Sun too close to zenith

radiance-general
1

Hi all,

I'm using Francesco Anselmo's "radmap" script to generate some annual skies for a fairly low latitude, and for a portion of the summer gensky throws me a warning: "sun too close to zenith, reducing altitude to 87 degrees". This adjustment creates a slight "bump" in the summertime corner of the analemma when you view a hemispherical fisheye projection of the annual sun positions. (The sun positions are being generated by the script, which is getting its info from an epw weather file.)

What is the cause for gensky relocating my suns to 87 degrees?

- Rob Guglielmetti

2

Hi Rob,

I'm not exactly sure why I put this in there, it was so long ago. I may have simply been duplicating someone else's algorithm, or I may have found problems related to the sun at zenith. The whole sky distribution is normalized to the zenith value, so having the sun too near could be a problem, or it might have been paranoia on my part.

You can try removing lines 218-225 in gensky.c and recompiling without:

  if (!overcast && altitude > 87.*PI/180.) {
    fprintf(stderr,
"%s: warning - sun too close to zenith, reducing altitude to 87 degrees\n",
        progname);
    printf(
"# warning - sun too close to zenith, reducing altitude to 87 degrees\n");
    altitude = 87.*PI/180.;
  }

Try a few data points where the sun is at the zenith and see what happens.

-Greg

···

From: Rob Guglielmetti <rpg@rumblestrip.org>
Date: December 7, 2005 1:59:46 PM PST

Hi all,

I'm using Francesco Anselmo's "radmap" script to generate some annual skies for a fairly low latitude, and for a portion of the summer gensky throws me a warning: "sun too close to zenith, reducing altitude to 87 degrees". This adjustment creates a slight "bump" in the summertime corner of the analemma when you view a hemispherical fisheye projection of the annual sun positions. (The sun positions are being generated by the script, which is getting its info from an epw weather file.)

What is the cause for gensky relocating my suns to 87 degrees?

- Rob Guglielmetti

3

Gregory J. Ward wrote:

Hi Rob,

I'm not exactly sure why I put this in there, it was so long ago.

You can try removing lines 218-225 in gensky.c and recompiling without:

    if (!overcast && altitude > 87.*PI/180.) {
        fprintf(stderr,
"%s: warning - sun too close to zenith, reducing altitude to 87 degrees\n",
                progname);
        printf(
"# warning - sun too close to zenith, reducing altitude to 87 degrees \n");
        altitude = 87.*PI/180.;
    }

Try a few data points where the sun is at the zenith and see what happens.

Thanks Greg, I'll check it out; thanks for getting back to me so quickly.

- Rob