"Nimbostratus" and "Altostratus", which cannot originate from human activity. The altostratus undulatus is a type of altostratus cloud with signature undulations within it. Altostratus can bring light rain or snow. Altostratus can be composed of ice crystals. The sun can be seen through thin altostratus, but thicker layers can be quite opaque. Often, when an altostratus cloud thickens and descends into lower altitudes, it will become nimbostratus. If the precipitation increases in persistence and intensity, the altostratus cloud may thicken into nimbostratus.