cloudiness

Cloudiness, or cloud cover, refers to the fraction of the sky that is obscured by clouds at a given location and time. It is a key component of the atmospheric state and is used in weather forecasting, aviation, climate analysis, and solar energy planning. Cloudiness is commonly reported in oktas, an eight-point scale where 0 means clear sky and 8 means completely overcast, and as a percentage of the sky covered by clouds. In weather observations, cloud cover is estimated by human observers at meteorological stations; remotely sensed data from satellites and ground-based lidar and radar also provide quantitative cloudiness measures expressed as cloud fraction.

Cloudiness depends on atmospheric moisture, temperature profile, and air lifting processes. Different cloud types contribute to cloudiness in distinct ways: low, stratiform clouds tend to blanket the sky, while towering cumulonimbus clouds can produce rapid, localized changes. The net effect of clouds on surface radiation is complex: they reflect and scatter shortwave solar radiation, reducing daytime heating, but they also trap outgoing longwave radiation, which can raise nighttime temperatures. Overall, cloudiness moderates diurnal temperature ranges and influences precipitation potential.

Globally, cloudiness is higher in tropical regions with frequent convection and along mid-latitude storm tracks, while arid regions exhibit low cloud cover. Seasonal cycles, large-scale circulation patterns, and phenomena such as El Niño–Southern Oscillation affect regional cloudiness. Accurate estimation of cloudiness is essential for weather prediction, climate models, and solar energy resource assessment.