Cirrostratus are sheet-like clouds composed of ice crystals. Though cirrostratus can cover the entire sky and be up to several thousand feet deep, they are relatively transparent, as the sun or the moon can be easily seen through them. Sometimes the only indication of their presence is given by an observed halo around the sun or moon Halos result from the refraction of light by the cloud's ice crystals.

[Image: halo through the cirrostratus clouds (58K)]

These high level clouds typically form when a broad layer of air is lifted to its Lifting Condensation Level by large-scale convergence. Cirrostratus clouds, however, tend to thicken as a warm front approaches, signifying an increased production of ice crystals. As a result, the halo is no longer seen and the sun (or moon by night) becomes less visible.

During sunrise and sunset, these clouds can appear in a magnificent array of colors as unscattered components of sunlight (red, yellow, and orange) are reflected by the underside of the clouds. The criss-cross pattern of cirrus streaks is commonly seen before an approaching warm front.

[Image: thickening cirrus and cirrostratus at sunset (65K)]

The cirrus streaks were aligned in a southwest to northeast direction, indicative of the advancing warmer air at higher levels. Lower on the horizon, thickening cirrostratus clouds effectively hide the sun, signifying changing weather ahead. With time, these clouds will thicken into altostratus and eventually into the lower and more dense nimbostratus clouds as the warm front gets closer and closer.