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Does high sky transparency translate to darker skies?

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#1 Darren Drake

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Posted 23 July 2019 - 09:37 AM

I've noticed that on the more transparent nights at my site I typically get around 21.2 on the sqm.  There have been other nights when there is higher humidity or some form of moisture in the air that the readings can be 21.3 or 21.4.  Have others  noticed this?  Does extra moisture prevent sky glow to some extent?  Thanks 



#2 George N

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Posted 23 July 2019 - 09:48 AM

You seem to be under some rather dark skies, with SQM running 21.2 to 21.4 - not 'perfect' - but very good.

 

When clouds pass over, do they look like 'black holes' or are they brighter than the clear sky background? My experience: SQM = 21.0, and a passing cloud is obviously brighter than the clear sky, but SQM = 21.85, and a passing cloud appears as a black spot with no stars.

 

I would think - just a guess based on experience - that in a dark location high haze will block natural sky glow and dim aurora, resulting in higher SQM readings.

 

However, in an area with even small amounts of Light Pollution (clouds look brighter than the clear sky) - high haze will reflect light back down, making the sky brighter.


Edited by George N, 23 July 2019 - 09:48 AM.

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#3 petert913

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Posted 23 July 2019 - 10:02 AM

If transparency is not optimal, then I think scatter would attribute to contrast (darker apparent background).



#4 TOMDEY

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Posted 23 July 2019 - 10:16 AM

Interesting --- and I'll speculate that it can go either way. Light pollution from localish ground bounces off clouds or high-haze, increasing the overall sky brightness and decreasing the SQM number. This is the situation where e.g. clouds actually look brighter than the sky behind them.

 

The converse is where the major effect is clouds and haze actually blocking natural sky light (stars plus air-glow, etc.) This is the situation where e.g. clouds look darker than the sky behind them. When I moved dark rural, many decades ago... I (fondly) recall looking thru the eyepiece, then up, and wondering, "where did the stars go?!" Only to realize that it had gotten cloudy... but that I couldn't even see the clouds... other than that they snuffed the starlight. I'm sure an SQM would have read  anomalously high.

 

In summary --- both effects are always present. In badly polluted areas, clouds and haze would likely lower the SQM reading; in nice dark zones... they would likely (anomalously) raise the SQM reading.

 

NOTE: Under really dark, unpolluted skies --- it's amazing how clouds can sneak up on you.    Tom


Edited by TOMDEY, 23 July 2019 - 07:25 PM.


#5 chadrian84

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Posted 23 July 2019 - 10:39 AM

I've noticed higher SQM readings when transparency is good.  Depending on transparency, on clear nights I usually get between 19.7 - 20.3 at my home with my SQM meter.  19.7 being below average transparency and 20.3 being good transparency.



#6 jura87

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Posted 25 July 2019 - 05:01 AM

Yes, tonight sky was full of transperent clouds and myst, I am at my cottage, there was moon outside and with bad transperacy and moon it was bortle 5...usually with transperant sky is bortle 4...I think that scatter is comming from cityes around, light from horison is reflected from myst...

Edited by jura87, 25 July 2019 - 05:02 AM.


#7 vsteblina

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Posted 25 July 2019 - 01:34 PM

I've noticed that on the more transparent nights at my site I typically get around 21.2 on the sqm.  There have been other nights when there is higher humidity or some form of moisture in the air that the readings can be 21.3 or 21.4.  Have others  noticed this?  Does extra moisture prevent sky glow to some extent?  Thanks 

I live in eastern Washington......and have a city site (20.4) and a dark site (21.6) 30 minutes away.

 

Relative humidity in summer runs between 10-20 percent. Relative humidity in winter runs 80-100%....night time fog is very common in winter.

 

I noticed the same thing that a dark, visually transparent sky reads brighter on the meter than a hazy, high humidity night. I have been very careful to meter the same segment of sky...region around the north star.

 

It is pretty dramatic how much darker a transparent sky is relative to a hazy sky, but the meter does not show that difference. 

 

Looking the south towards the city....the light dome is significantly lower during transparent skies. Yet, the meter doesn't seem to pick that up.  I have used both a SQ-L meeter and a SQ-M meter and get similar results.

 

That is the situation with my city site.....have not done enough readings at my dark site to see the effect there.  However, at my dark site, there a NO nearby lights and even with situations of close to 100% humidity.  It is pretty amazing to have a totally dark sky with fog visible on the meadow. The sky seems just as dark when it is transparent.

 

I don't know how sky meters work and interpret their data....but I suspect that is where the answer is found to your question.


Edited by vsteblina, 25 July 2019 - 01:37 PM.


#8 Darren Drake

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Posted 29 July 2019 - 12:06 PM

I suspect that if there is higher humidities near ground level which is not uncommon the fog lessens the amount of light that gets higher into the atmosphere.  This may result in darker skies in general for that area.  So even though the stars may be dimmer so is the sky glow.  Does this sound plausible??




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