The Hindu Business Line : Dense fog likely over weekend

The freak weather is best attributed to the north-to-northeasterly winds that blow from the hills coinciding with the passage of a western disturbance, one of which was witnessed on Sunday night, said Dr Akhilesh Gupta of the Department of Science and Technology.

Cold katabatic winds are frequently found in the early hours of the night when the solar heating has ceased and the ground cools by emitting infrared radiation.

Cold air from extra-tropical systems (which a western disturbance is) may contribute to this effect.

The warm front of the disturbance carries moisture, which gets dumped downhill by these winds. It gets collected as early morning fog in the plains, but gradually lifts as the day wears on.

But unlike in the case of a normal fog (radiative), this "advection" does not clear off easily, hanging instead as a low-level cloud.

This goes to even hide out the sun.

It rules out any scope for the air to get warmed and the cooler climes get extended in this manner.

The dense fog is now expected to repeat by the weekend (January 6/7) with the arrival of an intense westerly system.

According to Dr Gupta, it will also bring light rainfall to the plains in Punjab, Haryana, north Rajasthan and later to west Uttar Pradesh.