No. IV-18.4 - (IMD), Pune

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rainfall is mostly along and to the south of trough, the area to the north
getting relatively much less rainfall, in the case of the trough across the
Peninsula during this season, rainfall extends on either side of the trough. The
1 Limit of the cloudiness and rainfal] is upto the region of the dry air
of continental origin. This
limit between dry air and most air
is quite marked over the country during this period; the limiting zone can be
well delineated by the surface dew point temperatures and lower tropospheric
mixing ratio values. This moisture discontinuity line is an useful aid in analysis
and prognosis during this season (see para and Fig. 18.1).
12.3.3 The seasonal trough extends from the surface upto about 600 mb in the
mean charts with a southward slope with height, the average slope being about
1/100. The slope
of the trough line can be seen clearly in October when the trough is near about
13°N in the surface and there are enough upper air observations to the south of
the trough. However, later in the season when the trough shifts southwards, the
any
slope cannot be fixed as there is hardly upper air observation to the south of
trough line.
12.3.4 The east-west trough becomes well-marked
when the low pressure systems in the south Bay. and southeast Arabian Sea become
prcminent. This east-west trough is occasionally seen on the synoptic chart,
extending from south Arabian Sea in the west to as far east as southwest Pacific
across the south Indian Peninsula and south Bay and we notice a series of low
pressure systems embedded in the trough, spaced at nearly equal intervals (for
instance see Fig. 18.2 of Section 18).
12.3.5 The east-west oriented trough, the seasonal trough over the south
Bay and low pressure areas in the south Bay and southeast Arabian Sea are generally
inter-related phenomena, so that when one of them is seen, the others may
also be simultaneously present on the charts on many occasions.