I also recall pining for a grey day in Australia after weeks of blue skies. As well as their environmental importance, clouds have great aesthetic value, turning the sky into an art gallery.

If your knowledge of clouds only extends to 'dull and grey' or 'white and fluffy', then prepare to be amazed by this cloud atlas. Nature also produces clouds in all shapes, sizes and colours.

But what is the story behind the brocken spectre? The myth describes him as a Scottish Big Foot, said to loom large in shifting grey cloud on the UK's second highest mountain. The Big Grey Man of ...