Cirrus spissatus is a unique cloud type. It is the only cirrus cloud dense enough to hide the the disk of the Sun. Other cirrus allow the disk to be at least faintly visible. Most spissatus form by the breakdown of the anvil of a thunderstorm. Spissatus not formed by remnants of a thunderstorm anvil often appear as multiple patches of dense cirrus, often at different levels, like the cirrus in this photo.
The Weather Briefing Blog