As op compared with Ns
(Section 2.3.5.6.4)A low layer of Altostratus opacus (thick) is distinguished from similar Nimbostratus by:
- Presence of thinner parts through which the Sun is, or could be, vaguely revealed
- Being a lighter shade of grey
- Having a less uniform base
- Possible mamma
Visually distinguishing Nimbostratus from Altostratus on moonless nights is difficult, if possible at all. When rain, snow or snow pellets reach the Earth’s surface, the cloud is identified as Nimbostratus.