Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I know that's what it looks like, but here's the thing: when a ray intersects below the horizon, the luminance is modified by the ground reflectance. This reflectance is specified with the -g option to gensky (the default is 20%). For this reason, the best practice for creating Radiance skies is to ensure the weighted average adds to 1 for both hemispheres. Since we use greyscale models in OpenStudio, this is easy, as you have seen. ;) This business of specifying the intensity and optionally the color of the sky & ground is discussed at length in this post from the radiance-online listserv archive: sky definition.
This is a good time to remind you that there really is no substitute for using local geometry to represent the ground, since the ground hemisphere does not get shaded by the building and other local objects, can only be one continuous color, etc.
