in the northern parts of the country during the cold season.
Rainfall in Winter
Most parts of the country do not receive rainfall during the winter. Some areas which receive rainfall during the winters are
- Central and northern parts of the country get occasional rainfall during winter.
- Weak temperate cyclones cause rainfall in Delhi, Haryana, Punjab, and western Uttar Pradesh. This rainfall is beneficial for rabi crops.
- Northeastern parts of the country also receive winter rainfall.
- In October and November, the northeast monsoon picks up moisture while blowing over the Bay of Bengal and causes torrential rainfall over the coast of Tamil Nadu and the southern tip of Andhra Pradesh.
Also, Read Climate of India