Introduction

First of all one should know about different reasons of rain like, orographical rain, rain due to thermal convection, rain due to convergence, rain due to westerlies. In upper parts the recent spells are happening due to heating of Kashmir and Gb, this heating attracts winds which strike with foothills and cause orographical rain, however further spatial distribution of rain varies depending upon direction of stj position and direction.

Currently north to south moving jet the so called ridge does not allow upper parts rains to spread much due to lack of cooling in upper troposphere. Sub tropical westerly jet always override surface temperature gradient, increased temperature in baluchistan and kpk relative to adjacent plains of Punjab and sindh forces the stj to flow unconventionally north to south. The phenomena is exactly opposite to that of the trough when first temperature drops over western parts of the country. The north to south moving jet pushes thunderstorm from north to south when ts rise to the hight of jet so if the ms trough is over upper sindh and ts form with low ctt the said jet would drag them southward.  Why its raining in baluchistan when jet is n to s. As said earlier the heating of baluchistan attract winds from e (in current atmospheric conditions) the winds strike with foothills of Balochistan, orographical lifting cause thunderstorm, the added heating further fuels them and if ts reaches very low ctt and remains sustained it starts moving in the direction of jet.

The major difference between these tcs and those who form due to trough is post rainfall temperatures. Now coming to coastal sindh. Here the jet becomes more westward oriented (north east to south west) as the coriolis effect increases with time, but here it has less to do with rain of coastal sindh because the direction becomes generally similar to those of conventional easterlies which often effect south sindh from north east. Here the reason is different, during bob lpas, specially when they don’t reach coastal sindh, the ms axis hardly ever cause rainfall in Karachi though some isolated rains can reach se sindh but karachi hardly ever gets rains from ms axis of bob lpas, at least i didn’t see it in recent past. One possible reason as said above is that axis don’t reach close to karachi, secondly the low level convergence which happens due to convergence of ne and sw winds diminishes in presence of bob lpas because bob lpas cause coastal sindh winds to move more easterly along with already present coriolis force, the forcing reduces the likely hood of converge close to karachi.

Fahim Malik Dhurnal