(This story originally appeared in on Jan 04, 2018)

NEW DELHI: At 9.30am on Wednesday, an unbroken layer of fog was spread across more than 2,000km of the Indian subcontinent — starting from central Pakistan in the west, running right through the Indo-Gangetic plains and ending around Tripura.Experts said, in terms of territory covered, it was one of the largest simultaneously formed, single fog episodes over any land area in the world.The daytime multi-channel fog detection scheme image captured by Insat 3D shows the fog covering at least four countries — Pakistan, India , Nepal and Bangladesh. The fog actually began on Christmas Day in east UP and Bihar.The only comparable fog episode of this scale will have been from this region itself in previous years. Wednesday’s spread was certainly larger than other fog episodes in places such as central China, Italy or the California valley,” said R K Jenamani, head of the Met office at Delhi’s IGI Airport “After covering east UP and Bihar on Christmas, it started covering more areas in its west, including Delhi, by December 31. By January 2, it had covered Punjab and was continuing to move west into Pakistan. It has severely affected aviation, rail and road transport, and caused a sharp drop in day temperatures across the region,” Jenamani said.Ironically, however, although the fog reached its maximum spread at 9.30am on Wednesday, visibility at many places in north India, including in Delhi-NCR, was in fact better at that time than in the previous two days. “The satellite picture shows the extent of the fog. What it doesn’t show is that at many places, the fog wasn’t actually on the surface but a few hundred metres in the air, like a very low cloud. Technically, it’s still a fog,” said Jenamani.According to the IMD website, at least 15 airports in the country reported moderate to severe fog between 6.30am and 9.30am on Wednesday. These include Agartala, Patna, Gaya, Guwahati, Gorakhpur, Allahabad, Agra, Lucknow, Varanasi, Rae Bareli, Chandigarh, Jammu, Ludhiana and Pantnagar. The worst hit was Amritsar, which had visibility below 100m for 18 hours, from 6.30pm on Tuesday till 12.30pm on Wednesday.Delhi, however, had visibility of around 300m. The Met office said visibility would further improve over the next two-three days. Fog is formed when a cold, moist air current meets relatively warm air, leading to formation of fine water droplets.