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12.2 Low pressure areas and Low pressure waves<br />
12.2.1 During the northeast monsoon season, storms and depressions develop<br />
out of low pressure areas in the south Bay or southeast Arabian Sea. They<br />
(i.e. storms and depressions) also weaken into low pressure areas after they<br />
cross coast and move across the Peninsula. However, there are cases when the<br />
low pressure areas which form in the west central or southwest Bay move in a<br />
westerly direction without developing further and affect weather over south<br />
Peninsula. On an average, during October and <strong>No</strong>vember there may be 2 such low<br />
pressure areas in a month which do not develop further and in many years even 3.<br />
In December, they are slightly less — only 1 or 2.<br />
12.2.2 While some low pressure areas may be traced from across the Andaman<br />
Sea, they often form, in situ, in west central Bay and southwest Bay.<br />
12.2.3 Low pressure areas are delineated on the surface chart by a single<br />
closed isobar at 2 mb interval. At times, the closed isobar may only be an 'odd<br />
valued' isobar. When the area covered by the closed isobar is not wide and<br />
diffuse and the associated upper air circulation as far as could be judged from<br />
the coastal winds is also well—defined, the system is called a 'well-marked'<br />
low pressure area. When the satellite picture shows a heavy mass of clouding<br />
associated with the low, which can be categorised as Stage B, the system is<br />
always a well-marked low pressure area; at times it may be even a depression.<br />
About 40% of the low pressure areas are well-marked. When the closed isobar<br />
covers a wide area in the south and the adjoining central Bay of Bengal and the<br />
circulation is also seen in the lower troposphere, it is common practice to<br />
describe the situation only as "the seasonal low being well—marked". The seasonal<br />
low usually becomes well—marked as a prelude to the formation of a low<br />
or a depression.<br />
12.2.4 Over land areas, the low pressure area may not be seen as a closed<br />
isobar; it may be seen as a moving isallobaric centre on the 24 hrs. pressure
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