Definition: Overcast sky conditions occur when clouds cause low visibility conditions. Although fog can cause low visibility on the ground, overcast skies are those higher in the atmosphere. The National Weather Service uses the abbreviation OVC to indicate overcast conditions. Typically, individual clouds are not seen in an overcast sky.
In order to classify the sky as overcast, 90% to 100% of the sky is considered to be covered by clouds. The penetration of sunlight is noticeably less. Other conditions can lead to low visibility such as blowing snow, ash and dust from volcanoes, smoke, and heavy rain. On a weather station model, the center circle will show an estimate of cloud cover. In this case, the center circle would be completely darkened.
Sources:
- Globe.gov Estimating Cloud Cover: A Simulation
- Globe.gov Cloud Cover and Contrail Cover Protocol
- Science Netlinks Measuring Cloud Coverage
