There is great regional and temporal variation in the distribution of rainfall. Over 80% of the annual rainfall is received in the four rainy months of June to September. The average annual rainfall is about 125 cm. but it has great spatial variations.
(a) Areas of Heavy Rainfall (Over 200 cm): Assam, West Bengal, West Coast and Southern slopes of eastern Himalayas.
(b) Areas of Moderately Heavy Rainfall (100-200 cm): Western Ghats, eastern Maharashtra, Madhya Pradesh, Orissa, the middle Ganga valley.
(c) Areas of Moderate Rainfall (50-100 cm): Upper Ganga valley, eastern Rajasthan, Punjab, Southern Plateau of Karnataka, Andhra Pradesh and Tamil Nadu.
(d) Areas of Scanty Rainfall (Less than 50 cm): Northern part of Kashmir, Western Rajasthan and Punjab and Deccan Plateau. The two significant features of India’s rainfall is that (a) in the north India, rainfall decreases westwards and (b) in Peninsular India, except Tamil Nadu, it decreases eastward.
Variability of Rainfall:
The large variation in actual amount of rainfall from year to year is expressed in terms of co-efficient of variation as per the following formula:
Coefficient of Variation
(C)= Standard Deviation/Mean ? 100
The values of coefficient of variation show the change from the mean values of rainfall. The coefficient of annual rainfall in India generally ranges from 20 to 50%. A variability of less than 25 per cent exists on the western coasts, Western Ghats, north eastern peninsula, eastern plains of Ganga, north eastern India, Uttarakhand and H.P and south western parts of J and K. These areas have an annual rainfall of over 100 cm.
A variability of over 50% exists in the western parts of Rajasthan, northern parts of J & K and interior parts of Deccan plateau. These areas have an annual rainfall of less than 50 cm. The Rest of India has a variability of 25 to 50%. Higher the rainfall, lower is the variability of rainfall.
Land of Rain:
It is Cherrapunji is the land of rain, the village, which receives rain throughout the year is the rainiest place on earth for many centuries, is situated about 56kms from Shillong, the capital of Meghalaya in a height of about 4500ft above MSL in between Khasi, Gharo and Jaintia hills.
The Britishers pronounced the word Sohra as Chira. ‘Sohra’ means not suitable for cultivation, ‘punji’ means soil. ‘Cherrapunji’ means the soil not suitable for cultivation. This region has very poor topsoil has large reserves of coal and limestone beneath.
Therefore water is not available in the wells in this region although the topsoil is wet due to rainfall year round. The coveted place of Cherrapunji with an average rainfall greater than 1080 cms was taken over by another place in India some years back.
Mousinram, 6 km away from Cherrapunji was the place that captured the position. The Hawaiian Islands have also claimed the honour of being the rainiest spot on earth. But recently Cherrapunji has come back to regain the first place. Our neighbouring country Bangladesh prays not to have heavy rains in Cherrapunji for the reason that when heavy rainfall occurs in Cherrapunji many places in Bangladesh will get inundated.
Spatio Temporal Variation in the Rainfall:
1. Though the jet streams go a long way in explaining the origin of monsoon some questions remain unanswered. The great variation in the amount of rainfall both spatially and temporally, the high degree of uncertainty related to the date of arrival etc. are unexplained. Meteorologists have been trying to explain these phenomena from different angles relating to wide variety of generalisation.
They have been monitoring huge high pressure or anticyclone zones that form a few kilometers below the jet streams. This ridge hovers over south Goa. It has been noticed that if the ridge moves towards karwar in Karnataka it does not augur well for the monsoon. This high-pressure zone, it is reasoned, blocks the low flowing south westerly monsoon from intensifying over the west coast. When it is not positioned well, several meteorologists remain skeptical about the monsoon’s performance.
2. The unusual cooling of surface temperatures over the Arabian Sea by as much as 3 to 4 degrees before the onset of monsoon is another curious phenomenon. This is due to the cool Somali current. It pushes the cool waters of the Indian Ocean towards the Arabian Sea and the drop in temperatures seen to have an impact on the progress of the rains.
3. Just before the monsoon sets over south-east Asia the atmospheric pressure over the Indian Ocean drops. Simultaneously about 10,000 kilometers away in the South Pacific there is rise in pressure, when the rain is over, this reverses. This phenomenon called southern oscillations is key indicator of the south-west monsoon. When the pressure over Indian Ocean is low than normal it augurs well for the good monsoon.