[Radiance-general] using turbidity in gensky

[Greg Ward]

Hi David,

It may seem counterintuitive, but the way gensky works is that the solar intensity is unaffected by turbidity.  Decreasing turbidity actually reduces the sky zenith brightness using a simple model, which makes sense since no atmosphere would look like a night sky even in the daytime.  Thus, since we don't properly account for the loss in solar intensity as turbidity is reduced, the total illuminance goes up.

Generally speaking, you shouldn't bother with the turbidity setting at all.  Since it only affects the default sky brightness and not the sky distribution, it makes more sense to control these values directly using any of the -b, -B, -r, or -R options for setting radiance or irradiance.

Best,
-Greg

> From: David Appelfeld <d.appelfeld at gmail.com>
> Date: May 5, 2011 4:56:01 PM PDT
> 
> Hello Radiance users,
> 
> I am trying to generate sky distribution during sunny day for my model to compare the results with the measured data of illuminance. It seems that the day when the measurements were obtained was very clear and I would like to decrease the turbidity in gensky to get higher global and vertical illuminance.
> When I decreased the turbidity from the default of 2.45 to 1 then the illuminance decrease, while when I increase -t option then the illuminance increase too. I would actually expect opposite, and that the illuminance will increase with lower turbidity since the turbidity should scattered light more and thus block the direct contribution and increase the diffuse contribution form the sky.
> 
> Here is the gensky command I am using:
> gensky 9 5 $(printf "%02d:%02d" $h $m)CEST -a 55.964875 -o -12.4934 -m -15 +s -t 1 
> 
> Does someone have had this problem before? Or is this as gensky should works? 
> 
> Thank you 
> David