Lenticular clouds are stationary lens-shaped clouds that form in the troposphere, normally in perpendicular alignment to the wind direction. There are three types of lenticular clouds:
altocumulus standing lenticular (ACSL),
stratocumulus standing lenticular (SCSL), and
cirrocumulus standing lenticular (CCSL)
As air flows along the surface of the Earth, it encounters obstructions. These can be human-made objects, such as buildings and bridges, or natural features, like hills, valleys, and mountains. All of them disrupt the flow of air into eddies. The strength of the eddies depends on the size of the object and the speed of the wind. It results in turbulence classified as ‘mechanical’ because it is formed through the “mechanical disruption of the ambient wind flow”. Where stable moist air flows over a mountain or a range of mountains, a series of large-scale standing waves may form on the downwind side. If the temperature at the crest of the wave drops to the dew point, moisture in the air may condense to form lenticular clouds. As the moist air moves back down into the trough of the wave, the cloud may evaporate back into vapour. Under certain conditions, long strings of lenticular clouds can form near the crest of each successive wave, creating a formation known as a "wave cloud." Bright colours (called iridescence) are sometimes seen along the edge of lenticular clouds.
lenticular cloud seen in Palm Desert (which is adjacent to Palm Springs, CA) in April 2008. Source Wikimedia Commons (Jackie, 2008)