In meteorology, a reference to a cloud that is dense enough to obscure, to cover, or to cloud over the sun.

A cloud species which is 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 completely.

Cirrus clouds are composed of small particles, mostly ice crystals, that are fairly widely dispersed, usually resulting in relative transparency and whiteness and often producing a halo phenomena (colored or whitish rings, arcs, pillars or spots of light) which are not observed with other cloud forms.