This is a rare sight and I must admit that I had never heard of these type of clouds. These were seen in West Yorkshire yesterday.
Weird and very beautiful. It seems that these are caused by winds which pass over the Pennines causing air turbulants that produce 'standing waves'. Standing waves are all to do with resonant frequencies. They rarely happen in this country, but they can be seen in America near to mountains. Sometimes people think they are flying saucers!
