Moving into the high etage, over 20,000 feet, we come to the cirrus clouds. As with the two lower levels, there are a few basic cloud types that exemplify the upper etage. Today we’ll look at the one that is most representative of the type: basic cirrus. Cirrus form the mares’ tails in the old weather adage, “Mares’ tails and mackerel scales make lofty ships carry low sails.” This bit of weather lore is based on the fact that, while these high altitude clouds don’t bring any severe weather themselves, they often presage it. Since the winds are higher up there, the high level clouds precede the body of the weather system.
Cirrus clouds get their name from the Latin, “cirrus,” meaning a curling lock of hair. You can see why, with their wispy appearance. Resembling an artist’s brush strokes, cirrus clouds form from either super-cooled water droplets freezing into ice crystals, or super-cooled water vapor turning directly to ice, bypassing the liquid phase. Almost all cirrus is made of ice crystals, with a few instances of super-cooled water droplets that haven’t yet frozen. The ice crystals’ refractive properties interact with sunlight and moonlight in interesting ways. We’ll cover that in a future post.
Photo credit: NicholasT/Foter/CC BY
Some of the prettiest skies are made by cirrus clouds.