Friday 23 November 2007

Weather forecasting, Monsoon

The Weekend Observer, June 1992

Vagaries of weather forecasting

Shivanand Kanavi

The monsoon is not only a meteorological phenomenon for Indians but it deeply affects their literature, music, culture and the very psyche itself. Not only are the famous raga Megh Malhar and the poetic work Meghdoot expressions of it, but D.D. Kosambi, one of the great Indian encyclopaedists observed that the regularity of the cycle of seasons might have given rise to a fatalistic world view and even the myth of satyug, tretayug, dwaparyug and kalyug

Thus anybody who can predict and hopefully change the pattern of monsoons is always sought after, be they sooth sayers, astrologers, sadhus, performing yajnas and havans and even scientists. When the predictions or promised changes in weather do not come through, Indians seem to find any number of justifications for the failure of all the traditional wisdom but the meteorologist is never spared. He is the cartoonist’s delight. Who can forget R.K. Laxman’s cartoon of a long bus queue in pouring rain where everybody has provided himself with a raincoat or an umbrella except one fellow, and a guy whispering “must be from the weather bureau”!

All this is good for a laugh but when you meet a hardened weatherman like Dr. S Kumar, Deputy Director General of Meteorology heading the Colaba (Bombay) observatory and responsible for the western zone, and learn the rudiments of meteorology then you start appreciating the complexity of the subject.

The word monsoon owes its origin to the Arabic word mausam meaning season. It is believed to have been used by seamen, six or seven centuries ago to describe a system of alternating winds in the Arabian sea, these winds appear to blow from northeast for six months and from the southwest for another six months. Seasonal changes of wind are primarily the result of differences in the quantity of heat received from the sun by different parts of the earth.

As a consequence of its chemical composition and its soil structure, the conduction of heat into the earth is a comparatively slow process. Thus most of the solar energy received at the ground by the continents is used up in hating air rather than the earth’s surface. Whereas oceans are heated up to greater depths due to convection currents and a smaller part of the energy is available for heating the air, monsoon as a system of winds has the following notable features:
1. A system, with marked seasonal shifts, caused by the differential heating of the land and the sea.
2. A wind system that is largely confined to the tropics, that is the region between 20° N and 20° S latitudes on both sides of the equator.
3. Indian monsoon can be thought of as southeast trade winds which on crossing the equator are deflected to the right by the earth’s rotation (Coriolis force) and hence approach the land from a south-westerly direction.
4. The trade winds of the northern and southern hemispheres are divided by the Inter Tropical Front (ITF) which is a region of considerable cloudiness and rainfall. The southwest monsoon, originates in the ITF and moves northwards, due to the pull of a low pressure area in the hot Indo-Gangetic basin, but of the subcontinent after reaching the southern tip of India around June 1, it splits into two branches; the Arabian sea branch, and the Bay of Bengal branch.

The Arabian Sea branch gradually advances northwards to Bombay. The advance from Trivandrum to Bombay takes about ten days and is fairly rapid.

The Bay of Bengal branch moves northwards into the central Bay of Bengal and rapidly spreads over most of Assam by the first week of June. On reaching the Himalayan barrier, the bay branch of the monsoon is deflected westwards. As a consequence, its further progress is towards the Gangetic plains of India rather than towards Burma. The arrival of monsoon at Calcutta is lightly earlier than at Bombay. By mid-June, the Arabian Sea branch spreads over Saurashtra-Kutch and the central parts of the country. Thereafter the two branches tend to merge into a single current. The remaining parts of western UP, Haryana, Punjab and the eastern Rajasthan experience the first monsoon showers by the first of July. Some times the first showers at Delhi arrive from the east as an extension of the Bay of Bengal branch and sometimes from the south that is from the Arabian sea branch. Often it is a race between the two. By mid July it will extend to Kashmir and remaining parts of the country but only as a feeble current because by this time it has shed most of its moisture.

The normal duration of monsoon varies from two to four months. The withdrawal is much more gradual than its onset. Generally the monsoon withdraws from northwest India by the beginning of October and from the remaining parts of the country by the end of November. Though theoretically it seems possible for both the southwest monsoon and the northeast monsoons to co-exist in the southern half of the peninsula in October, in reality such situations are rare.

This in brief is the story of the monsoon but there are any number of disturbances of local and regional origin that can upset the text book schedule for example a cyclone in the Arabian sea that starts drawing the moisture away can lead to delays and dissipations.

The short term forecasts deal with a period of twenty four to seventy two hours which are mainly done with the help of data from over 500 weather stations spread all over the country, the data from the ships in the ocean, the satellite pictures from he NASA polar satellite NOVA which scans India every six hours, pictures from the geo-stationary INSAT satellite and even input from airline crews.

But a satellite picture, as Dr Kumar points out, is like a X-Ray photograph in the hands of a physician. It needs interpretation which is bound to be subjective. This is where the years of experience of our weathermen count.

Attempts are on to developing computer programmes to forecast weather in the medium term that is three to ten days at the Super Computer facility in Delhi.

The long term forecasting that is from ten days to a few months, is being attempted by the group in Pune. Over sixteen phenomenons all over the globe are being watched by this group and correlated with the Indian monsoon. Some of tem are the total snowfall over Eurasia during the previous winter, the convective wind between Darwin in the southern hemisphere and Tahiti islands in the pacific, the El Nino oceanic current off the coast of Peru in south America etc.

Considering the enormity of a weather system like the Indian monsoon, and the usual constraints of funds and technology and the very nature of a field where controlled experiments are well nigh impossible, our weathermen are doing a competent job, to say the least.

1 comment:

kochuthresiamma p .j said...

very interesting, informative blog. have book marked this site.
being a keralite, monsoon evokes nostalgic feelings, which i have recorded in my blog:
even today, to a larg extent,our lives are dictated by the monsoons.