From this ocean-bound city I recently glanced up at late afternoon sunlight on the underside of a continuous then sheet of altostratus cloud, to see a strange light. A solitary huge ripple, looped in cross section like the Greek letter omega or the ruck in a disturbed carpet, stretching almost from one horizon to the other, rolled slowly across the cloud sheet. The rest of the cloud, in front of and behind the feature, remained calm. I estimated its front-to-back width to be roughly 100 metres, and its speed to be between 5 and 10 kilometres per hour. What was this?George Farrant, Auckland, New Zealand