Iridescent Wave Clouds, Denver, Colorado ~ Image by Silvia Maria February 10, '08. ©Silvia Maria, shown with permission.

These look for all the world like stratospheric nacreous clouds but that cannot be. They are rarely seen so bright when the sun is up and the stratosphere over Denver would have to be very unusually cold indeed. A temperature of -85 Celsius is needed that is sometimes attained in Polar and near polar regions.

These Denver wave clouds are more likely lower and warmer. Airflow over the Rockies induces vertical wave motion in their lee.

An air pocket in a rising section of wave expands and cools. Droplets condense.

As the air pocket subsequently descends it heats again and the droplets formed only seconds earlier evaporate.

Clouds of droplets with near identical histories have similar sizes in particular areas of the cloud. They scatter and diffract sunlight to produce these filmy colours.


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