The Hindu Business Line : Fog envelops plains as skies clear up in northwest

Thiruvananthapuram, Dec. 21 A strong anti-cyclone moving in from West Asia to India’s northwest border would be the significant atmospheric feature with a major say on the unfolding regional weather.

This would happen as the prevailing western disturbance over Jammu and Kashmir and the weather-creating trough in the westerlies move eastwards.

The scattered rain or snowfall over the western Himalayan region may last for another day, an India Meteorological Department outlook (IMD) said on Sunday.

Minimum temperatures that are above normal by 4 to 6 deg Celsius over northwest, central and adjoining parts of east India are likely to fall by 3 to 5 deg Celsius over these areas during this period.

But IMD sees another western disturbance approaching the region mid-week this week.

This could lead to reversing of the lowering trend in the mercury, though briefly.

MERCURY TO FALL

The homing-in of the massive anti-cyclone would drive away any approaching trough from the northwest and push the prevailing anti-cyclone over central and east-central India further to the southeast and over the Bay of Bengal.

The dry, cold and sinking northeasterlies from the latter would keep at bay any easterly surge from the Bay of Bengal and ring in a phase of little or nil precipitation over the peninsula.

The night temperatures are likely to record a fall in the region.

The easterly movement of the disturbance clears the skies and brings light northwesterly winds into play over the region.

Moisture in the lower atmosphere left behind by the westerly trough is conducive for formation of fog over northwest India during morning hours over the next four days.

FOG ALREADY

An IMD update on Sunday said that foggy conditions have already been prevailing over parts of Punjab, Haryana, west Rajasthan, Uttar Pradesh, Bihar, Jharkhand and West Bengal.

The passage of a western disturbance is followed by fall in night temperatures at many places by three to four deg Celsius.

This creates ambient conditions for the welling up of fog.

Fogs in northwest India is largely caused by advection (wind), a process in which moist air passes over cool ground and gets in turn cooled.

Local winds blowing in from the hills (katabatic winds) slow down over the valley to produce convection, with the inherent moisture getting converted into fog.

Katabatic winds are those that flow from high elevations of mountains, plateaus, and hills down their slopes to the valleys or plains below.

MORE FREQUENT

Frequency of fog incidence in the north Indian belt has jumped by as much as 50 per cent since 1996, according to Dr Akhilesh Gupta, Senior Adviser to the Department of Science and Technology.

This could be attributed to stronger advection pattern and increasing vegetation cover in Rajasthan.

The green cover in Rajasthan has aided the propagation of cold wave conditions farther to the east.

Isolated rain or thundershowers are likely over Gujarat and east Rajasthan during the next 24 hours and mainly dry thereafter.

Mainly dry weather is likely over central India.

Minimum temperature will begin to head southwards from Monday onwards.

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