|Cirrus Radiatus at White Pass, Washington on December 31|
|Here is Eric Sloane's cloud map from his book Skies and the Artist|
Our first Cloud of the Week comes from the genus cirrus: cirrus radiatus. Cirrus is a Latin word meaning "fiber" or "hair." In general, these clouds are delicate and wispy, not unlike strands or locks of hair. They are composed entirely of ice crystals (not water droplets) and occur at high altitudes--16,500-45,000 feet above the earth.
Cirrus radiatus is one of five species and four varieties of cirrus distinguished by the appearance and orientation of their streaks. This cloud's streaks appear in bands, usually aligned with the wind, that seem to converge toward the horizon. In fact, they do not converge at all. These clouds are actually parallel to one another. It took me a very long time to see this and to override the illusion of convergence. My head still spins when looking at cirrus radiatus like those in the photo above.
In this photo, you can see some tiny clumps within the streaks. These are the ice crystals that form the initial cloudlettes that are almost instantly blown about by the winds (fast winds of up to 180 m.p.h.) As the heavier-than-air ice crystals descend, they encounter slower winds and lag behind the initial clump. The lagging ice crystals create the classic cirrus streaks. These clouds are sometimes called "jet-stream cirrus" as they are caught up in the fast winds of the jet stream.
Weatherwise: In isolation, cirrus radiatus, indicate high atmospheric pressure and--because of the orientation of their streaks--the direction of the winds aloft. Wind direction is useful to know as it will help you know where your future weather is coming from. Being the Accidental Naturalist, I took the photo posted here while cross-country skiing at White Pass. I "accidentally" looked up from the trail and saw these clouds. I travel with a camera, but alas, not a compass. So I do not know which way the wind was blowing. Literally.