This dramatic photograph was taken by Pete Williams in the spring at sunset near the Sierra Nevada mountains and we have asked Tom Bradbury, one of the world’s most respected Met men, to tell us just what produced this effect.

Tom says “This seems a splendid example of a dark grey rotor cloud in shadow topped by a fine bar of wave cloud still in sunshine. The grey rotor cloud has a typical rough-topped cumuliform summit. The air is often turned upseide down by the rotation resuting in warmer air tucking in below cold air and so becoming particularly unstable. This is why cumuli are mixed into the cloud bar. However, the churning motion doesn’t allow time for dome-shaped tops to develop and the cloud looks ragged. "The sunlit bar of cloud above is high enough to be clear of any rotor affects and the flow through it is smooth, giving the typical lentiucular shape. There is a faint suggestion of along-wind striations under the base of this lenticulaer, possibly due to the vertical wind shear which can be concentrated near the top of a wave system. “The zig-zag pattern in the far distance may be a reflection of bends in the mountain range and the rougher (but still sunlit) patches below the lenticular might be associated with more distant and less obvious rotor flow. There is something interesting going on in the far distance but I cannot make it out. One longs for a powered aircraft to explore the pattern - and also for a time lapse camera on the ground to reveal just how the wave is changing. “Sunset can be a good time to watch waves because the evening cooling of the ground halts low level thermal turbulence and makes it easier for the airflow to follow the lee slopes down to the valley instead of breaking away”. (If anyone else has an interesting weather picture we would like to feature it with Tom's Met explanation.)