Total height of light pollution on atmosphere?

[N3ptune]

Hello I have a little question.

At what height in the atmosphere do you believe the light pollution becomes invisible? (The light pollution dome of a large city for instance.)

Thanks.

 

[vlaiv]

Complex question. I'm certain that there is no single answer to it as it depends on so many factors. It might be that there is a rule of a thumb that goes like this: "On most nights for most of the practical purposes light dome can be approximated to be X km/miles up".

If you think about it, LP depends on:

1. Number of light sources (all having certain frequency / wavelength signature) - this is not static thing, changes with time, people turn on and off lights.

2. Amount of stuff in atmosphere that reflects / absorbs light - also frequency / wavelength dependent - changes with conditions (fog, polluted air - smog, natural things - pollen and such - this changes with time of the year). It also has a gradient - like atmospheric pressure - so you can think about it in "layers" of density.

So when you start to "simulate" what happens with LP - you have to think in terms of this layers - light can be scattered, absorbed or go thru certain layer - in last case you have to repeat the process for next "layer". Also some of the light that scatters on one layer, can be scattered in "upward" direction - so you have to include its contribution in next layer.

This means that if there is heavy ground fog - light dome will not be as high as on relatively transparent night in spring when there is a lot of pollen in the air. But LP "on the ground" will be much more severe.

Also the notion of "invisible" is somewhat ambiguous - it can mean for naked eye unable to distinguish level of contrast between "background" and LP - but there is no precise line where LP stops, so how can you tell what is background? Or it can mean less than N photons per arc seconds squared of the sky falling on aperture of N cm squared - still not "invisible" but there is upper bound to it.

[N3ptune]

Thanks vlaiv for your answer.  Basicly just want to measure roughly the domes of the major cities from various observation points to evaluate the obstructions in the worst-case scenario. Perhaps that map could provide extra information.

Last night I was at point C (correction, not point A)  but it's in the mountains so it's not clear if I even could identify the dome of Montreal at all... point C (correction, not point A)  is 85 kms away from the white border of pollution, I noticed the sky was much better from Point C (correction, not point A) then from point B.

[PeterW]

Difficult as you don't know how well cutoff the local lights are. Light pollution maps only show light going UP or model the expected pollution based on population. If the lights are all well cutoff then the spread of light (from light emitted near the horizontal) will be greater. If you can get hills in the way and go out when the air is clean it will maximise your viewing.

 

good luck

 

peter

[N3ptune]

(On my previous messages I misplaced point A and C, correction in red)

I don't know how much of a problem the local lights of the point C are, I know the sky is much better then at point A where I literally see a wall of light in front of me. (Passing thought dust and humidity). Has I understand, considering the various factors, the light dome of a large city can vary in size from one day to another so it's somewhat hard (or irrelevant) to try to evaluate its size.

Now I guess the best rule of thumb would be to get away has far has possible from any light? And maybe even hide behind a hill.

[jetstream]

http://www.blue-marble.de/nightlights/2012

[N3ptune]

Hem thanks for the map Jetstream!

[jetstream]

On 2017-02-06 at 18:00, N3ptune said:

Hem thanks for the map Jetstream!

The Blue Marble, an SQM-L and some gas will show you where the pockets of darkness are. Pick objects that get those big domes at your back ie I would head south if you want a southern view-maybe between Stanstead and Norton? This map is so sensitive it reveals lights that may not dome...the brightest concentration of lights are big ones for sure. I mapped my area using the SQM-L and it will show you the nearest dark holes as well as the edges of light domes ( or the dome itself).

[N3ptune]

On 2/7/2017 at 23:55, jetstream said:

The Blue Marble, an SQM-L and some gas will show you where the pockets of darkness are. Pick objects that get those big domes at your back ie I would head south if you want a southern view-maybe between Stanstead and Norton? This map is so sensitive it reveals lights that may not dome...the brightest concentration of lights are big ones for sure. I mapped my area using the SQM-L and it will show you the nearest dark holes as well as the edges of light domes ( or the dome itself).

Ok that's really interesting, I should get a SQM-L to map the light pollution myself... it would answer many questions.

But Jetstream at 100 kms north from Montreal, you believe the domes from Ottawa and Montreal could still be an obstacle on southern objects? these domes seem quite small.

[jetstream]

1 hour ago, N3ptune said:

Ok that's really interesting, I should get a SQM-L to map the light pollution myself... it would answer many questions.

But Jetstream at 100 kms north from Montreal, you believe the domes from Ottawa and Montreal could still be an obstacle on southern objects? these domes seem quite small.

I don't know, if there is haze in the air it reflects the light and makes light domes much worse. Arizona has done many studies on light domes and at one point I had the major cities light dome profiles. I can say this, from one of my dark sites 40 miles from a small town the dome can be visible under poor transparency.

The SQM-L is very accurate when properly used. Does the Milky Way show defined structure between Ottawa and Montreal, near "B" on your map?

[jetstream]

http://azdailysun.com/news/local/images-show-flagstaff-s-dark-skies-success/article_b9d0f493-556b-584c-b510-74f3a3006119.html

[N3ptune]

@jetstream

That's an interesting experience to measure the light pollution, although I am not sure of what to extract out of that experience exactly. The LEDs are killing the sky, that's a depressing fact.

Light pollution is affecting my mood... more and more. Maybe I should think less of this and drive my car north more often simply...

[jetstream]

One could possibly extract a few things.

1: light domes can grow under poor transparency

2: light pollution control does work as evidenced by the images, 2A has much smaller domes

3: severe light pollution can reach over 30 deg high- answering the title of the thread

4: light domes can overlap,Cheyenne and Fort Collins are very close to overlapping

So from my experience seeing the small dome at 40 miles under sketchy transparency, it might be possible that Ottawa's and Montreal's domes might overlap under certain conditions.

A good indication of sky conditions might be revealed by the question I asked...does the Milky Way show defined structure around "B" on your map or anywhere you have observed from?