Rest your weary head, child, on the pillows of angels, Nebraska,USA, photo by Brettnickeson.
Mammatus Clouds are pouch-like cloud structures and a rare example of clouds in sinking air. Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form - a commonly held misconception. In fact, mammatus are usually seen after the worst of a thunderstorm has passed.
Australian photographer Ken Prior snapped the nearly visible brushstrokes in the above sky over Scotland. The photo is the result of a mysterious, as yet unclassified cloud type that's been showing up all over the United Kingdom and New Zealand, and wherever else Prior happens to have his camera pointed at the sky. Meteorologists are calling them asperatus clouds, while people who aren't insecure about being confused with the weatherman are using the more straightforward Jacques Cousteau clouds.