Middle Level Clouds
The mid level clouds 6,500-23,000 ft (2,000-7,000 m) typically have the prefix "alto" added to the two genera in this category. Altostratus clouds appear as a uniform bluish or gray sheet covering all, or large areas of the sky. The sun or moon may be totally covered or shine through very weakly. These clouds are complex as they are usually layered, with ice crystals at the higher, top layers, ice and snow in the middle, and water droplets in the lower layers. Altostratus clouds often yield precipitation. Altocumulus are elliptical, dense, fluffy balls. They are seen as singular units or as closely bunched groups in a clear sky. When the sun or moon shines through these clouds, one can sometimes see the "sun's rays" or corona.