Sky Type Characteristics and Daylight
Overcast skies are distinct from clear skies and partly cloudy skies in significant ways. The diffuse inter-reflectivity of light within the white, cloud-laden environment of an overcast sky creates general illumination from all directions. It is not uncommon for illumination levels to vary from one overcast day to another. At worst, overcast cloud conditions provide on average some S00 footcandles (fc) of illumination whereas at best, an overcast day may produce illumination levels approaching 1000-1200 footcandles. The brightest zone of the overcast sky vault is at the zenith and darkest zone is at the horizon, typically in a ratio of 3:1. Orientation is not typically considered significant under fully overcast skies as light generally comes uniformly from all directions. The overcast day is often considered the "darkest" or "worst case" condition for daylighting design.
Clear skies are just the opposite. The absence of clouds in the sky means that the dark blue sky vault reflects much less light than the overcast vault. The sky vault portion of the clear sky is brightest at the horizon and darkest at the zenith, typically at a ratio of 7:1. On a clear day however, illumination levels and light distribution and characteristics are dominated by the presence and location of the intense and highly directional solar disk (sun). Direct sun can produce illumination levels above 10,000 footcandles.
A partly cloudy sky is, in fact, often considered the brightest of days because it combines light distribution characteristics of both overcast and clear sky conditions. The intensity of direct sunlight is combined with the presence of clouds and therefore the inter-reflection of light from the sky dome to exceed the illumination levels of even a clear sky. Because of the ever-changing density and pattern of the cloud cover and its relationship to the solar disk the partly cloudy sky is also considered the most dynamic sky type and therefore the most difficult to model and study.