Dark Cloud north of Dorking

Almost everyone in the UK would already know that today (Sunday) there was a huge explosion of a fuel depot in Hemel Hempstead, Hertfordshire. It is wierd as although Dorking is miles away I noticed the dark cloud as soon as I woke up, it was like a characterless straight black band of cloud. It looked even more strange as it was a cloudless bright sunny day. As the day went on it got darker as the smoke spread throughout the South East of England.