Altocumulus Lenticularis over Norway imaged by Marcin Grzybowski (photography site) of Poland.  ©Marcin Grzybowski, shown with permission.


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Lenticular clouds form where there are atmospheric waves, usually downwind of mountains.

They are very dynamic objects. Whilst they appear stationary and peaceful, air is rushing across them and the droplets that comprise them live for no more than a few seconds.

Air in atmospheric waves rises and falls. As an air pocket rises it expands and cools. If its temperature and humidity are right some of its water vapour condenses to form a mist or cloud. This happens as the air pocket is rising. As the pocket falls again it contracts and heats and its droplets evaporate. The layers of a lenticular cloud represent the upper parts of particular atmospheric waves where condensation and evaporation are occurring.

Atmospheric optics? Wherever small droplets form quickly and are ‘young’ they have similar sizes. When these clouds are thin (not as here!) their droplets diffract sunlight to give vivid iridescent colours.