clearsky

Clearsky, or clear sky, refers to atmospheric conditions with minimal or no cloud cover, allowing the sky to appear blue during the day and dark at night. In meteorology, clearsky denotes regions where cloudiness is negligible, though thin cirrus can occasionally be present without significantly obscuring the sky.

Cloud cover is commonly described using oktas; a completely clearsky condition is 0 octas. Clearsky tends to occur under high-pressure systems with stable air, low humidity, and limited convection. Variable factors such as air pollution and humidity can still affect brightness under a visually clear sky.

In solar energy and atmospheric science, 'clear-sky' conditions are the baseline used to estimate solar irradiance when clouds are absent. Clear-sky models, including widely used approaches such as the Ineichen–Perez and Bird models, account for Rayleigh scattering, ozone and water vapor absorption, and aerosol content to predict direct and diffuse solar radiation. These estimates support solar-resource assessment and photovoltaic design.

Measurement and observation rely on ground observers, all-sky cameras, ceilometers, and satellite data. The term 'clearsky fraction' describes the proportion of sky free of clouds. Even with a visually clear sky, aerosols, dust, and pollution can reduce irradiance and influence visibility and color.

Clearsky conditions influence outdoor activities, astronomy, and daylight-energy planning. They are transient and can be broken by thin clouds, haze, or changing atmospheric conditions that introduce scattering or absorption.