A thick blanket of haze covered pockets of Delhi and its surrounding areas as mercury dipped to 6 degrees Celsius on Thursday morning. The India Meteorological Department (IMD) predicted dense fog over Delhi, Uttar Pradesh, Haryana and Punjab till December 31. The weather office issued an advisory, a day after a thick blanket of haze, which covered much of north India, delayed over 110 flights and 25 trains in Delhi.