New Delhi, Feb 22 : Dense fog enveloped the national capital on Wednesday, with the Air Quality Index (AQI) continuing to be under the "poor" category for a second consecutive day. According to the India Meteorological Department (IMD), visibility levels dropped to 50 metres at the Palam Observatory near the Indira Gandhi International Airport. "A cyclonic circulation has developed over Punjab and Haryana under the influence of a western disturbance affecting the western Himalayan region. Warm and humid easterly winds and cool northwesterly winds are interacting over the region due to the induced cyclonic circulation. The increase in humidity and condensation are leading to fog formation. Such weather is not common in February," said Mahesh Palawat, a senior meteorologist at Skymet Weather. Meanwhile, the overall AQI in Delhi was recorded at 201, according to data from the System of Air Quality and Weather Forecasting And Research (SAFAR). Both the major pollutants PM 2.5 and PM 10 were recorded at 201 (poor) and 161 (moderate), respectvely. The air quality is likely to improve on Thursday. 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". /IANS