Plains of North India have remained fog-free for the last 3 days. Capital city Delhi, otherwise most susceptible to fog has witnessed bright mornings and a fair amount of sunshine, throughout the day. Airport Palam visibility has not dropped below 1500 meters, a very rare occasion. Rather, it improved to beyond 2000 meters early hours of the day. Amritsar, Jalandhar and Pathankot, the fog hubs in Punjab witnessed flashy mornings, instead of murky and blurred days, with Jammu, Jaipur, and Ambala having similar unusual conditions to start the day. With shining hours, most places breached mercury levels of 20°C.
Northern plains and the national capital received winter showers on two successive days, the 04th and 05th of February. Earlier, decent rains were observed at the start of the month as well. During the winter months, after the cessation of rains, thick and dense fog is observed in the morning hours and poor visibility hours extend till noon or even later. Disruptive weather conditions impacting air, rail and road traffic were earlier observed for many days. But the envelope of fog went missing precariously over a large area for longer durations.
Precise prediction of fog is a challenge for meteorologists. It always has a mixed bag of success and failure. The formation of fog needs stable atmospheric conditions. Three ingredients are very essential for the fog formation. Light winds, high humidity and moderately cool temperatures induce fog formation, more frequently seen during late night and early morning hours. Any of the three requisites, if not behaving, may lead to disruption of obscurity, including fog.
Over the plains of north India, the air near the earth surface needs to become nearly saturated for fog formation. It means the relative humidity has to be in excess of 90% and preferably even higher. Fog formation also needs some mixing action and therefore light winds are very essential for this phenomenon. Absolute lull or stronger winds do not stand favourable for dense fog formation. The air parcel near the surface of the earth needs to be adequately cool so the mercury reading between 5-10°C triggers condensation and in turn fog formation. Clear skies at night times can help escape terrestrial radiation and achieve necessary cooling near the surface and lower layers.
Plains of North India covering parts of Punjab, Haryana, Rajasthan and Delhi have been experiencing strong surface winds during the wee hours to start with and carry on through the day. These strong winds are a part of ‘Nocturnal Jet’. A nocturnal jet is a fast-moving air current in the lower atmosphere during night time when the skies are clear. Nocturnal jets are stronger in the early morning hours and are more speedy in the lower layers of the atmosphere, rather than at the surface. Such high-speed winds are the byproduct of a strong pressure gradient which builds up over the northern plains after the passage of western disturbance, lashing the mountains with heavy snow. High-speed winds cause churning and turbulent mixing of air, in the friction levels of the atmosphere. Prevalence of Nocturnal Jet precludes the fog formation.
Strong winds on account of this jet are likely to prevail today and tomorrow. A twist in the weather conditions will change the airflow, commencing sometime on 10th Feb or slightly later. A strong cyclonic circulation is likely to come up over the central parts. The circulation starting from parts of Maharashtra, Madhya Pradesh will travel to parts of South Rajasthan and Gujrat. Later, this will shift more towards parts of Madhya Pradesh and Uttar Pradesh. This feature will trigger some weather activity also over the central parts. Weather activity in real terms will not reach Delhi and the northern plains, but the reversal of airflow is likely over these parts. Consequentially, the visibility conditions will become poorer anytime from 10th Feb onward and persist for the next 4-5 days.
