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49<br />
2.5 to 3.0 mb for the season).<br />
v) On the northern side of the low, there was considerable cross—isobaric<br />
flow and moderate to strong northerlies/northeasterlies prevailed. This<br />
feature makes it difficult to uniquely fix the central region of the moving<br />
low pressure system from the observations over south Peninsula.<br />
vi) Although it was not a well-marked system, it caused considerable rainfall<br />
in Tamil Nadu, particularly in the southern districts. The gradient in<br />
rainfall was quite strong in the north; on 8th, when Cuddalore, Nagapat—<br />
tinam and Vedaranniyam received rainfall ranging from 6 cm to 21 cm, there<br />
was hardly any rain at Madras (about 150 km to the north). The rainfall<br />
extended well to the north of the low pressure system, in the region of the<br />
strong northerlies. In contrast to the well-marked low pressure area discussed<br />
in Sec. 16, in this case there was no good evidence of a northsouth<br />
oriented trough to the north of the low, moving from east to west.<br />
18. East-west oriented trough across the Peninsula<br />
- 25 to 26 October 1971<br />
18.1 During the first half of the northeast monsoon season, particularly in<br />
October, a trough of low pressure extends from south Arabian Sea across the<br />
south Peninsula to south Bay and south Andaman Sea. The trough may also extend<br />
further east across Malaysia. The trough is seen on the surface chart and in<br />
the lower tropospheric levels. An east-west oriented dumb-bell shaped isobaric<br />
configuration over the Indian Peninsula and the Indian Sea areas, is the common<br />
feature of pressure distribution during this period, with a low pressure area<br />
over the west central Bay and another over southeast Arabian Sea. Low pressure<br />
areas may form in this extended trough over these sea areas, and move from east<br />
to west. Rainfall over the south Peninsula is generally light to moderate when<br />
such a trough system overlies the area. But, when a low pressure area (embedded<br />
in the trough) moves across, it causes localised heavy to very heavy rains. The<br />
movement of the low pressure areas may be traced better in the upper air charts<br />
(aided by vertical time sections of stations across the path of the low pressure
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