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Vinson Kurian

Thiruvananthapuram, Jan. 6 Cold wave and fog in the plains of the northwest can only get worse as deep westerlies continue to empty their moisture over the cooler surface marked by little or even nil wind activity.

The warm and rising motion of air in front of the westerly system aids convection, but the sinking air from the seasonal anti-cyclone traps the moisture to within the colder lower atmosphere.

The temperature inversion (wherein air gets warmer with height unlike what conventional wisdom suggests) triggers the formation of fog. Anti-cyclone (high pressure) brings ‘stable’ weather conditions against the ‘instability’ heralded by cyclonic (low pressure) circulations.

It is the incoming low pressure systems (in the form of westerly troughs and disturbances) that set up the instability in a seasonally ‘stable’ weather regime brought about by the anti-cyclone.

Lack of ventilation effect – either vertical (capped by sinking air) or horizontal (lack of high winds) – is what traps the fog in the lower atmosphere, said Dr K.J. Ramesh, Senior Adviser to the Department of Science and Technology.

Outlook by the European Centre for Medium-Range Weather Forecasts has indicated the arrival of a deep westerly system in another week’s time. According to Dr Ramesh, such frequent arrival of deep systems may indicate change in facilitating conditions over the northwest border.

This would also include change in ‘weather guard’ systems – semi-permanent troughs or ridges, as the case may be. While ridges generate cold and dry conditions over the northwest, troughs set up warm and humid or wet climes.

Meanwhile, the IMD has mounted a watch for rains breaking over central India and southern peninsula during the weekend and into the next. A prevailing cyclonic circulation would bring isolated rain or thundershowers over Madhya Pradesh and Chhattisgarh.

A facilitating trough over southwest and adjoining southeast Bay of Bengal has shifted entirely to the southwest Bay, off Sri Lanka and south Tamil Nadu. This would drive rains into eastern Sri Lanka and coastal Tamil Nadu. A cyclonic circulation has been persisting over Lakshadweep over the past few days.

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