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

Banich, Howard Howard.Banich at nike.com
Wed Sep 15 17:10:03 UTC 2010

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."

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


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

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:


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
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