In the hundreds of folders of photographs that I've got filed away on my computer, there's a good chance that in each there'll be at least one photograph of the sky. The ever-changing colours and forms of the sky are always a great subject; there's a reason the 'cloud porn' hashtag on Instagram is so popular. A few days I stumbled across these bizarre cloud formations - 'undulatus asperatus', as named by the founder of the Cloud Appreciation Society, who is hoping for it to be classified as a separate cloud formation. The pictures are weird enough (the photograph above is by B. J. Bumgarner on Flickr, where there's a lot more photographs of the formation which I didn't use due to their copyright status!) but the film is just incredible. Alex Schueth captured the clouds in timelapse footage as they rolled over Lincoln, Nebraska, earlier this year. The clouds are obviously slower moving in real time, but are still completely surreal - like looking up at the waves from underneath the sea. It's not a common formation, especially in Britain, so I'm thankful to the proliferation of digital recording equipment which allows us to see incredible footage of rare events like this.