0 a type of stratus (= a flat, grey cloud) formed in thin layers and found at medium height, above the level of stratus -- 层云（灰白成层的云雾，见于中等高度）
High altostratus clouds and night flying conditions prevented him from making any visual contact.
Fractonimbus exist only under precipitation clouds (such as nimbostratus, altostratus or cumulonimbus), and don't produce precipitation themselves.
The sun can be seen through thin altostratus, but thicker layers can be quite opaque.
If the precipitation increases in persistence and intensity, the altostratus cloud may thicken into nimbostratus.
These include middle-tage layers of "altocumulus" and "altostratus" as well as low "stratocumulus", and "stratus".
Altostratus is usually composed of water droplets but may be mixed with ice crystals at higher altitudes.
The best-known is cumulonimbus with mammatus, but the mamma feature is also seen occasionally with cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus.
However, only two are seen with altostratus and stratus nebulosus whose uniform structures prevent the formation of a perlucidus variety.
