Moderate fog cloaked Delhi on Wednesday morning, lowering visibility to 400 metres and affecting road and rail traffic.

A railway spokesperson said around 18 trains were running late.

Operations at the Delhi airport remained normal, an official said.

However, he said, three flights were returned or diverted to the Delhi airport due to bad weather in Chandigarh, Varanasi and Lucknow on Tuesday night.

The India Meteorological Department (IMD) said the Palam airport logged the lowest visibility level of 400 metres at 2:30 am, while it dropped to 500 metres at the Safdarjung airport at 5:30 am.

On Tuesday, visibility levels had plunged to 50 metres at both these places.

An IMD official attributed the improvement in visibility to south westerly winds at the middle tropospheric level and a consequent increase in temperatures in the past 24 hours.

However, amid low temperatures, high moisture and still winds, a layer of dense to very dense fog persisted over Punjab, Haryana, northwest Rajasthan, Uttar Pradesh, Bihar and parts of Uttarakhand.

At 5:30 am, visibility levels stood at zero in Bhatinda; 25m in Ganganagar, Amritsar and Bareilly, and 50m in Varanasi, Bahraich and Ambala.

According to the 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 Sadarjung Observatory, the primary weather station in Delhi, recorded a minimum temperature of 7.1 degrees Celsius, a notch below normal. The maximum temperature is likely to settle around 22 degrees Celsius.

The minimum and maximum temperatures are likely to drop to five degrees Celsius and 20 degrees Celsius in the next few days.