A subdivision of cloud genera based upon transparency and the arrangement of cloud elemets. Cloud varieties are:

duplicatus (du) - more than one layer at different levels
intortus (in) - irregular or tangled
lacunosus (la) - thin cloud with regularly spaced holes, net-like
opacus (op) - completely masks sun or moon
perlucidus (pe) - broad patches with some (small) gaps allowing blue sky to be seen
radiatus (ra) - broad parallel bands convergind owing to perspective
translucidus (tr) - translucent enough to permit the sun or moon to be seen
undulatus (un) - sheets with parallel undulations
vertebratus (ve) - looking like ribs or bones

The standard abbreviations consist of the first two letters. None, one or more of the terms may be applied to describe an individual cloud type (genus).

Related features:
Cloud classification
Cloud types (genera)
Cloud species
Clouds - sentry of the sky