The word ‘Monsoon’ is derived from the Arabic word ‘Mausim’ ‘Monsoon is flow pattern of the general atmosphere circulation over a wide geographical area, in which there is a clearly dominant wind in one direction in every port of the region concerned, but in which this prevailing direction is reversed (or almost reversed) from winter to summer and from summer to winter
There is a broad unity in the climates of India. The social and cultural life in the country revolves round the seasonal rhythm of the climate. Different types of crops are grown during different seasons and along with these a number of festivals and cultural functions are organized.
But there are regional variations in the climatic conditions too. Assam in the northeast is damp with high rainfall. Rajasthan in the west is dry with low rain; Kerala in the south is hot and humid while Punjab in the northwest is continental and less rainy.
Barmer in Rajasthan records a day temperature of 48°C to 50°C in summer, while Dras and Kargil in Jammu and Kashmir record a minimum temperature as low as -40°C in winter night. Cherrapunji and Mawsynram in Meghalaya receive a rainfall of over 1,080 cm in a year, Jai salmer in Rajasthan rarely gets more than 12 cm of rain during the same period.
Origin and Development of Monsoon:
The seasonal change in the direction of wind is a principal feature of the Indian climate. With greater continentality and less oceanic influences, the northwestern part of India becomes the center of high pressure in winter and low pressure in summer.
The cold winds blowing from this part of the country in winter are known as the north- east monsoon, and the warm moist winds drawn in summer are known as the southwest monsoon. The word ‘monsoon‘ implies season and it implies seasonal change of prevailing winds. The winds are mainly off shore in winter and on shore in summer. This change in pressure from high to low is gradual.
The Jet Streams(westerly, easterly ):
Besides the lower level winds which change their direction seasonally, there are upper level winds which blow above 6 to 7 km from the surface and attain greater velocity at higher levels. These winds blow upto a height of 12 to 16 km within the troposphere. The areas over which they blow vary latitudinally with seasons and height. At 12 to 13 km e upper level winds attain speeds exceeding 180 km per hour and are known as the jet streams.
They are of two types: westerly jet streams which blow from west to east at a height of 12 km and easterly jet streams which blow from east to west at 13 km from the earth surface. The position of these jet streams changes seasonally. The easterly jet stream, blows to the south of 25°N parallel during the southwest monsoon.
In June it blows over the southern part of the peninsula and has a maximum speed of 90 km per hour. In August it is confined to 15°N parallel and in September upto 22°N parallel. The easterlies normally do not extend to the north of 30° N parallel in upper troposphere.
The western disturbances move across north India even in the hot weather season. Then they are associIated with violent dust storms over Pakistan and northwest India and with violent North wester over northeast India and Bangladesh. Several Posts and articles are found in internet for further informaiton.
During the south- west monsoon season they cease to travel along the southern periphery of the Himalayas. Whenever these troughs move eastwards, over the Ganga valley, the extension of the monsoon currents into Bihar and Uttar Pradesh are prevented and the monsoon experiences breaks.
They are not very frequent during the season of retreating monsoon, but are responsible for re-curvature of the tropical cyclones of the Bay of Bengal and the Arabian sea, Thunderstorms, the most common form of tropical storms are always associated with unstable or very strong vertical movement of air reaching heights, from 5 to 15 km, greatest in summer.
The average diameter of thunderstorms varies from 8 to 40 km. Precipitation from mature storm is generally heavy and composed of large drops of rain and hailstorms. Caused by surface heating over land they are most common in summer afternoon.
Monsoon Seasons: Two distinct seasons in the cli- mate of India are clear. They are the southwest mon- soon and the northeast monsoon. The former brings rain which is essential for agricultural economy of the country. The latter brings a spell of dryness over
INTER TROPICAL CONVERGENCE:
Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade Winds converge and so it is a zone where air tends to ascend. In July, the ITCZ is located around 20° to 25° N latitudes (over the Gangetic plain), sometime called monsoon trough. This monsoon trough encourages the development of thermal low over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40° and 60°E longitudes and start blowing from southwest to northeast due to Coriolis force. It becomes south- west monsoon. In winter, the ITCZ moves southward, and so the reversal of Winds from northeast to south and southwest, takes place. They are called northeast monsoon.
SALIENT FEATURES OF THE INDIAN CLIMATE:
Seasonal Reversal of winds Winter season- Winds blow from NE to SW.
Formation of Alternatively High and Low Pressure Areas over the land. Winter season-due to low temperature conditions high-pressure area is formed. Summer season-Intense heating of the land leads to the formation the thermally induced low-pressure cell over NW part of the country.
Seasonal and Variable Rainfall Over 80% of annual rainfall is obtained during the five month of the rainy season. There is variability in rainfall so far time and place are considered. There is considerable variation in the general distribution of rainfall.
Plurality of Seasons-That is constantly changing weather conditions.
Characterised by National Calamitities: The season of general rains thus is characterised by the alternating breaks which affects badly the growth of standing crops. The season is also associated with innumerable tropical cyclones which originate in the Bay of Bengal and the Arabian sea and move into the land. They give heavy rain and cause enormous dam- age by floods. Over three-fourths of the total rain in the country is received during the southwest monsoon
season.
Retreating Monsoon Season: By the end of September the southwest monsoon becomes weak as the low pressure trough of the Ganga plain starts moving southward in response to the southward march of the sun. The monsoon ceases by the first week of September in western Rajasthan and by the end of the month.
It is drawn from Rajasthan, Gujarat, the western Ganga plain and the Central Highlands by the beginning of October, the low pressure centers over northern parts of the Bay of Bengal and by early November, it moves over Karnataka and Tamil Nadu. By the middle of December, the centre of low pressure is completely removed from the peninsula.
This change begins in early October and is completed by early December. October and November, thus, is a period of transition leading to the conditions of dry winter. The retreating southwest monsoon season is marked by dry weather in north India but is associat ed with rain in the eastern part of the peninsula. Here, October and November are often the rainiest months of the year.
The wide spread rainfall in this season is often associated with the passage of cyclonic storms which form in the sea and advance westwards and northwards on the peninsula. A few cyclonic storms also strike the coast of West Bengal, Bangladesh and Myanmar. Such cyclonic storms are less frequent in the Arabian sea.
Distribution of Rainfall: Rainfall is the most important element for Indian agriculture. Agricultural prosperity of the country depends very much on timely and adequately distributed rainfall. If it falls agriculture is very badly affected. For this reason, it is often said that the Indian agriculture is a gamble in hands of the monsoon. There is a great regional and temporal variation in the distribution of rainfall.
Over 80 per cent of the annual rain is received in the four rainy months of June to September. But there is some rain in other months as well. During the cold weather season, northwestern India gets about 25 cm of rainfall. The southeastern peninsula also gets about 25 cm of rainfall from the retreating southwest monsoon.