Cloud cover was the percentage of the sky that was obscured by clouds. Life on a cloud covered world was difficult for most humanoids to survive in.
In 2286, the call of the Whale Probe transformed ocean water into cloud vapor on the planet Earth. This increased cloud cover resulted in an increase of severe weather across the planet. Cities across the globe reported the increasing percentage of cloud cover to Starfleet. For cities that had 100% cloud cover, like Tokyo and Leningrad, these places were reduced to drawing power from the Planetary Reserve after losing main electrical power. (Star Trek IV: The Voyage Home)