From Glossary of Meteorology


  1. After U.S. weather observing practice, the state of the sky when it is cloudless or when the sky cover is less than 0.1 (to the nearest tenth.)

    In aviation weather observations, a clear sky state is denoted by the symbol "O."

  2. The character of the sunrise or sunset when the disk of the sun is visible at these times.

    Compare cloudy.

  3. To change from a stormy or cloudy weather condition to one of no precipitation and decreased cloudiness.

  4. In popular usage, the condition of the atmosphere when it is very transparent (as opposed to hazy, foggy, etc.) and accompanied by negligible cloudiness.

    In weather forecast terminology, the maximum cloudiness considered is about 0.2.
    Compare cloudy, fair.