[SQM] What's the SQM reading for an *overcast* dark site

Jaime Zamorano jaz at astrax.fis.ucm.es
Thu Sep 16 12:38:31 UTC 2010

Hello !

To have an idea of the measures of sky brightness at the darkest sites
you should read the scientific papers submitted by the personel of
astronomical observatories. They use astronomical photometric

Read for instance: http://arxiv.org/PS_cache/arxiv/pdf/0709/0709.0813v1.pdf
a paper about the night sky at Calar Alto (Spain) with a table with
values at some well known observatories. The darkest measure ia about
V=21,9 magn/sq. arcsec at zenith and without Moon.

Out of the atmosphere the principal contributor is the zodiacal light.
Approximate Zodiacal Sky Background as a Function of Helio ecliptic
Latitude and Helio ecliptic Longitude (in V magnitude arcsec-2):
You can see that the darkest sky (out of the atmosphere) is 23.3
magn/arcsec2 in V.

Remember that there is a conversion factor between SQM readings and V
but measures of 23 mag/arcsec2 from ground are not expected on a clear
cloudless sky.

On the other hand since the clouds reflect light overcast skies should
be brighter in case of light pollution. The albedo of the snow is very
high so the light reflected by the snow to the sky and reflected again
to the ground by the clouds is again higher. The comparison picture of
Steinar shows a difference higher than expected. Please try to repeat
using raw format instead of  jpeg images.

I hope that this information is useful. Some data has been provided by
David Galadí.

All the best, Jaime

  Jaime Zamorano                     E-MAIL: jaz at astrax.fis.ucm.es
  Dept. Astrofisica   Phone: 34-91 394 4590    FAX: 34-91 394 4635
  Fac CC Fisicas // Universidad Complutense // 28040 Madrid (Spain)
    http://guaix.fis.ucm.es/   - http://www.ucm.es/info/Astrof/
 Logics will get you from A to B,
 imagination will take you everywhere. (Albert Einstein)

On Wed, Sep 15, 2010 at 5:10 PM, Banich, Howard <Howard.Banich at nike.com> wrote:
> I have Steinar. At the Oregon Star Party last month we had an overcast night and I pulled out my SQM for a reading because it was so dark. The reading I got was around 23.7 or so, but since there were several RV's nearby with light on I assume the actual SQM reading would have been higher without them. Even so, as my Dad would have said, "it was as dark as the inside of a cow."
> Howard
> Portland, Oregon
> -----Original Message-----
> From: sqm-bounces at unihedron.com [mailto:sqm-bounces at unihedron.com] On Behalf Of Steinar Midtskogen
> Sent: Wednesday, September 15, 2010 1:06 AM
> To: sqm at unihedron.com
> Subject: [SQM] What's the SQM reading for an *overcast* dark site
> Hello
> Has anyone tried to measure the light at a site with near zero light
> pollution when it's overcast?  One could think that the reading will
> be much higher than the perfect clear sky, i.e. higher than 22, but
> I'm not sure.  I found a table somewhere stating that the starlight in
> overcast conditions equals 0.0001 lux, which should translate to just
> over 26 mag/arcsec² (assuming that I've been using the right
> conversions).
> In my experience it's still bright enough to see things relatively
> well, especially if there's snow on the ground, even if it's cloudy
> and and artificial light sources are far, far away.
> Do clouds glow?
> Something else: I'm new to this list and while searching the archives
> I found some discussions regarding the effect of snow cover.  A while
> ago I made this composite picture to illustrate the effect:
> http://voksenlia.net/galleri/lysforurensing/lysforurensing-20081030.jpg
> which I think is pretty interesting.  The picture is made up of two
> pictures taken in Oslo.  The right part was taken 28 October 2008, the
> left part was taken 30th October after the first snow, precisely 48
> hours later.  The exposure was exactly the same (same aperture, same
> exposure time).  The only difference is the snow and cloud cover.
> --
> Steinar Midtskogen
> http://voksenlia.net
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