Luminance and Ice Crystals

Sometimes looking at the sky brings surprises. This afternoon we had a very nice example of luminance a relatively rare coloration of the clouds. High clouds were overspreading the sky from the west. The first surge of cloudiness featured cirrocumulus which were painted pink and purple by luminance.  The soft glow of color was caused by sunlight being refracted by ice crystals. The ripples in the cirrocumulus look like waves on a lake or the ocean. In fact the waves are caused by the undulation of the air. The waves move through the air like waves on the ocean. Aircraft flying through these waves get a bumpy ride. 

 Cirrocumulus and cirrostratus with luminance - Photo by Craig Johnson

Cirrocumulus and cirrostratus with luminance - Photo by Craig Johnson

The luminance of cirrocumulus was soon followed by cirrus fibratus. These fibers took a dramatic pose against the deep blue of the sky. The fibers are streams of ice crystals brushed over the sky by winds aloft. 

 Cirrus fibratus, cirrostratus, and cirrocumulus   photo by Craig Johnson

Cirrus fibratus, cirrostratus, and cirrocumulus   photo by Craig Johnson

Within 30 minutes the sky changed again. Instead of the cirrus fibers a gray pall had overspread the sky. Ice crystals were falling filling the layer with a broad layer of precipitation hiding the more fibrous ice crystals above. The sky was smeared with a diffuse layer of ice without a indistinct base, cirrostratus.

It is amazing how fast our sky can change in a matter of minutes. Try looking up several times a day. The show is free for all attendees. The different cloud types reveal a variety of processes and air movement. Try to imagine what might be causing the different cloud shapes that appear and watch here for more explanations in the future. 

 Cirrostratus   photo by Craig Johnson

Cirrostratus   photo by Craig Johnson