Cirrus spissatus

From Glossary of Meteorology
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cirrus spissatus

(Previously called cirrus densus, cirrus nothus;
also called thunderstorm cirrus, false cirrus.) A cloud species unique to the genus cirrus, of such optical thickness as to appear grayish on the side away from the sun, and to veil the sun, conceal its outline, or even hide it.

These often originate from the upper part of a cumulonimbus, and are often so dense that they suggest clouds of the middle level.