A variety of cloud (sheet, layer, or patch), the greater part of which is sufficiently dense to obscure the sun (betweeen 10 and 20 optical depths). This variety is found in the genera altocumulus, altostratus, stratocumulus, and stratus. (Note: cumulus and cumulonimbus clouds are inherently opaque. ) In the case of altocumulus opacus or stratocumulus opacus the elements stand out in true relief at the cloud base, rather than as a consequence of varying degrees of opacity. Also, in these cases, this variety usually modifies the species stratiformis. See cloud classification.