Ablanket of ”dense” fog covered parts of Delhi on Sunday morning, lowering visibility and affecting traffic movement, the India Meteorological Department (IMD) said. Rains on Saturday increased moisture content in the air that led to ”dense” fog in parts of the city, head of the IMD’s regional forecasting centre, Kuldeep Srivastava, said.

The Safdarjung Observatory, which provides representative data for Delhi, recorded fog that reduced visibility to 200 metres in the morning hours, the IMD said. ”Dense fog at the Palam weather station lowered visibility to just 100 metres,” Srivastava said and added that ”moderate to very dense fog is predicted for the next two days”.

”Very dense” fog is when visibility is between zero and 50 metres and ”dense” when between 51 and 200 metres. When visibility is between 201 and 500 metres it is ”moderate” fog and ”shallow” when visibility is between 501 and 1,000 metres, according to the IMD. Light rains in parts of Delhi under the influence of a Western Disturbance brought the mercury down by a few notches. On Sunday, the minimum temperature settled at 11.5 degrees Celsius. The mercury is likely to dip below 10 degrees Celsius on Monday with the wind direction changing to northwesterly.

These winds blow from snow-laden western Himalayas towards the plains. Delhi’s air quality was recorded in the ”very poor” category on Sunday and is likely to improve due to favourable wind speed, a government forecasting agency said.

The city’s air quality index (AQI) was 351 at 9 am. The 24-hour average AQI was 356 on Saturday and 295 on Friday. An AQI between zero and 50 is considered ”good”, 51 and 100 ”satisfactory”, 101 and 200 ”moderate”, 201 and 300 ”poor”, 301 and 400 ”very poor”, and 401 and 500 ”severe”.