Very dense fog enveloped parts of over Uttar Pradesh, Bihar and Odisha reducing visibility to as low as 25 metres ahead of the advent of the two-week peak fog season of the Northern Hemisphere winter from around mid-December, as causative western disturbances move from West to East over the region.

Very dense fog descended at a few pockets over East Uttar Pradesh, Bihar and Odisha while it was isolated over West Rajasthan, West Uttar Pradesh and Madhya Maharashtra. Less intense (dense) fog hung over Punjab, Assam, Meghalaya, West Bengal, Sikkim and Tripura aligned with movement of the western disturbances.

Rains may return to Andamans, TN from Dec 15

Warm air over cooler surface

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 the wind/air. When the moist, warm air makes contact with the cooler surface air, water vapor condenses to create fog.

Prolonged foggy weather could trigger fungal diseases in the rabi crop, affect dispersion rate of beneficial insects and therefore the time required for pollination. Insecticides and liquid fertilisers cannot be spayed under thick fog conditions. Reduced the solar radiation could impact evapotranspiration and photosynthesis.

'Steaming engine' up front

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.

Next western disturbance

This is even as an incoming western disturbance was located over Afghanistan en route to North-West India on Friday afternoon. It has already been 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.

The IMD has forecast fairly widespread to widespread rainfall/snowfall over Jammu & Kashmir, Ladakh and Himachal on Saturday as well while it would be isolated over Uttarakhand. Isolated to scattered rainfall is also forecast over Haryana, Chandigarh, Delhi and Punjab during same period.

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