New Delhi, Jan 14 : Delhi-NCR on Sunday woke up to a thick blanket of fog that led to over 100 flights and 22 trains getting delayed due to poor visibility. The national capital recorded a minimum temperature of 3.5 degrees Celsius, four notches below the seasonal average, the India Meteorological Department (IMD) said. Delhi on Sunday morning was blanketed by dense fog with lowest visibility at zero at Safdarjung at 8 a.m. while the Palam also recorded zero visibility at 5 a.m. The IMD classifies fog intensity into four types: shallow, moderate, dense and very dense fog. The visibility ranges from 999 m to 500 m, 499 m to 200 m, 199 m to 50 m and less than 50 m, respectively. According to Indian Railways, a total of 22 trains were running late by hours due to fog and cold wave. The traffic on the roads was extremely slow as fog reduced the visibility to a few feet. Meanwhile, several areas in Delhi were experiencing 'severe' air quality, prompting the weather office to issue a red alert due to the prevailing cold and fog conditions. The IMD forecast indicated dense fog to persist in Delhi until Tuesday. On January 12, the city encountered the first cold wave day of the season, with temperatures dropping to 3.9 degrees Celsius. Saturday marked the coldest night so far, recording a season-low temperature of 3 degrees in Aya Nagar, the last village on the Mehrauli-Gurgaon Road in Delhi. /IANS