The word monsoon is derived from the Arabic word ‘Mausim’ which means season. Monsoon refers to the seasonal reversal in the wind direction during a year.
During summer, the interior parts of North Indian Plains covering Rajasthan, Punjab, Haryana, and Western Uttar Pradesh are intensely hot. The daily maximum temperature in some of these parts is as high as 45° to 47°C.
Air has weight and this weight exerts pressure on us, which is known as air pressure. There is an inverse relationship between temperature and air pressure, i.e. if the temperature of any area is high, then the air pressure will be low and vice-versa. Difference in the air pressure is responsible for the attraction of the winds.
The average maximum temperature is above 33°C in the month of May at Delhi and Jodhpur. Such high temperature heats up the air of that region. Hot air rises, low pressure area is created under it. This low pressure is also known as monsoonal trough. It lies between Jaisalmer in the west and Balasore in Odisha in the East.
On the other hand temperature over Indian Ocean is relatively low, as water needs more time to get heated as compared to land. So a relatively high pressure region is created over the sea. Thus, there is a difference of temperature and resultant pressure over North Central Indian Plains and Indian Ocean.
Due to this difference, air from high pressure region of the sea starts moving towards the low pressure region of North India. Thus, by mid June the general movement of air is from equatorial region of Indian Ocean to the Indian subcontinent and the direction of these winds in general is from South-West to North-East.
This direction is exactly opposite to that of the trade winds (North-East to South-West) prevailing during winter in India. This complete reversal of wind direction from North-East to South West and vice-versa is known as monsoons.
These winds originate over warm seas. Therefore, they contain a lot of moisture. When these moisture laden winds move over the Indian sub-continent they cause wide spread rain throughout India and from June to September. About 80% to 90% of the total rainfall in India is confined to these four months only.
Characteristics of the Monsoon
- Monsoons are not steady winds. They are irregular in nature affected by different atmospheric conditions i.e. due to regional climatic conditions. Sometimes monsoon comes early or some times late.
- Monsoons are not equally distributed. Coastal areas like Kerala, West Bengal and Odisha receive heavy rain fall, whereas interior regions like Haryana, Madhya Pradesh, receive less rainfall.
- When monsoon arrives, it gives heavy rainfall which continues for several days. This is known as ‘burst of monsoon’. This occurs mainly at Kerala coast where it reaches first.