As op compared with St neb op

(Section 2.3.5.6.6)

Altostratus opacus (thick) is distinguished from Stratus nebulosus opacus (thick) by:

• The layer being thin enough somewhere to reveal the  position of the Sun
• The type and intensity of precipitation; snow is common to both clouds but will be of a greater intensity with Altostratus; rain and ice pellets are unique to Altostratus; weak drizzle and snow grains are unique to Stratus
• Possible mamma

In the absence of thin parts near the Sun, precipitation or mamma, it can be difficult to distinguish between these two clouds. A continuous watch will assist, as Stratus opacus usually does not occur in combination with other cloud genera. Altostratus rarely forms without the presence of high-level clouds.