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26<br />
change chart or as a region of negative pressure departure. Sometimes, it may be<br />
seen only in the lower tropospheric wind field.<br />
12.2.5 In October, we can delineate the low pressure areas even over the sea<br />
a:rea unambiguously as they usually form at a slightly northerly latitude and a<br />
few observations over Sri Lanka and the south Bay indicate a westerly wind.<br />
But later in the season, when the seasonal trough shifts to extreme south Bay,<br />
the central region of the low pressure systems may be to the south of 8°N or<br />
even further south. Under such circumstances, we may not be able to draw any<br />
closed isobar in our chart which is mainly due to the non-availability of observations<br />
in the equatorial region. In such cases, a trough may also extend from<br />
the low northwards to off Tamil Nadu coast and may appear to be more prominent<br />
on the charts than the main low to the south, which again is mainly on account<br />
of lack of observations in the south to define the low. However, satellite picture<br />
may help the analyst on such occasions to delineate the low in the south.<br />
Figs. 12.a, 12.b, 12.c, give the spatial distribution of centres of low pressure<br />
areas, monthwise, in the northeast monsoon season. The centres have been<br />
taken as the geometric centres of the closed isobar defining the low pressure<br />
a:reas. The low pressure areas are seen in the south Bay, Comorin and Laccadives<br />
a::ea in October and <strong>No</strong>vember. Their hiqher frequency in Laccadives area is<br />
noteworthy. By December they are seen mainly to the east and south of Sri lanke.<br />
and in Maldives area.<br />
12.2.6 Over land, the low pressure areas are much less in number and they<br />
also occur only in October. Many of the lows over the land are the remnants of<br />
depressions and storms. It is also noticed that whenever a well-marked system<br />
approaches Tamil Nadu coast from the east, it induces a low pressure area in the<br />
Laccadives region.<br />
12.2.7 On an average, there are about 30 days in each year during the northeast<br />
monsoon with a low pressure area on the chart in the south Bay, south
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