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Is light pollution less bad later at night?

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#1 RobH2020

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Posted 15 January 2020 - 04:59 AM

I've tried searching the forums and Google for this, not sure it's got much coverage.

 

Does anyone find that light pollution diminishes as the night wears on? As the general population goes to bed, turns off house lights, cars stop being driven, shops and factories shut down... 

 

In my area, they even turn off most of the street lights on quiet residential streets after midnight.

 

However, I'm 30 mins train ride from the centre of London - this is lit up so brightly at all times that when I lived there and got used to the brightness I was actually confused about the darkness at night when I visited my parents at Christmas. It was so unfamiliar to walk around in the dark! 

So anyway I'm not sure if the proximity of London means that local lights being turned off have little effect.

 

What's anyone else's experience?


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#2 Tony Flanders

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Posted 15 January 2020 - 05:34 AM

Does anyone find that light pollution diminishes as the night wears on? As the general population goes to bed, turns off house lights, cars stop being driven, shops and factories shut down...


Yes, I have measured the effect frequently with my Sky Quality Meter. In or near cities, the skyglow typically falls by roughly 1/2 magnitude between early evening and the early a.m., after which it flattens out. The drop is less in semi-rural areas, but it's still quite substantial.


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#3 LDW47

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Posted 15 January 2020 - 08:33 AM

Up here in the wide open spaces of Northern Ontario in our small city of 50,000 since they installed LED lighting about 6-8 yrs. ago it is even more pronounced. From my back yard here in town all the major constellations are well defined even the Big Dipper, which hangs out over the city core, is no problem defining, on a good nite of transparency / seeing M 80-81 can be picked up with ease high above, what many call those d*mn LED’s ! I love LED lighting in comparison to the days of HPS / MV that shone like a spot light high into the night skies ! 


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#4 happylimpet

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Posted 15 January 2020 - 11:11 AM

In Southampton the LED streetlights dim noticeably at 2355. I had some imaging across this time and noticed a ~5% drop in background level....obviously much of my light pollution isnt local. But some is.

 

It is definitely darker in the wee small hours than at, say, 10pm.


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#5 PXR-5

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Posted 15 January 2020 - 12:24 PM

Yes!
By 2AM it's better, then by 4AM is much better.
I can't really explain why though.

#6 John Fitzgerald

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Posted 15 January 2020 - 12:28 PM

Lots of house lights, car lights, and storefronts turn off lighting after 10 PM, especially in the winter after the holidays.  In summer and during holiday sales it seems to go on a little later.  Christmas lighting makes LP noticeably worse here for several weeks before the holiday and a week after.


Edited by John Fitzgerald, 15 January 2020 - 12:28 PM.

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#7 MartinPond

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Posted 25 January 2020 - 11:59 AM

Yes!
By 2AM it's better, then by 4AM is much better.
I can't really explain why though.

There are particulates, fumes,  and faint plumes of moisture.

There is less of each at night as homes and car and factories turn down.

Thing is, it takes time to clear.  Also, if there is a weather 'inversion'

things like wood smoke can get trapped all night.

 

But, on average:

One great thing about a late clear night:

  radiational cooling makes the tree-limbs, and then

   the ground, colder than the air around them.

  With a moderate steady breeze, this sucks vapor and

   the attached particles out of the air, gives you a 'cleaning' of the lower

   few thousand feet. 

 

Smog (ozone making gunk with hydrocarbons from exhaust or pine trees)

   peaks in the afternoon on a bright day and

   dissipates finally 2-3 hours after sundown.  Until then, it can drift

   over you from as much as 200-300 miles away.


Edited by MartinPond, 25 January 2020 - 12:00 PM.

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#8 KI5CAW

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Posted 29 January 2020 - 12:06 PM

Last night I had trouble sleeping so at 3 a.m., I got up to look for the supernova in NGC 4636. I was immediately struck by how much darker my suburban sky was, especially to the west, where Albuquerque lurks on the other side of the Sandia Mountains. I suppose everybody has turned off most lights by 3 a.m.? Any way, I found the galaxy and the supernova in my 8" RFT...the old fashioned way...by star hoppinglol.gif



#9 Napp

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Posted 29 January 2020 - 12:27 PM

Yes, as businesses close and people go to bed a lot of lights are turned off.  I observe at a dark sky site about 40 miles west of a city with a big light dome.  The light dome starts to noticeably dim after about 11:00 PM.  



#10 sg6

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Posted 29 January 2020 - 04:45 PM

As some places in the UK have auto switch off at either midnight ot here 1:00, then less lights so less pollution.

People usually turn off lights and I suppose companies turn off lights. Ours used to turn off at midnight, now 1:00. They didn't change when the clocks (DST) changed.

 

Suppose car parks and other places also turn off lights - when some of the supermarkets here close around 22:00 I guess the car park lights get reduced an hour later.

 

So would expect a reasonable reduction in light numbers that are on and so a reduction in pollution.



#11 AaronF

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Posted 30 January 2020 - 07:11 PM

Does anyone find that light pollution diminishes as the night wears on? As the general population goes to bed, turns off house lights, cars stop being driven, shops and factories shut down... 

 

In my area, they even turn off most of the street lights on quiet residential streets after midnight.

You might be interested in this experiment (report is written in English) performed by students from the university in Barcelona, where I live.

 

They put an SQM meter on two boats (and designed a cool gimbal mount specifically for the purpose!) and measured sky brightness as they moved away from the shore.

 

Here's a relevant quote:

 

[...] the global urban  light  emissions  are  not  constant but tend to decrease at a variable rate throughout the night (roughly of order 4.5%  per  hour [2])

 

2. Falchi  F,  Cinzano  P,  Duriscoe  D,  Kyba  CCM,  Elvidge  CD,  Baugh  K,  Portnov  BA,  Rybnikova NA, Furgoni R. The new world atlas of artificial night sky brightness. Sci. Adv. 2016

 

I also find this to be true.

In Barcelona they switch off a lot of public lights at midnight.

After about 0300h it's actually not bad, considering it's supposed to be a Bortle 9. I'd say it's more a Bortle 8 after 3 in the morning.

 

I used to live in London. The light pollution from the city swamps most of south-east England.

The nearest place to you is the north-eastern corner of Norfolk. Otherwise head down to Dartmoor, or to Wales or Scotland. The Pennines are probably still dark, too :-D

 

Another option is to emigrate - anywhere in Europe's still available until the end of the year! ;-D


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#12 Jon Isaacs

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Posted 31 January 2020 - 10:21 AM

In my experience skies tend to become slightly darker as the evening advances.  The reduction in artificial lighting is definitely a factor but in some locations there's more going on.

 

Our place in the high desert has a bright light dome to the west because of the vast majority of the population is within 20 miles of the Pacific Ocean.  The sky brightness is very much governed by the light from the city and if the coast cities are covered by a layer of clouds, the skies in the high desert will be significantly darker. 

 

What often happens along the coast is that a layer of clouds roll in off the ocean sometime after sunset, this darkens the skies in the mountains where you are above the marine layer.  Driving in, the marine layer of clouds can be very apparent.  From the passes along the freeway, the skies can be bright blue, just gorgeous, but to the west. the lower elevations are covered by the clouds.

 

Jon



#13 rogan

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Posted 02 February 2020 - 12:46 PM

I have commented on this in the past on CN. I have noticed that at 10 PM it is almost as if someone throws a switch and the night sky becomes noticeably darker. While I’m sure it is an ongoing process as lights go out and businesses close I have noticed a more abrupt darkening at 10PM.

#14 Marty0750

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Posted 08 February 2020 - 12:55 AM

Yes sky darkens a few tenths mag after midnight. My roof mounted SQM with data logging confirms this. I presume it is the reduced traffic and industrial activity kicking up less light scattering aerosols. Be aware the Milky way needs to be accounted for. An SQM cam record the MW up to 0.5 brighter sky when it passes over the SQM's field of view.


Edited by Marty0750, 08 February 2020 - 12:55 AM.

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