Very dense fog from overnight to early this (Friday) morning over Uttar Pradesh, Bihar and Odisha reduced visibility to 25 metres at a number of places even ahead of the advent of the two-week peak fog season of the Northern Hemisphere winter from around mid-December.
India Meteorological Department (IMD) said less intense (dense) fog was reported from areas over the rest of the plains in upstream Punjab and downstream West Bengal, Assam, Meghalaya and Tripura relative to the movement of the causative western disturbance.
Rains may return to Andamans, TN from Dec 15
UP towns worst affected
The IMD said that visibility ranged 200 metres or less with the worst-affected being Bareilly, Baharaich, Lucknow, Sultanpur, Chandbali and Gaya-25 metres; Varanasi, Babatpur, Patna, Kolkata (Dum Dum) and Digha-50 metres; Purnea, Diamond Harbour, Bankura, Amritsar, Dibrugarh, Malda, Kailashahar and Dibrugarh-200 each.
Fog forms when warm, moist air passes over the ground cooled by the winter. This process is called advection, a scientific name describing the movement of fluid. In the atmosphere, the fluid is wind. When the moist, warm air makes contact with the cooler surface air, water vapour condenses to create fog.
Cyclonic circulation
This is even as an incoming western disturbance was located to West Iran and adjoining Afghanistan on Friday morning. It is prompting a low-pressure area over the South-East and adjoining South-West Arabian Sea to spray some of its warm, moist air into Central and adjoining North-West India.
Fairly widespread to widespread rainfall/snowfall would continue over Jammu & Kashmir, Ladakh and Himachal Pradesh while it would be isolated over Uttarakhand today and tomorrow (Friday and Saturday). Isolated to scattered rainfall is forecast over Haryana, Chandigarh and Punjab. A cyclonic circulation over Haryana would assist the spread of the moisture around the region.
Moisture transport along the way
This moisture would cause dense to very dense fog to hang over Uttar Pradesh until Monday. Dense fog is also forecast over Bihar, Odisha, West Bengal, Sikkim and Tripura today (Friday); and over Punjab, Haryana, Chandigarh, Delhi and Uttarakhand on Sunday and Monday.
A western disturbance carries its ‘steaming engine’ upfront as it chugs all the way from the Black Sea-Mediterranean into North-West India across Iran-Afghanistan-Pakistan. In fact, it mops up moisture all along the way — the Mediterranean, the Black Sea, the Caspian Sea and even the Arabian Sea, as is currently the case.
After traversing long distances, it runs into the heights of the cold and mighty Himalayas, the impact of which causes it to suddenly cool down and break up into rain or snow over the hilly terrains of North-West India and a part of the plains of North India, helping the rabi crop or replenishing the Himalayan glaciers.