Visibility problems return to South Asia as cold air sinks across the region from the Himalayas.

Parts of northern India awoke to a thick blanket of fog on Sunday. Visibility was severely reduced across cities throughout the Plains, throwing normal life out of gear and causing widespread travel disruption.

The dense fog shrouded New Delhi, where the temperature dropped to 8 degrees Celsius. Traffic was reduced to a crawl, with many drivers resorting to the unsafe practice of driving with their hazard lights on.

Rail services were also affected by the fog, with passengers in Kanpur city waiting for hours at the train station.

Some people lit bonfires to keep themselves warm, but the smoke combined with the fog forming smog, making the visibility even worse.

Temperatures in northern parts of the Indian subcontinent, skirting the Himalayan foothills, dip drastically during the months of December and January.

Cold air drains down from the higher ground, and areas of high pressure, which act like a lid on the atmosphere, tend to trap any fog and smog over the Northern Plains. Light winds mean that the poor-quality air can remain in place for weeks at a time.

South Asia’s winters are not as cold as other regions, such as North America, but the millions of poorer people here are hit harder because they live in the open and do not have enough warm clothes, often leading to deaths.