This month the weather has been a mixed bag of rain and sun. One evening I made this shot after a few hours with rain and thunder. Just before sunset the weather cleared up and the sky turned out to get quite dramatic and for a few minutes I saw this cloud formation called Mammatus cloud, extending from the base of the mighty Cumulonimbus thundercloud.
Strange clouds such as these are probably behind one of the most perplexing phenomena of modern times - the UFO sighting. They look like an expensive special effect from a horror movie, 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. Named after the Latin for breast, Mammatus clouds are associated with the powerful storms which can occur in the summer, and are a sign of the massive quantities of water vapour that are held in the languid summer air. (Wikipedia)
(Nikon D700, Carl Zeiss ZF.2 100mm 2,0 T* Makro-Planar, iso 200, handheld)