Geoscience Reference
In-Depth Information
Figure 11.32
Mean daily rainfall (mm)
along the west coast of India during the
period 16 May to 30 September 1971,
showing a pronounced burst of the
monsoon followed by active periods
and breaks of a periodic nature. All
years do not exhibit these features as
clearly.
Source
: After Webster (1987b). Copyright
© 1987. Reproduced by permission of
John Wiley & Sons, Inc.
Figure 11.33
The yearly drought area index for the Indian subcontinent for the period 1891 to 1988, based upon the percentage of
the total area experiencing moderate, extreme or severe drought. Years of extreme drought are dated. The dashed line indicates the
lower limit of major droughts.
Source
: After Bhalme and Mooley (1980). Updated by courtesy of H. M. Bhalme, reproduced by permission of the American Meteorological
Society.
clouds over central and eastern Tibet in July. Late
afternoon rain or hail showers are generally accompanied
by thunder, but half or more of the precipitation falls
at night, accounting for 70 to 80 per cent of the total
in south-central and southeastern Tibet. This may
be related to large-scale plateau-induced local wind
systems. However, the central and eastern plateau also
has a frequency maximum of shear lines and associated
weak lows at 500 mb during May to September. These
plateau systems are shallow (2 to 2.5 km) and only 400
to 1000 km in diameter, but they are associated with
cloud clusters on satellite imagery in summer.
5 Autumn
Autumn sees the southward swing of the equatorial
trough and the zone of maximum convection, which lies
immediately to the north of the weakening easterly jet
(see Figure 11.16). The breakup of the summer circu-
lation systems is associated with the withdrawal of
the monsoon rains, which is much less clearly defined
than their onset (Figure 11.34). By October, the easterly
trades of the Pacific affect the Bay of Bengal at the 500-
mb level and generate disturbances at their confluence
with the equatorial westerlies. This is the major season
