The word "overcast" describes skies that are at least 90 percent covered by clouds. Overcast skies occur when clouds form at a relatively high level of the atmosphere.
Comprehensive weather observations combine information such as the temperature and dew point with a description of the sky conditions at a particular location. When it is completely sunny during the daytime or there are no clouds whatsoever at night, skies are said to be clear. The technical term for a sky that is minimally covered with clouds is "few," though phrases such as "mostly sunny" are used when describing the weather to the general public. Progressively higher levels of cloud cover include scattered, broken and overcast.