Friday, July 2, 2010

Rainfall characteristics of Nilgris in Tamilnadu

South West Monsoon

Greater part of the total annual rainfall (88 cm out of 170 cm) is brought by the south-west monsoon which blows from June to September and the heaviest falls occur in the stations of the which are furthest west and thus are the first to receive this monsoon. As it travels eastwards the current rapidly deposits the moisture with which it is laden and every succeeding station to the east gets less and less rain from it. Thus at Devala which is just at the top of the Western Ghats and receives the full force of the monsoon straight from the sea and the total annual rainfall is nearly 411 cm against the district average of 170 cm and of which rainfall during the south-west monsoon is 336 cm against the district average for that period of 88 cm. Gudalur is much sheltered from the south-west by the spurs on the northern boundary of the Ouchterlony Valley and there the annual rainfall drops to 229 cm of which nearly 177 cm are received during the south-west monsoon. But at Naduvattam which is above these spurs and on the very crest of the plateau and so just at the spot where the monsoon receives a sudden check in its progress the total rainfall rises again to nearly 258 cm of which 201 cm come with the south-west monsoon.

As the monsoon travels eastwards across the plateau it gives less and less rain. The fall during its course at Paikara (only four miles east of Naduvattam as the crow flies) drops to 131 cm, at Ooty (eight miles again east) to 57 cm and at Coonoor which is sheltered by the big spurs of the Dodabetta range to only 40 cm. This Dodabetta range checks the current and causes it to deposit the greater part of the moisture which it has left and all north east the recording stations to the east of it (Coonoor, Wellington, Kothagiri and Kodanad) receive less rain from it than they do from the north-east monsoon which blows in the directly opposite direction from October to December.

North East Monsoon

During the NE monsoon the above conditions are all reversed and the stations on the east of the district fare better than those on the west for the same reason as before namely that this monsoon reaches them first before it has deposited much of its moisture. At Coonoor which stands at the head of a ravine the mouth of which faces east and so collects the damp winds like a funnel the fall between October and December is as high as 78 cm. Wellington, which lies further within the plateau and is somewhat sheltered by the hills to the east of it receives only 54 cm. Kothagiri and Kodanad which are on the eastern crest of the plateau get 71 and 62 cm respectively and Kilkundah, where the rain driving up the Bhavani valley is checked receives 57 cm. But at Ooty which lies right under the protecting mass of Dodabetta, the fall during this monsoon is only 37 cm at Naduvattam, farther west only 32 cm and at Gudalur, down under the lee of the plateau only 27 cm.

Between January and March its the driest season of the year Coonoor, the other stations on the east of the district and Kilkundah all receive some benefit from the last showers of this north-east monsoon; but nowhere else is the fall in these three months as much as 5 cm. In April and May showers appear impartially all over the country and every station gets from 18-26 cm.

This unequal distribution of the rainfall, as is pointed out elsewhere in this book is of the greatest importance from an agricultural point of view plants and trees which will do well on the moisture west side refusing to flourish on the drier eastern slopes and also provides the resident in the district with a wide choice of climates. Its greatest extremes do not appear in the official statistics, for there are no recording stations in either the wettest or the driest parts of the district. Probably the annual rainfall on parts of the Kundahs is as much as 508 cm and that on the south eastern slopes above the Coimbatore district as little as 102 cm. The average annual fall in the district as a whole is raised by the heavy rain in its western stations and is thus larger than in any other collectorate except the two on the west Coast proper Malabar and South Canara.

In Ooty itself the total rainfall is only 123 cm which is less than that of Chennai 125 cm and very little more than the figures for the littoral districts (Chengelpet 115 cm, South Arcot 111 cm and Tanjore 114 cm) . Yet Ooty is popularly classed as a rainy spot. The chief reason for this is the fact that nearly two-fifths of its total fall is received during the three months (May — July) during which it is full of visitors and that this arrives in lighter showers than anywhere else in the district and is spread on an average of over 38 of the 92 days in those three months. A visitor who finds that more or less rain has fallen on 40 per cent of the days he has spent in the station and classes the place as damp locality.

Source: W. Francis, Asian Educational Services
Photo:-My Collections

1 comment:

anand said...

Excellent information. Very valuable.