Why is there so big difference between CIE sky and Perez sky

I compared the simulated results under CIE cloudy sky model and Perez sky model, and I found the difference is very big.Below is the horizontal illuminance distribution under the CIE and Perez sky model,
CIE model
perez model

In addition , I also generated the sky distribution of CIE overcast and Perez sky model with direct irradiation of 0 w/m2 and diffuse irradiation of 228 W/m2 and found that the solar existed in Perez sky model even if I set the direct irradiation to 0 W/m2.

Even if the two skies were normalised to the same diffuse horizontal irradiance/illuminance (which might not be the case in your example), you’d likely find sizeable differences in the internal light distributions – especially if the calculation point can ‘see’ the circumsolar region of the clear sky model. The discussion here may help:


John Mardaljevic PhD FSLL FIBPSA

Daylight Experts Ltd.
Expert Witness | Simulation | Measurement | Conservation

Associate Editor Lighting Research & Technology

Hi, John

Thanks for your reply!

Now I understand the sizeable differences enven if the both skies are normalised to same diffuse irrdiance.But I am still confused why solar present in Perez sky when direct solar irradiance absent.That seems not reasonable.

I don’t know how your sky is generated (I don’t use GH/Ladybug/etc.), but the false-colour image may just be showing the circumsolar region. Perhaps you mean: why is the circumsolar present at all when direct is set to zero? Which is a fair point for the Perez All Weather sky. And, if that’s the case, I don’t know the reason why.

two comments:

  1. Gendaylit has implemented also a luminance efficacy model from Perez, not only the Perez sky luminance distribution model. In gensky you feed-in irradiance only in the visible range, that means you need to apply a luminance efficacy in advance (and that value typically varies with the sky type and its turbidity). Typical measured irradiance (or provided in weather files like in EPW) are for the full solar spectrum and not only in the visible range. That might explain also your deviations.

  2. If you have 0 direct irradiation, the sun disk has also zero radiance, see below the output of gendaylit. Epsilon is calculated to 1 and therefore pure overcast sky. For your sky probably delta is calculated as “very bright” which means the sky gets also brighter in sun direction, even the sun itself is not visible. In general this makes sense, although we all know that sky models can differ for individual timesteps dramatically (especially for intermediate sky that depends on the cloud distribution on the sky), but over the coarse of the year things balance out and for many climates it fits pretty well. Be also aware that the Perez model also depends on the dew-point and that you can feed the dew-point temperature with the option -d Dewtemp - that increases the accuracy of the luminance efficacy calculation.

Here the output of gendaylit when applying it with 0 direct-irradiation:

gendaylit -ang 20 -45 -W 0 228

gendaylit_dew -ang 20 -45 -W 0 228

Local solar time: 0.00

Solar altitude and azimuth: 20.0 -45.0

epsilon, delta, atmospheric precipitable water content : 1.0000 0.4673 2.0000

void light solar
3 0.0 0.0 0.0

solar source sun
4 0.664463 -0.664463 0.342020 0.533000

void brightfunc skyfunc
2 skybright perezlum.cal
10 2.544e+01 5.664e+00 0.091877 -0.241465 5.424682 -2.333236 0.183553 0.664463 -0.664463 0.342020


It seems that the radiance installed on my computer doesn’t support -d option considering dew-point temperature’s impaction on the accuracy of the luminance efficacy calculation.What version do you use?My version is “RADIANCE 5.2.dd0f8e38a7 NREL 2018-10-09”.

this was changed at the beginning of this year, around February. So you need to download a newer version from here:

another thing: gendaylit is bit sensitive to the order of the parameters, you probably need to apply like this:
gendaylit -ang 20 -45 -d 11.09 -W 0 228
or gendaylit 10 14 13.5 -d 11.09 -W 0 228 (if you provide month, day, hour instead)

Thanks for your expert adivce, Jan! And I kown you are the author of DGP, so I have another problem about DGP which is I got a high DGP (0.47) with the view facing the southern window when used Perez overcast sky with direct irradiation of 0 w/m^2 and diffuse irrdadiation of 228W/m^2.I suspect the validity of this result, because according to our life experience, dicomfort glare rarely happens under overcast sky. Moreover, I used solar total irradiance monitor to measure the outdoor direct and diffuse irradiation which are 57 W/m^2 and 251 w/m^2, and I also measured the vertical eye illuminance that is 2100lux with the view facing the southern window of a classroom. vertical eye illumiance of 2100 should appromaximately correspond to DGP of 0.35.
Below is the simulated HDR image: