In mountain areas, powerful forces are unleashed in the atmosphere on a windy day. For example, most keen hillwalkers and pilots are familiar with the gusts and lulls that occur, which are a symptom of the turbulence generated as the air flows over hills and ridges. Higher up, something more remarkable occurs in the right conditions: the formation of mountain waves. These are largely invisible, but sometimes clouds form in the rising air around the crest of each wave. Known as lenticular clouds, they are often elongated and smooth-faced like a lens, and there may be a series heading off Continue Reading