Distribution of Rainfall in India

Rainfall is the important element of Indian economy. Although the monsoons effect most part of India, the amount of rainfall varies from heavy to scanty on different parts. 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.

1. Areas of Heavy Rainfall (Over 200cm) : The highest rainfall occurs in west costs, on the western Ghats as well as the Sub-Himalayan areas in North East and Meghalaya Hills. Assam, West Bengal, West Coast and Southern slopes of eastern Himalayas.
2. Areas of Moderately Heavy Rainfall (100-200 cm) : This rainfall occurs in Southern Parts of Gujarat, East Tamil Nadu, North-eastern Peninsular, Western Ghats, eastern Maharashtra, Madhya Pradesh, Orrisa, the middle Ganga valley.
3. Areas of Less Rainfall (50-100 cm) : Upper Ganga valley, eastern Rajasthan, Punjab, Southern Plateau of Karnataka, Andhra Pradessh and Tamil Nadu.
4. Areas of Scanty Rainfall (Less than 50 cm) : Northern part of Kashmir, Western Rajasthan, Punjab and Deccan Plateau. The two significant features of India's rainfall is that
   i. in the north India, rainfall decreases westwards and ii. in Peninsular India, except Tamil Nadu, it decreases eastward.

Facts About Indian Monsoon
Rainfall occurs in summer Rainfall is erratic and unpredictable Rainfall is unevenly distributed Rainfall affects Indian economy

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 temperature seen to have an impact on the progress of the rains.
3. Just before the monsoon sets over south-east Asia the atmosphere 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 phenomena called southern oscillations is key indicator of the south-west monsoon. When the pressure over Indian Ocean is lower than normal it augurs well for the good monsoon.

Global Atmospheric Research Programme and Monex

The World Meteorological Organisation (WMO) and the International council of Scientific Union (ICSU) organized a Global Atmospheric Research Programme (GARP) in 1969. Under the aegis of this programme, a Global Weather Experiment was conducted for one full year beginning on 1 December 1978. it was one of the biggest ever international experiments, on a global scale, for observing the earth's atmosphere from land and ocean based data collection platforms, and by weather satellites, which now monitor the restless atmosphere, was launched after several years of intensive preparations and planning. Some idea of the dimensions of the experiment may be gleaned from the fact that in May of 1979 as many as fifty two research ships were deployed over the tropical oceans between 10oN and 10oS, While 104 aircraft missions were successfully completed over different parts of the Pacific, the Atlantic and the Indian Ocean. Of considerable interest to India was a special programme of the Global Weather Experiment. This was the Monsoon Experiment (MONEX). Its purpose was to study the influence of monsoon winds on the general circulation of the atmosphere. In view of its economic impact, the Indian scientists were naturally interested in improving their capacity to predict the vagaries of this seasonal phenomenon, which occurs year after over the landmasses of Asia and parts of Africa. In view of its seasonal characteristics, the monsoon experiment (MONEX) was designed to have three components:- Winter MONEX from 1 December 1978 to 5 March 1979 to cover the eastern Indian Ocean and the Pacific along with the land areas adjoining Malaysia and Indonesia. Summer MONEX from 1 May to 31 August 1979 which covered the eastern coast of Africa, the Arabian Sea and the Bay of Bengal together with the adjacent landmass. It also covered the Indian Ocean in the belt extending from 10oN to 10oS. A West African Monsoon Experiment (WAMEX) over western and central parts of Africa from 1 May to 31 August 1979 International MONEX Management Centers (IMMC) were set up in Kuala Lumper and in New Delhi to supervise the winter and summer components of the experiment. A large number of scientists from different countries came and worked at these Centers to plan and implement this international project.