Delhi's air quality turned "severe" on Saturday with slow wind speed allowing the accumulation of "locally generated" pollutants, according to India Meteorological Department (IMD) officials. However, it is expected to improve by Monday as the wind speed is likely to pick up.At 10 am, the city's air quality index (AQI) was 401, which falls in the "severe" category. The 24-hour average AQI was 382 on Friday, 341 on Thursday, 373 on Wednesday, 367 on Tuesday, 318 on Monday and 268 on Sunday.Air pollution in the neighbouring cities of Ghaziabad (AQI 446), Greater Noida (401) and Noida (406) also entered the "severe" zone. 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".The wind speed remained slow allowing the accumulation of pollutants generated locally, V K Soni, head of the IMD's environment research centre said. "Easterly winds carrying moisture also led to the formation of secondary particulate matter. All these factors together pushed the air quality in the 'severe' zone," he said.However, Soni said, the air quality is likely to improve to the "poor" category by Monday as the wind speed is expected to pick up. According to the IMD, the maximum wind speed is likely to be 8 kmph on Saturday, 10 kmph on Sunday and 15 kmph on Monday.The minimum temperature in Delhi was 11 degrees Celsius on Saturday and the maximum is expected to settle around 28 degrees Celsius, the weather department said. However, the minimum temperature is likely to dip again with the wind direction changing to northwesterly by Monday, according to IMD officials.Westerly and northwesterly winds blow from snow-laden western Himalayas towards the plains. Shallow to moderate fog is also predicted over the next two days, the IMD said.The minimum temperature this season has remained 2-3 notches below normal in the absence of a cloud cover on most days, according to IMD officials. Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.The central government's Air Quality Early Warning System for Delhi said the city's ventilation index -- a product of mixing depth and average wind speed -- is likely to be 2,500 m2/s on Friday. Mixing depth is the vertical height at which pollutants are suspended in the air. It reduces on cold days with calm wind speed.A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for the dispersal of pollutants.