Dense fog conditions have descended on north-west India to coincide with the ‘critical period’ of mid-December to mid-January known for their most frequent occurrence.
The trigger for fog is when the temperature, humidity and wind values combine to reach a threshold level.
Dense to very dense fog has been observed at a few places over Punjab, Delhi, Haryana, Uttar Pradesh and north-west Rajasthan during the 24 hours ending in the morning.
This is only a ‘pilot’ of more similar events, including wet spells, expected during course of the week and beyond, according to global models.
Outlook for the next couple of days said that fog to shallow (low-lying) fog conditions would recur over Punjab, Haryana, Delhi, Uttar Pradesh and east India during morning hours.
FOG WARNING Intensity and duration of fog would only increase, an India Met Department update said on Monday evening.
A weather warning for Tuesday and Wednesday said that dense to very dense fog (visibility below 200 m) would occur at a few places over Punjab, Haryana, Delhi, Uttar Pradesh and Bihar.
Meanwhile, cold wave conditions prevailed over some parts of interior Odisha and Madhya Maharashtra. It will take a suitably endowed western disturbance to extend its influence to these areas for the cold wave conditions to lift.
COLD WAVE This is because cloud forming rising up front of the western disturbance will help trap terrestrial radiation from escaping into the atmosphere and keep the surface warm.
The absence of a western disturbance allows the cold carried in by northwesterly winds to sink in to ground level and trigger a cold wave. Prevailing minimum temperatures were below 5 deg Celsius at many places over western Himalayan regions until Monday morning.
They ranged between 5 deg Celsius to 10 deg Celsius over Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, Jharkhand, north eastern States and interior Maharashtra and at a few places over Odisha, Bihar, Chhattisgarh and Rajasthan.
The lowest minimum of 3.6 deg Celsius recorded at Muzaffarnagar in Uttar Pradesh.