Sky Condition

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Hemispherical View Clear Sky (Images: Max Foley; UW IDL)
Overcast Sky (Images: Max Foley; UW IDL)
Annual average cloud cover percentage Seattle, WA; (Images: Autodesk Ecotect Weather Tool/UW IDL)

“Sky condition” commonly refer to the character of the sky at a particular point in time relative to percentage of cloud cover. Sky conditions are product of sun position, weather, and atmospheric conditions including the presence of water vapor and other particulate in the atmosphere.

Technically sky conditions are defined by:

Sky Condition Percentage of Cloud Cover
Clear Less than 10%
Partly Cloudy 10-50%
Mostly Cloudy 50-90%
Overcast Greater than 90%

The two primary sky types are clear skies and overcast. Clear skies allow for direct beam sunlight, cast sharp shadows, and generally produce levels of illumination from 50,000 to 200,000 lux at horizontal unobstructed surface during daylight hours (depending on time and location). Distributions of light under clear skies are highly dependent on sun position and orientation. Overcast skies tend to deliver diffuse light, dominantly from overhead, and provide a range of 6,000 to 50,000 lux at noon (depending on latitude), however illumination ranges can vary widely due to cloud density. Generally the brightest source of illumination under overcast skies is the zenith (directly overhead).

A range of sky model descriptions have been developed to represent common sky conditions. These include the CIE Overcast Sky, Clear Sky, Intermediate Sky, and Uniform Sky. Other sky models have been developed to facilitate annual or “climate based” simulation of sky conditions.

Section Key Resources
  • Baker, N., Steemers, K., (2002). Daylight Design of Buildings. James and James
  • Boyce, P., Hunter, C., Howlett, O. (2003). The benefits of daylight through windows. Troy (NY): Lighting Research Center.
  • Brown, G. Z., Dekay, Mark (2000). Sun, Wind & Light: Architectural Design Strategies. Wiley; 2nd Ed.
  • Hopkinnson, R. G., Petherbridge, P., Longmore, J. (1966), Daylighting. William Heinemann, LTD.
  • Lam, W.M.C., (1986) Sunlighting as Formgiver for Architecture, Van Nostrand Reinhold

Lead Author(s): Chris Meek

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