- Overcast sky is the sky in which the sun's position cannot be detected because of the high density of the clouds.
- It is characterized by diffuse light.
- It is three times brighter at the zenith than the horizon (evenly distributed illumination around the zenith).
- As a result, most illumination comes from the sky dome.
- The amount of its illumination can vary according to the density of the cloud cover.
- Not including the sun the overcast sky is brighter than the clear sky.
- Advantage of overcast skies for daylightin: the high light energy from the sun has been spread across the very much larger area of the whole sky.
- Essence of daylighting strategy: to expose the interior to as much of the overcast sky as possible. The more of this sky that can be seen from an interior, the brighter the interior will be.
- Usually there is no need to limit the amount of sky that can be seen, since the illumination from such skies is the norm to which people are accustomed. In anyway blinds can be used in the opposite case.
- Roof lights provide more light than vertical windows since the overcast sky is brighter at the zenith.
- Orientation do not play so important role, the differnce between a window facing the equator and a window facing the poles is very little.
- There might be the need of protection of overheating in days where the sky is clear. (and surely there will be)
- Diffuse light is coming from overcast skies. As a result, the point will receive light from all parts of the sky that it can see. The greater the area of sky that is visible from the point the better the lighting will be.
- The most important part is the area of the sky visible directly from the point. The intension of any reflections (which are also important) will be reduced.
- However, internally reflected component is very important and the reflectances of the internal syrfaces should be high.
- Protection from glare is often required (easy to achieve on south and more difficult to achieve on east and west).
- The average daylight factor should be above 2 to prevent a room appearing gloomy. If above 5 the room will appear brightly lit and cheerful. For deep rooms, a check should be made that the ratio of the average daylight factors of the back and front halves of the room is not less than 1:3.
REFERENCES
[1] Brawn G. and DeKay M., 2001. Sun, Wind and Light. Architectural Design Strategies. USA: John Wiley & Sons, Inc.[2] WSA, 2013, Passive Design Strategies: Overcast skies [pdf] Cardiff: Cardiff University
[3] WSA, 2013, Passive Design: Daylight introduction [pdf] Cardiff: Cardiff University
IMAGES
[1] Brawn G. and DeKay M., 2001. Overcast sky. [Image] In: Sun, Wind and Light. Architectural Design Strategies. USA: John Wiley & Sons, Inc, p.27.
[1] Brawn G. and DeKay M., 2001. Sun, Wind and Light. Architectural Design Strategies. USA: John Wiley & Sons, Inc.[2] WSA, 2013, Passive Design Strategies: Overcast skies [pdf] Cardiff: Cardiff University
[3] WSA, 2013, Passive Design: Daylight introduction [pdf] Cardiff: Cardiff University
IMAGES
[1] Brawn G. and DeKay M., 2001. Overcast sky. [Image] In: Sun, Wind and Light. Architectural Design Strategies. USA: John Wiley & Sons, Inc, p.27.