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）
Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus.
This cloud typically forms from altostratus in the middle altitude range, but it tends to thicken into the low altitude range during precipitation.
The variety "undulatus" (having a wavy undulating base) can occur with any clouds of the species stratiformis or lenticularis, and with altostratus.
Altostratus and nimbostratus clouds always have this physical appearance without significant variation or deviation and, therefore, do not need to be subdivided into species.
They also can occur under altostratus cloud preceding a warm or occluded front, when cumulus usually lose vertical development as the sun's heat decreases.
The altostratus velum cloud appears dark in comparison to its parent cloud, and can persist even after the cumulonimbus has disintegrated.
A further thickening of the altostratus is often accompanied by virga and the arrival of precipitation is imminent.
However, only two are seen with altostratus and stratus nebulosus whose uniform structures prevent the formation of a perlucidus variety.
