Chandigarh, Dec. 20 -- As dense fog persisted in Chandigarh for the third consecutive day on Friday, the city's day temperature dropped below the 20degC mark for the first time this season, intensifying the winter chill. According to the India Meteorological Department (IMD), visibility dropped sharply through the morning, falling from 250 metres at 5.30 am to just 120 metres by 8.30 am at the Sector 39 observatory - the lowest recorded this season. However, like the previous two days, visibility improved by late morning to around 2,500 metres by 11.30 am, before dipping again to 1,800 metres in the afternoon. As per IMD, a "dense fog" day is declared when visibility drops between 50 and 200 metres. Visibility below 50 metres is classified as "very dense fog", while readings between 200 and 500 metres fall under "moderate fog". The prolonged fog cover, which blocks the sun, is leading to a noticeable drop in temperatures. The maximum temperature fell from 21.9degC on Thursday to 19degC on Friday, settling 1.6 degrees below normal. The minimum temperature also declined, dropping from 9.9degC to 7.9degC, though it was 0.2 degree above normal. This made Chandigarh colder than several hill stations, including Solan, where the day temperature was 21.2degC and Dharamshala, which recorded a high of 20degC. Even at night, the city was colder than Shimla, where the minimum temperature stood at 12.2degC, along with Dharamshala (8.8degC) and Kasauli (11.7degC). Explaining the phenomenon, IMD Chandigarh director Surender Paul said fog was typically observed over the plains and not the hills, causing daytime temperatures in cities like Chandigarh to dip lower than in higher altitudes. He added that a Western Disturbance that was affecting the region recently had a greater impact on Himachal Pradesh, raising night temperatures there. Meanwhile, the city's air quality also continued to remain poor for the third straight day. According to the daily bulletin issued by the Central Pollution Control Board (CPCB), Chandigarh recorded an AQI of 254 on Friday, placing it in the "poor" category for the third day in a row. Panchkula, however, saw an improvement, with its AQI dropping to 145, which is considered "moderate", after remaining in the "very poor" bracket for two days. Delhi's AQI stood at 374, categorised as "very poor". The CPCB bulletin is based on a 24-hour average of pollutant levels recorded across all monitoring stations in a city, picked at 4 pm daily. An AQI between 201-300 can cause breathing discomfort to most people on prolonged exposure, while that between 301-400 can cause respiratory illness on prolonged exposure....