New Delhi, Dec 21 (IANS): Dense Fog engulfed the Indo-Gangetic plains, including Delhi, for the third morning on the trot on Wednesday, lowering visibility in the national capital and affecting traffic movement.

The visibility dropped to 500 metres at the Indira Gandhi International Airport (IGI) at 5.30 a.m.

"Flights are being diverted or are returning to Delhi due to bad weather in Chandigarh, Varanasi, and Lucknow," stated the advisory issued for flyers by Delhi International Airport Limited (DIAL) on Wednesday.

Around 20 trains were delayed due to the thick fog and low visibility that blanketed the national capital this morning at the New Delhi Railway Station.

"INSAT 3D Rapid Satellite imagery shows continuation of dense fog layer from Punjab & Northwest Rajasthan to East UP across Haryana. Gray circled cloud patch, over central parts of West UP, is Medium & High clouds associated with middle tropospheric level westerly," tweeted the India Meteorological Department (IMD).

While sharing the lowest visibility report as per cities, the IMD said that visibility recorded 8.30 a.m. showed 25 metre visibility in Amritsar, Hissar, Ganganagar, Pantnagar, Bareilly, Bahraich, Gorakhpur while 50 metre visibility was reported at Ludhiana, Karnal, Churu, Varanasi, Patna, Purnea, Agartala and 200 meter visibility at Ambala, Bhiwani, Lucknow and Fursatganj.

The IMD has issued a 'cold wave' alert in many North Indian states.

A drop in temperature has been also recorded in states of North India, including Delhi, Uttar Pradesh, Punjab, and Bihar

Meanwhile, the Sadarjung Observatory, the primary weather station in the national capital, had recorded a minimum temperature of 6.3 degrees Celsius, two notches below normal. The maximum temperature is likely to settle around 22 degrees Celsius.