Author Topic: New Light Pollution Atlas w/o Snow Cover  (Read 375 times)

chionewssesu

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New Light Pollution Atlas w/o Snow Cover
« on: December 27, 2017, 12:17:55 PM »
I'm sure most of us have used the World Atlas of the Artificial Night Sky Brightness to help us gauge the quality of our skies and to help locate new sites for observing. This atlas is based on Defense Meteorological Satellite Program (DMSP) satellite measurements of light sources on the earth's surface and a model of how these light sources affect the amount of light scattered downward to an observer's eyes (see here for more detail). While the maps are imperfect, they are nevertheless an excellent resource for finding dark skies, particularly for gauging the <em class="bbc">relative brightness of sites.This post concerns the possible effect of snow cover on this light pollution atlas. The idea is not new--the authors of the atlas themselves have pointed out the potential impacts of snow cover, as has Tony Flanders in his blog. The satellite measurements used to make the sky atlas were taken in the following three time periods:1. March 16-23 1996, 2. January 5-14 1997, 3. February 3-12 1997 (from Elvidge,C.D., Baugh, K.E., Dietz, J.B., Bland, T., Sutton, P.C., Kroehl, H.W. 1999. Radiance Calibration of DMSP-OLS Low-light Imaging Data of Human Settlements. Remote Sensing of Environment 68(1), pp. 77-88.)Snow cover data is available from the National Snow and Ice Data Center (NSIDC) on a weekly basis here. The three weeks that correspond most closely to the three time periods above are:1. March 18-24 1996, 2. January 6-12 1997, 3. February 3-9 1997The attached figure shows the number of weeks with snow cover on the ground from the NSIDC data. 100% means that all three weeks had snow cover, 67% means 2 out of three weeks had snow cover, etc. As you can see, much of the northern third of the US had snow cover during the entire period when the light data was taken. This snow cover will dramatically increase the amount of light sensed by the satellite and will thus make the light pollution atlas brighter than it would otherwise be. How much of an effect does snow cover have on the atlas? It turns out there is additional DMSP satellite data taken from September - November 2001. Except for a relatively small amount of snow centered over northern Wyoming and western South Dakota during the November new moon, this entire period was snow free. (This new data is only available online for the lower 48 states, unfortunately.) In the posts below, I calculate a new light pollution atlas using this 2001 data. First, I try to re-calculate the current atlas with the original 1996/1997 data to make sure the new atlas is a fair comparison.

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notaslasof

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #1 on: December 29, 2017, 02:09:40 AM »
Here I briefly describe where I obtained the data/model to re-calculate the night sky atlas and how my re-calculated version compares with the original atlas:The "radiance-calibrated" DMSP satellite data for 1996/1997 are available online here. This DMSP data is special because it includes some observations taken when the satellite's gain setting is reduced so that urban cores are not saturated.

The light pollution model used by Cinzano et al was derived by Roy Garstang in the following two articles:

-Garstang, RH: Model for artificial night-sky illumination, Publications of the Astronomical Society of the Pacific, 98 (601): 364-375, Mar 1986
-Garstang, RH: Night-sky brightness at observatories and sites, Publications of the Astronomical Society of the Pacific, 101 (637): 306-329, Mar 1989

I programmed the Garstang model myself using the parameters given in another Cinzano paper.A comparison of the original atlas (top) with the atlas re-calculated by me (bottom) is shown in the attached figure. A higher resolution version of my figure can be found here and a higher resolution version of Cinzano's atlas is here. There's very good agreement between the two, but if you look closely you can see differences. I've looked over my code many times and I do not think I made an error. I give some possible reasons for the discrepancy here. Anyway, the differences in these plots are well within the uncertainties involving the assumptions of the model.

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John Daniels

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #2 on: December 29, 2017, 04:30:42 PM »
The new radiance calibrated satellite data is available here. Like the 1996/1997 data, this data also includes some observations taken when the satellite's gain setting is reduced so that urban cores are not saturated. This data is only available on a map projection that includes the lower 48 states and some surrounding areas (for example, areas in south-central Canada are included but Vancouver, B.C. is cut off).I used the same model in the previous post to calculate a new Atlas of Artificial Night Sky Brightness for the lower 48 states (see attached). The atlas calculated using the original data is on top, while the new atlas calculated using data from Fall 2001 is on the bottom. (As before, higher resolution versions are available here.)Looking at the areas from Virginia south along the Atlantic Coast to Florida and then west along the Gulf coast, you see fairly good agreement in the light pollution derived from these two datasets. If you look in the northern US and Canada, on the other hand, the differences are dramatic. For example, in Minnesota, Wisconsin, Quebec and northern New England you tend to be a full light pollution "zone" darker in the 2001 data. In the 1996/1997 dataset, the only area black in the eastern US is a tiny area in the Boundary Waters Canoe Area in northern Minnesota. In the new dataset, this Minnesota region expands and new black regions appear in upper peninsula of Michigan and northern Maine. The black regions in Montana and Idaho are also much bigger. (You can't compare the maps in northwest Washington because Vancouver is not included in the Fall 2001 data.)

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Chad Shepard

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #3 on: December 31, 2017, 10:56:47 AM »
To better see the differences between the 1996/1997 and 2001 atlases, you could divide the brightness in 2001 by the brightness in 1996/1997. This plot is very noisy. Therefore I have smoothed the results before dividing (and also compare logarithms). The steps are:1) For each atlas, take the logarithm base 3 of the brightness values (except for the black/gray transition, the boundary between all color zones in the Cinzano Atlas is a power of three).2) Smooth the "maps" from (1) (I use a Gaussian kernel with a "standard deviation" of about 18 pixels).3) Subtract the 1996/1997 smoothed "map" from the 2001 smoothed "map".The resulting map is attached. Because I take the difference of log base 3, the maps given the change in light pollution "zone" in going from the 1996/1997 to the 2001 atlas. A value of -1 means that you are about one light pollution "zone" (or "color") darker in the 2001 atlas, while a value of +1 means that you are about one light pollution "zone" (or "color") brighter.As you can see, in Canada and in the northern part of the US, the 2001 map is about one "zone" darker than the 1996/1997 map. Across the southern portion of the US, there tends to be relatively little change in the light pollution "zone". The change in light pollution zone is broadly similar to the snow cover map I showed in the first post, which suggests that snow cover is playing an important role. The differences between the snow and change in zone could be due to any number of things: 1) increase in number of lights over 4-5 years, 2) More foliage on trees in Fall 2001 than Winter 1996 and 1997 (1 and 2 might tend to offset each other), 3) we do not know which of the three periods in 1996/1997 the DSMP satellite had a cloud free overpass (did the satellite 'miss' the week with snow or not?) 4) depth or age of snow (deep and/or new snow is more reflective than a dusting/old snow) 5) growth in natural gas production (for example, western and southwestern Wyoming), 6) inter-calibration issues with the satellites (see here). The last effect would have a uniform effect across the entire map, I believe. Therefore, it cannot account for the huge differences in space that we are seeing.So...Once again, the high resolution maps can be found here. If anyone wants the actual brightness data plotted on the maps, let me know. Thanks for reading and I hope you enjoy the new atlas!-Dave

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Marlon Hilzer

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #4 on: January 04, 2018, 09:45:40 PM »
Very, very interesting, important work.In addition to eliminating snow, you have produced two maps separated in time by 4 years, which may be enough to detect changes due to population and streetlight growth. Have you looked at it from that angle?It's also very helpful to have a map without boundaries overlaid. Most major population centers are on coasts, so the boundaries in Cinzano's released maps tend to blot out most of the interesting detail in places where many of us actually live.Is any data available after 2001?

trimarnado

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #5 on: January 08, 2018, 10:50:06 PM »
On a personal note, this has changed the zone for most of my customary observing spots. For instance, my country home which used to be in the yellow zone is now in the green. My club's observing field, which used to be on the red/orange border is now deep in the orange.Going by the new zones, the correlation between the predicted sky brightness for each zone and my own measurements is much better. So is the correlation between the zones and the Bortle descriptions.This also helps explain why people in the South complain how terrible their red-zone sites are, whereas I have so far said "Oh, the red zone isn't so bad." It's because what I thought was red zone should actually be orange.

charnhoupito

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #6 on: January 09, 2018, 08:16:09 PM »
I wonder what caused the huge increases in southeastern Oregon and southeastern Utah. There's kinda like nothing there in either place. Having just been in southeastern Oregon (Steens Mountain) I can vouch that it's still super-dark.

Bryan Sonian

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #7 on: January 11, 2018, 06:50:08 AM »
Hi Tony,

I don't believe there is more data that is not saturated in the urban cores. The set of all available data products for this satellite can be found  here. There is more recent data if you're not interested in changes in the urban cores.

Detecting population and street light growth is very important but it is probably more difficult to get from this data. The reason is intercalibration. I think I'd expect population/street-lighting growth to be more uniform than the snow effect I emphasized here. If it is more uniform, then it can be hard to disentangle the satellite calibration effect from population/street-lighting growth.

Here are some maps of population change in the US:
http://www.census.go...t/gallery/maps/

There does not appear to be a strong correlation, although the census data is for changes over the last decade.

-Dave

noneanoncrag

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #8 on: January 12, 2018, 10:18:08 AM »
Quote
This also helps explain why people in the South complain how terrible their red-zone sites are, whereas I have so far said "Oh, the red zone isn't so bad." It's because what I thought was red zone should actually be orange.
This was my impression too! What motivated this study was a comparison of sites in different parts of the country (not using measurements--just my impression when deep sky observing). I live in Wisconsin, which doesn't have the best skies, but it seemed like my skies were darker than what the light pollution atlas was saying.

rubnirootcount

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #9 on: January 12, 2018, 10:39:03 AM »
Quote
I wonder what caused the huge increases in southeastern Oregon and southeastern Utah. There's kinda like nothing there in either place. Having just been in southeastern Oregon (Steens Mountain) I can vouch that it's still super-dark.
I noticed this too because these are some of the darkest ares in the lower 48 (I've always wanted to explore Steens Mountain!). You can see few more light sources in the new maps, but I'm not sure what's going on.The differences can be relatively big in the dark areas when looking at the ratio of new to old (or similarly for the difference of logarithms). This is because when you add a little bit of light to almost nothing, the ratio of new to old can be very big even if the difference in brightness is very small. So for the areas with very little light pollution to begin with, a difference rather than ratio of brightness may be a more appropriate metric.-Dave

kocewaffre

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #10 on: January 12, 2018, 07:57:27 PM »
Great job. Thanks.

ecidjapa

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #11 on: January 13, 2018, 01:09:39 PM »
Now all we have to do is to keep that pesky snow away. Any ideas?

togoseera

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #12 on: January 15, 2018, 11:03:02 AM »
I work for a municipality, and this additional analysis is very useful as we continue to evolve our outdoor lighting policies to address light pollution.Thanks.

cicacating

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #13 on: January 15, 2018, 02:31:12 PM »
Quote
Now all we have to do is to keep that pesky snow away.

I was thinking this over, and concluded that while snow certainly increases skyglow, it probably doesn't increase it as much as it affects the satellite data.

Remember, these satellites are <em class="bbc">not measuring skyglow; they're measuring the light that goes out into space, which is just a crude proxy. Insofar as light goes out into space, it's not coming back into your telescope.

For instance, a spotlight shining straight up will send the satellite sensors wild while producing very little skyglow. Turn that same spotlight horizontal, so it's scattering through miles of dirty, ground-level air, and it will produce huge skyglow while being almost invisible to the satellite.

Imagine a city lit with fully shielded, down-pointing streetlights. These are good in every way, producing little skyglow and also almost invisible to the satellite.

Now let's say that there's a fresh snow. All those down-pointing streetlights suddenly become highly visible to the satellite, because most of their light is now reflected upward. But because the light source (the snow) is at ground level, the ability of the light to travel horizontally and generate skyglow is limited.

Andre Stubblefield

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Re: New Light Pollution Atlas w/o Snow Cover
« Reply #14 on: January 20, 2018, 04:50:15 PM »
I think Tony's right.

Within the context of the model used to calculate the light pollution atlas, you can even quantify the relative contributions of reflected light and direct light to the total light pollution. The effect depends on distance from the source.

In the attached plot, I show brightness as a function of distance from a point source of light (purple). The y-axis tells how the light polution zone changes relative to the zone at the origin (more negative implies darker). The red and blue curves show the contribution of reflected and direct to the light polution. Near the source they are comparable, but as one travels further from the source the direct contribution dominates:

At  15km from the source, reflected light = 25% of total
At  50km from the source, reflected light = 19% of total
At 100km from the source, reflected light = 13% of total

These numbers depend on assumptions in the model and so should not be taken as precisely accurate. Nevertheless, I'm sure the basic result that reflected light is most important near the source still holds. So, as Tony stated, the satellite is <strong class="bbc">overestimating[/b] the effect of snow. This is especially true when you are relatively far from major light sources.

-Dave

(Two side comments:
1. This is a log plot, so even though the red plus the blue curve add up to the purple, it is not apparent on the plot.
2. Technically this is not a point source, but a source with a size of about 0.5-1.0km.)

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