India's capital New Delhi and surrounding areas in northern India woke up to a thick fog on Wednesday, affecting trains and flights.

Air traffic monitoring portal Flightradar 24 showed more than 100 flights had been delayed at New Delhi's main airport. Local media said more than 40 trains in northern India had also been affected.  

The Indian Meteorological Department issued an orange alert, its second-highest warning level, and predicted dense to very dense fog in many areas. 

Fog common in winter

Visibility at Delhi's airport was between zero to 100 meters (328 feet), the agency said.

Indigo Airlines, the country's largest air carrier said "Low visibility and fog over Delhi may lead to some delays."

The low visibility also slowed down traffic, with local media showing footage of cars moving at a snail's pace on the city's roads. 

Temperatures as low as 7 degrees Celsius (44.6 degrees Fahrenheit) coupled with poor air quality also made the fog conditions worse. 

Fog is common in the low-lying Indo-Gangetic plains during winter.

On Wednesday, Delhi's air pollution showed a reading of 437 or "hazardous" according to live rankings by Swiss group IQAir. The city is the world's most polluted, and air quality deteriorates more during winters.

The Indian capital was clouded in a dense smog in November that brought much of the city to a halt.

tg/ab (EFE, Reuters)