A spectacular display of Cumulonimbus with Mammatus clouds at sunset. Seen from a hill in Prezganje, in the hills east of Ljubljana, Slovenia. Mammatus clouds are the most distinctive of all cloud formations consisting of pendulous globules that hang from the underside of the anvil of a thundercloud. They are an indication of severe weather conditions. The anvil of a mature cumulonimbus cloud is known to spread out over hundreds of miles, or kilometres, and thus the centre of the storm is often far away. However, the mammatus formation usually occurs soon after the cloud has reached maximum growth and intensity, signalling a particularly intense storm. This was likely the edge of a big hail storm in Bovec, western Slovenia.