The India Meteorological Department on Tuesday said that dense to very dense fog was observed at isolated areas in Uttar Pradesh, Delhi, Haryana and Chandigarh and is likely to continue the next day. Same conditions also prevailed in pockets of Bihar, West Bengal, Sikkim, Assam and Meghalaya.

In its weather forecast, the IMD said that “very dense fog” is expected over Uttar Pradesh and “dense fog” over Punjab, Haryana, Delhi and Chandigarh on Wednesday. Dense fog was predicted in isolated pockets over Mizoram, Tripura, Assam and Meghalya in the next five days and West Bengal in the next 2-3 days, the agency said.

“The season of fog has started,” said Kuldeep Srivastava, head of IMD’s regional weather forecasting centre, according to the Hindustan Times. “Easterly winds that were blowing over Delhi till Sunday noon brought moisture. High moisture content combined with low wind speed resulted in heavy fog. On Tuesday, we are expecting dense fog at Safdarjung while very dense fog at Palam.”

There was moderate fog at New Delhi’s Palam with a visibility of 300 metres on Monday morning. It was 200 metres on Tuesday, the weather agency said.

The fog also affected the air quality in the national Capital. The air quality index, or AQI, at 1 pm was 394 in Delhi, according to Central Pollution Control Board data. In the National Capital Range, the AQI in Gurugram’s Sector 51 was 322, while it was 396 in Noida’s Sector 62 at 2 pm.

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”.

Very light rain is likely in Delhi on Saturday because of a western disturbance, which will affect the whole of northwest India. “Till at least December 12, there will be no significant change in the morning and night temperatures,” Srivastava said. “Only after the passing of the WD [Western Disturbance], there could be a drop in temperature.”

IMD forecast also showed that isolated or scattered rainfall was likely in Haryana, Chandigarh and Punjab on Friday and Saturday.