No. IV-18.4 - (IMD), Pune

50
area) than in the surface isobaric analysis. In this section an instance of this
type of aynoptic situation will be discussed.
18.2 From 23rd October 1971 onwards, the seasonal trough was well-marked
over south Arabian Sea, south Peninsula and south Bay of Bengal. A low pressure
area (clearly seen in the upper air) moved westwards across the south Peninsula
between 23 and 25 and reached Laccadives area on 25th. On this day,
line also ran roughly along 9°N on the surface (Fig. 18.1) and in the lower troposphere,
from Laccadives to extreme south Vietnam coast
Extended
charts showed that the trough extended as far east as southwest Pacific with a
number of embedded lows in it (Fig. 18.3). The trough was seen upto 3.6 km
(Fig.18.3)
without any noticeable tilt with height./ Both the easterlies and westerlies (on
either side of the trough line) were moderate to strong, the strength being on
an average about 15/20 kts. Even though the low pressure area had moved to
laccadives, still an elongated closed isobar could be drawn over south Bay and
south Andaman Sea. The rainfall on this day was fairly widespread over the
Peninsula south of Lat. 13°N. The significant amounts were close to the trough
line and they were moderate to rather heavy. Monsoon was active in Tamil Nadu.
This increased activity was partly due to the low pressure area moving westwards;
across the area in the
24 hrs.
18.3 Cloudiness and light precipitation extended in the north upto about
Lat. 20°N. The upper air charts showed that air was considerably dry at all
levels to the north of Lat. 20°N. To the south of Lat. 20°N, the easterlies we::e
more or less uniformly moist., This was also reflected in the surface dew point
distribution. Thus,there was a pronounced moisture discontinuity across the
country along Lat. 20°N, which was also the northern limit of weather and cloud
belts.
18.4 On the 26th also the situation was nearly similar except that the
trough extended upto 6.0 km. On this day also significant rainfall amounts were