The Hindu Business Line : Fog conditions not unusual for this time of the year: Expert

Mid-December to mid-January is a ‘critical period' with ample mischief potential, says Dr Akhilesh Gupta, leading operational forecaster and adviser to the Department of Science and Technology.

The trigger is when the temperature, humidity and wind values combine to reach a threshold level, Dr Gupta told Business Line.

This threshold has been drawing nearer from December 15 with a steady fall in mercury. Moisture feed from repeat western disturbances as also that straying from the Arabian Sea had helped surcharge the atmosphere.

The dense fog blanketing the stretch extending from northwest India right up to West Bengal is categorised as ‘advection fog,' which, Dr Gupta said, is difficult to simulate.

Advection fog is a type of fog caused by the horizontal movement of moist air over a cold surface and the consequent cooling of that air to below its dew point.

This essentially means that the lower layers of the air get cooled down to below the temperature at which fog forms. This type of fog can throw up irrespective of the direction of the flows – warm southwesterly or cold northwesterly.

Advection fog travels with the prevailing dominant flows, which is east to southeast in this case. This is quite unlike ‘radiation fog' which is triggered entirely due to local conditions and is usually predictable.

Katabatic winds

Radiation fog is formed on clear, still nights when the ground loses heat by radiation, and cools. The ground in turn cools the nearby air to saturation point, thus forming fog.

An interesting component in the fog scenario being witnessed over the last decade in the northwest is the ‘katabatic wind' flows from the Himalayan slopes.

‘Katabatic wind' is a generic term for the wind that occurs when cold and dense air, chilled by radiation cooling, usually at night, moves down-slope gravitationally beneath warmer, less dense air.

The fog thus generated will hang heavy over ground, but with the sun managing to break out, it will get transformed as low-level clouds.

Meanwhile, India Meteorological Department (IMD) has warned on Sunday that cold day conditions would continue in Punjab, Haryana, Uttar Pradesh and Bihar during the next two days as well.

Fog will occur over parts of Punjab, Haryana, Chandigarh, Delhi and West Uttar Pradesh and dense fog will occur over parts of east Uttar Pradesh and Bihar.

Light rain, snow

Light to moderate rain/snow has been forecast at a few places over the western Himalayan region during next 24 hours. Light rain or thundershowers are likely at a few places over parts of plains of northwest India as well.

Minimum temperatures were below normal by 4 to 6 deg Celsius over parts of Jharkhand, Chhattisgarh, Gangetic West Bengal and Orissa and by 2 to 4 degree Celsius over parts of east Uttar Pradesh, Bihar and Vidarbha.

During the last 24 hours ending Sunday morning, the lowest minimum temperature of -0.8°C was recorded at Amritsar. Cold wave conditions prevailed over parts of Jharkhand, Chhattisgarh and Orissa.

Cold day condition has been prevailing over most parts of Punjab, Haryana, Uttar Pradesh and Bihar. Isolated to scattered light rainfall has occurred over Himachal Pradesh, Haryana, Rajasthan and Gujarat, the IMD update added.

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