Chandigarh, Dec. 18 -- Three departure flights were cancelled and two arrivals were diverted as Chandigarh experienced the first spell of dense fog on Wednesday when visibility dropped to as low as 150 metres. As per India Meteorological Department (IMD), fog season has officially started in the city and is expected to get worse in the coming days. An orange alert has been issued for Thursday and Friday. As per the IMD observatory in Sector 39, the visibility at 5.30 am was 800 metres which plunged to 150 metres by 8.30 am before improving to 3,500 metres by 11.30 am. Fog is considered to be dense when visibility drops between 50 metres to 200 metres. Below 50 metres, it is classified as very dense while visibility between 200 metres and 500 metres is considered to be moderate fog. IMD Chandigarh director Surender Paul said, "Dense fog is usually expected in the second half of December. Earlier, we had dry North Westerly winds blowing in the region, however, due to the recent Western Disturbance (WD), moisture laden easterly winds are now blowing in the region. The moisture, coupled with low temperature during the day, has led to the fog and we expect it to continue for the next few days." Due to the fog, the air quality index (AQI) in the city has also declined. According to the Central Pollution Control Board (CPCB), the city's AQI on Wednesday was 275 which falls in the poor bracket. This is only slightly lower than Delhi where AQI was 334. This is also a jump from Tuesday, when AQI was 126, increasing over two times in just a day due to the fog. Panchkula, with an AQI of 330, was in the very poor category, and close to Delhi. The CPCB bulletin is prepared by taking average AQI values from all stations in the city. The AQI values at 4 pm are taken, which are also prepared by taking a 24-hour average of pollutant levels in that area. AQI between 201-300 is considered poor and can cause breathing discomfort to most people on prolonged exposure. AQI between 301-400 is considered very poor and can cause respiratory illness on prolonged exposure. As per Paul, fog doesn't directly lead to a higher AQI, but it leads to drop in day temperatures which leads to a phenomena called temperature inversion, increasing the concentration of pollutants in the area. Further, there are chances of smog formation in the city if the concentration of pollutants increases further....