# CIE Overcast sky and time?

Sir,

I have some question about Illuminance calculation in Diva for grasshopper.

I don't understand why, in calculation of illuminance with a CIE Overcast sky, I can choose a date and time of the year wheareas i am in overcast sky.
And so, I get for illuminance in overcast sky, 16800 Lux where I should find a value of 10000 Lux.

Thank you

Tomas

Elioth SCE

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### Replies to This Discussion

How do you know that your illuminance should be 10000 lux?

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It has been said that the overcast sky produced an illuminance on a horizontal plane that shall be 10000 lux.

The CIE overcast sky can therefore be fully
characterized by the horizontal illuminance, usually given in lux. A realistic horizontal illuminance for a (brightish) overcast sky is 10,000 lux   (extract from daylight simulation from Radiance)

CIE overcast day (10000 Lux) This option is similar to 5-CIE overcast day described above but with scaled with a standard sky illuminance at the zenith of 10,000 Lux such that daylight factors can be calculated simply as working plane illuminance values divided by 100. This option is frequently used for daylight factor calculations. (from design builder).

I am just wondering why I don't get the right value.

Thank you

Tomas

SCE Elioth

Ahh, you mean outside and as a test, ok. Hmm, are you sure that you have no geometry whatsoever in your file but the plane? when I run it with just a plane, I get about 11000 lux, and the gensky options are right. That's pretty close to what you expect.

I have just only one plane, as a test (as you said)

But I can't understand why can't we have just 10000 Lux or around this value (about +/- 10 lux).

Is there a way to enter manually the value of the illumination.

Thank you

Tomas

Elioth SCE

Dear Tomas,

The total value for a CIE sky (global horizontal value) will vary with climate. However, we use a CIE overcast sky model for our Daylight Factor calculation in DIVA where the total value is forced to be 100. You could set this value to 10000, and run this as your illuminance calculation. It should work reliably.

Just edit the file, c:\DIVA\Daylight\CIE.Overcast.Sky.rad. The line to edit is,

> !gensky 12 21 12.00 -c -a 42.300 -o 71.100 -m 75.000 -B 100

where the -B parameter sets the gh-value.

After this, just run a daylight factor analysis, and you will get illuminance values under the sky defined in that file.

Best,

Alstan

Oki. I should make some tests now.

But I think it could be very interesting to have this parameter inside GH.

Thank you

Tomas

ELioth SCE

Hi Alstan,

Why do we have a location in the CIE sky file that can contradict the location specified in the weather data file?

!gensky -ang 45 0 -c -B 55.865921

This was extracted from the Radiance Daylight Simulation manual by John Mardaljevic and represents a 10,000LUX sky. however it does not have a location assigned. We're just confused when the location is being used as we're also specifying one in the weather file on DIVA?

Hope you can help.

Many thanks

Arthur

J. Alstan Jakubiec said:

Dear Tomas,

The total value for a CIE sky (global horizontal value) will vary with climate. However, we use a CIE overcast sky model for our Daylight Factor calculation in DIVA where the total value is forced to be 100. You could set this value to 10000, and run this as your illuminance calculation. It should work reliably.

Just edit the file, c:\DIVA\Daylight\CIE.Overcast.Sky.rad. The line to edit is,

> !gensky 12 21 12.00 -c -a 42.300 -o 71.100 -m 75.000 -B 100

where the -B parameter sets the gh-value.

After this, just run a daylight factor analysis, and you will get illuminance values under the sky defined in that file.

Best,

Alstan

Attachments:

Hi Arthur,

The reason is that gensky needs some form of angle or location input, but -c -- the CIE cloudy sky -- doesn't actually vary with location on the Earth (besides perhaps in intensity, but it does not vary in relative distribution). So by setting -B 55.865921, John Mardaljevic is making sure the total contribution of the sky will be 55.865921 * 179.0 = 9999.999859 lx. A 10,000 lx overcast sky in Norway will be the same as a 10,000 lx sky in Singapore, so it doesn't matter in this case.

Once you actually account for solar position, this doesn't work obviously.

Edit: DIVA is always using the location and time of day data to set the total intensity of the overcast sky. The only time this is not the case is when daylight factor is calculated, which is all the c:\DIVA\Daylight\CIE.Overcast.Sky.rad is used for.

Alstan