A layer of dense fog enveloped the Indo-Gangetic plains including Delhi Thursday morning, affecting road and rail traffic.

A railway spokesperson said 20 trains were running late by 1:30 to 4:30 hours. Flight operations remained unaffected, an airport official said.

According to the India Meteorological Department (IMD) 'very dense' fog is when visibility is between 0 and 50 metres, 51 and 200 is 'dense', 201 and 500 'moderate', and 501 and 1,000 'shallow'.

The Met office said the Palam and Safdarjung airports logged visibility levels of 200 metres at 5:30 am. On Tuesday, visibility levels had plunged to 50 metres at both these places.

South westerly winds at the middle tropospheric level and a consequent increase in temperatures had led to some improvement in visibility on Wednesday.

Amid low temperatures, high moisture and still winds, a layer of dense to very dense fog persisted over Punjab, Haryana, northwest Rajasthan, west and east Uttar Pradesh and Bihar.

"Satellite imagery shows continuation of dense fog layer from Punjab and northwest Rajasthan to Bihar," an IMD official said.

At 5:30 am, visibility levels stood at zero in Bhatinda; 25m in Ganganagar, Chandigarh, Gorakhpur and Bareilly, and 50m in Amritsar, Churu Bahraich and Ambala.Visibility levels are likely to improve from Wednesday night/Thursday morning due to decrease in relative humidity at the lower tropospheric level over the Indo-Gangetic plains, the IMD said.

The Sadarjung Observatory, the primary weather station in Delhi, recorded a minimum temperature of 7.2 degrees Celsius, a notch below normal.

The maximum temperature is likely to settle around 21 degrees Celsius.

On Wednesday, the city logged a maximum temperature of 21.2 degrees Celsius, the lowest so far this season. The minimum and maximum temperatures are likely to drop to 5 degrees Celsius and 20 degrees Celsius in the next few days.

Fog formation over the Indo-Gangetic Plains and north-Indian region is common in December-January every year with around 60 percent of days being foggy.

Recent studies on fog in India have raised significant socioeconomic concerns due to increase in frequency, persistence and intensity of fog occurrence over the northern parts of the country.

A study published in the Nature journal earlier this year attributed the intensification of fog over the Indo-Gangetic plains to intense moisture incursion from the Arabian Sea onto the west coast of India.

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