Dense fog enveloped parts of Delhi on Wednesday morning, reducing visibility to 100 metres in the early hours.The mercury settled at a low of 8.4 degrees Celsius, 1.5 notches below the seasonal average.The weather office has forecast fog during the day. The maximum temperature is expected to settle around 22 degrees Celsius.Delhi's Air Quality Index stood at 333 in the 'very poor' category, according to the Central Pollution Control Board.