Theo Lobo’s Post

Here are many observations about cirrus clouds from the international cloud atlas: Dense cirrus patches or cirrus in tufts may contain ice crystals large enough to an acquire an appreciable terminal velocity. Trails of considerable vertical extent fall from the base of cirrus. Sometimes, infrequently, ice crystals in the trails melt into small water droplets and this makes the trails more greyish and a rainbow is possible. The trails curve irregularly or slant as a result of wind shear and of the variation in the size of constituent particles; consequently, Cirrus filaments near the horizon do not appear parallel to it. Cirrus tufts with rounded tops often form in clear air. Fibrous trails may appear under the tufts; the tops then gradually lose their roundness. Then, the tufts may disappear and the clouds are then in the form of filaments (of species uncinus or fibratus). Cirrus in the form of filaments may also develop from dense cirrus patches, from altocumulus castellanus and floccus and, at very low temperatures, from cumulus congestus. Cirrus not close to the horizon is mostly white at all times of day, white than most other clouds at that part of the sky. With the sun close to the horizon it is whitish while lower clouds might be more orange or yellow. After the sun is below the horizon, cirrus high in the sky is yellow, then pink, then red and finally grey. The color sequence is reversed at dawn. Cirrus near the horizon often takes a more yellow to orange tint due to the great thickness of air traversed by the light in passing from the cloud to the observer. These tints are less conspicuous in clouds in the middle to low levels. When observing from below the cloud cirrus has no distinct structure but it can have white and delicate filaments, patches or narrow bands. When observing within the cloud, cirrus is made of ice crystals, the observer sees a glitter of these crystals in the sunlight. Small halo phenomena is possible too. When observing from above the cloud cirrus appears very bright in sunlight. Thin cirrus may resemble the upper surface of a layer of haze, while dense cirrus has a milky appearance. Other clouds below and the ground are frequently visible through a cirrus. A subsun (undersun) may sometimes be visible from above a thick cirrus. Check the comments of the latest post to get the links to our weather videos. Enjoy our cirrus cloud photographs below.