Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals. They are difficult to detect and if capable of forming halos the cloud takes the form of thin cirrostratus nebulosus. Its presence indicates a large amount of moisture in the upper atmosphere. Cirrostratus clouds sometimes signal the beginning of a warm front if they form after cirrus and spread from one area across the sky and thus may be signs that precipitation might follow in the next 12 to 24 hours.
A circumhorizontal arc is an ice-halo formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. As with all halos, it can be caused by the Sun as well as the Moon. The misleading term "fire rainbow" is sometimes used to describe this phenomenon, although it is neither a rainbow, nor related in any way to fire. More
A sun dog, or parhelion, is an exceedingly bright spot on a solar halo. A sundog is an atmospheric optical phenomenon primarily associated with the reflection or refraction of sunlight by small ice crystals making up cirrus or cirrostratus clouds. Often, two sun dogs can be seen (one on each side of the sun) simultaneously.