a grey appearance; indicates some upward movement of air in the upper troposphere. Stratiformis (Str) "Sheet-like": Horizontal cloud sheet of flattened stratocumuliform (Sc str, Ac str, Cc str); indicates very slight airmass instability. Uncinus (Unc) "Hook-like": Cirriform (Ci unc) with a hook shape at the top; indicates a nearby backside of a weather system. Volutus (Vol) "Rolled": Elongated, low or mid-level, tube shaped, stratocumuliform (Sc vol, Ac vol). The division of genus types into species is as shown in the following table. The genus types (including some cumulus sub-types) are arranged from top to bottom in the left column in approximate descending order of average overall altitude range. The species are sorted from left to right in approximate ascending order of instability or vertical extent of the forms to which each belongs: Stratiform species, Cirriform species, Stratocumuliform species, Cumuliform species, Cumulonimbiform species. These ordinal instability numbers appear in each box where a particular genus has a particular species. WMO varieties Opacity-based Opacus Latin for "Opaque". A thick sheet of stratiform or stratocumuliform cloud. Perlucidus "Semi-transparent". Sheet of stratocumuliform cloud with small spaces between elements. Translucidus "Translucent". Thin translucent patch or sheet of stratiform or stratocumuliform. Pattern-based Duplicatus Latin for "Double". Closely spaced often partly merged layers of cloud in one of several possible forms. Intortus "Twisted". Curved and tangled cirriform. Lacunosus "Full of holes". Thin stratocumuliform cloud distinguished by holes and ragged edges. Radiatus "Radial". Clouds in one of several possible forms arranged in parallel lines that appear to converge at a central point near the horizon. Undulatus "Wavy". Stratiform or stratocumuliform cloud displaying an undulating pattern. Vertebratus "In the form of a back-bone". Cirriform arranged to look like the back-bone of a vertebrate. The following table shows the cloud varieties arranged across the top of the chart from left to right in approximate descending order of frequency of appearance. The genus types and some sub-types associated with each variety are sorted in the left column from top to bottom in approximate descending order of average overall altitude range. Where applicable, the genera and varieties are cross-classified to show the species normally associated with each combination of genus and variety. The exceptions comprise the following: Altostratus that have varieties but no species so the applicable boxes are marked without specific species names; cumulus congestus, a species that has its own altitude characteristic but no varieties; cumulonimbus that have species but