Simulating colors of clear and partly cloudy skies.

A model (SKYCOLOR) is developed that simulates the light and color of the sky and open cloud decks in the vertical plane including the Sun and the observer and animates the changes as the Sun goes down. Model skylight consists of sunbeams that are scattered toward the observer, but depleted by scattering and absorption in the Chappuis bands of ozone. SKYCOLOR includes the Earth's curvature, atmospheric refraction, cloud shadows, and solar eclipses. Scattering is given a wavelength (lambda) dependence of lambda(-4) for air molecules (Rayleigh scattering), lambda(-1) for tropospheric aerosols, and lambda(+1) for volcanic aerosol particles. Multiple scattering is calculated directly in clouds but is parameterized in clear air by decreasing the scattering rates of sunlight and of skylight in the Earth's shadow by 30%.