Dark, brooding and menacing. This apparition, which materialised over the skies of Hertfordshire, looks like an expensive special effect from a horror movie.
The strange globular clouds, which resemble drops of oil poured into a murky pond, appear to herald the appearance of an alien spacecraft, here to cause havoc.
In fact, this picture shows a rare but perfectly natural (and perfectly harmless, provided you do not try to fly through it in light aircraft) meteorological phenomenon known as a mammatus cloud.
This picture was taken by Ian Pattison, 31, an assistant librarian, outside his home in St Albans last week. He said: 'I got a new phone last week and when I saw this amazing sky, I just grabbed it and started snapping.'