[Radiance-general] using turbidity in gensky

[David Appelfeld]

Hi Greg and Thomas,

thank you for the answer. I was not sure if I can use -R option, because I
have available only global horizontal irradiation and illuminance and it
seems from gensky manual that I need to have direct irradiance measured
horizontally without the diffuse contribution.
In the case I can use -R option, will the gensky looks simple like this
gensky 9 5 12:30CEST -a 55.964875 -o -12.4934 -m -15 +s -R 600

thank you
David

On 5 May 2011 17:02, Greg Ward <gregoryjward at gmail.com> wrote:

> 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
>
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