These stacked lenticular clouds were observed from the A5 Road just east of Tryfan, Wales, in the mountain pass between the Carneddau and Glyderau ranges. Photographer Graham Stephen; Graham’s website
Normally found in the lee of mountains, altocumulus lenticularis are a type of stationary wave cloud that results when air is forced to rise over elevated terrain. Water vapor within the wave layer condenses at the wave crest if the temperature is sufficiently cool (dew point temperature), forming a smooth, lens shaped cloud. The cloud evaporates due to adiabatic heating when it sinks into the drier wave trough. In this way, clouds appear at intervals closely related to the wavelength. Photo taken on June 26, 2011.