Cirrostratus clouds are high-level atmospheric formations that exist in the upper troposphere, typically forming above 20,000 feet. Because the temperature at this altitude is well below freezing, these clouds are composed entirely of tiny ice crystals rather than water droplets. Their appearance can sometimes be so faint that they are barely noticeable, yet they are one of the most identifiable high-altitude cloud types.
Identifying Characteristics
Cirrostratus clouds appear as thin, expansive, veil-like sheets covering large portions of the sky. They are generally milky or hazy, giving the sun or moon a diffused look while still allowing the light source to be clearly visible. The cloud layer is so transparent that objects on the ground still cast discernible shadows, distinguishing them from lower, more opaque cloud types like altostratus.
The structure is often uniform and featureless, known as Cirrostratus nebulosus. They can also appear fibrous with delicate, hair-like streaks, classified as Cirrostratus fibratus. Regardless of the subtype, the cloud is characterized by its sheet-like nature, spreading across the sky rather than appearing in distinct, patchy formations. This consistency makes the entire sky look slightly washed out or pale.
The Signature Trait: Solar and Lunar Halos
The signature trait of a cirrostratus cloud is the atmospheric optical phenomenon it creates: the 22-degree halo. This luminous, circular ring of light symmetrically surrounds the sun or the moon. The halo is caused by the way light interacts with the hexagonal ice crystals suspended within the cloud layer.
Light from the sun or moon is refracted, or bent, as it enters one face of a hexagonal ice crystal and exits through another. This double refraction acts like a prism, consistently deviating the light at a minimum angle of 22 degrees from its original path. This specific physics creates the ring observed at an angular radius of 22 degrees away from the light source. The sky inside the halo often appears noticeably darker because no light is refracted at angles less than 22 degrees. The inner edge of the halo can show a faint reddish tint, while the outer edge may appear slightly bluish due to the dispersion of light wavelengths.
What These Clouds Predict
The presence of cirrostratus clouds carries a message about impending weather changes. They are frequently observed ahead of an approaching warm front, where warm, moist air slowly rises over a cooler air mass. This movement allows the cloud layer to form high in the atmosphere before the rest of the weather system arrives.
If these high-level clouds gradually thicken and lower over several hours, it signals the warm front is continuing its advance. This shift suggests that continuous precipitation, whether rain or snow, is likely to begin within the next 12 to 24 hours. The cirrostratus layer will transition into lower and thicker cloud types, such as altostratus and nimbostratus, as the front moves closer.