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General Astronomy >> Light Pollution

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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
New Light Pollution Atlas w/o Snow Cover
      #3867711 - 06/16/10 04:07 AM Attachment (246 downloads)

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 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 1997

The 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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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3867712 - 06/16/10 04:09 AM Attachment (206 downloads)

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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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3867713 - 06/16/10 04:10 AM Attachment (180 downloads)

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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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3867717 - 06/16/10 04:18 AM Attachment (177 downloads)

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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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3867906 - 06/16/10 09:02 AM

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?


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Tony Flanders
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Reged: 05/18/06

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Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3868020 - 06/16/10 10:07 AM

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.


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Tony Flanders
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Reged: 05/18/06

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Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3868031 - 06/16/10 10:13 AM

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.

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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3868276 - 06/16/10 12:20 PM

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.gov/popest/gallery/maps/

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

-Dave


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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3868286 - 06/16/10 12:26 PM

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.


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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3868302 - 06/16/10 12:30 PM

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


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s58y
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Reged: 12/12/04

Loc: Eastern NY
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3868404 - 06/16/10 01:11 PM

Great job. Thanks.

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star drop
Snowed In
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Reged: 02/02/08

Loc: Snow Plop, WNY
Re: New Light Pollution Atlas w/o Snow Cover new [Re: s58y]
      #3868508 - 06/16/10 01:56 PM

Now all we have to do is to keep that pesky snow away. Any ideas?

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Michael Cook
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Reged: 05/30/08

Loc: Newcastle, ON, Canada
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3868596 - 06/16/10 02:35 PM

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.


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Tony Flanders
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: star drop]
      #3868626 - 06/16/10 02:48 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 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.


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DaveL
member


Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3868859 - 06/16/10 04:46 PM Attachment (110 downloads)

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 overestimating 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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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3869052 - 06/16/10 06:20 PM

I've just realized that my post directly above is not really the right way to look at the problem. It does correctly give the relative roles of reflected versus direct light (when there's no snow), but how this translates to the snow problem and the satellite bias is not clear from the above graph. I'll have the revised figure soon, but I have to go.

The short answer is:
when there is snow on the ground the satellite over-estimates the light pollution by a factor of 1.6 near the source, a factor of 3 at 50km, a factor of 4 at 170km etc. (This is if you simply apply the light pollution model without changing the parameter related to the reflectivity of the surface.)

-Dave


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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3870524 - 06/17/10 01:37 PM Attachment (102 downloads)

I now have results for how snow cover affects the satellite compared to how it affects "reality" (i.e. the light pollution model but properly taking into account the reflectivity of snow). The original light polution model has the albedo of the ground as a parameter, so changing this from the default setting to a snow-like setting will give the effect of snow.

The albedo of snow is:
Fresh snow, dry = 0.85
Fresh snow, wet = 0.80
Old snow, dry, clean = 0.70
Old snow, wet, clean = 0.60
Old snow, wet, medium dirty = 0.50
Old snow, wet, heavily dirty = 0.40

I used the new wet snow (0.80) and the old, wet, clean snow (0.60). The effect of snow in "reality" with these parameter settings is given by the purple and blue curves. (In my previous post I used an albedo of 0.85.) The snow effect is largest closest to the source, and decreases to about 25% larger at a distance of 300km.

If the satellite predominantly measures light directed nearly straight up, then it is measuring mostly reflected light. To make the light pollution map from a satellite, you use this "straight up" light as a proxy for the light in all directions. If you assume the wrong surface albedo (which is what you do if you apply an asphalt/concrete albedo to snow covered ground (as in the original atlas)), then you essentially multiply all the light by a certain factor independent of direction. This is unlike reality, where snow only changes the reflected component. The factor that you wrongly apply to all light is simply the albedo of snow divided by default, asphalt/concrete albedo (=0.15). The effect of snow wrongly estimated from the satellite is shown by the red and orange lines.

So as Tony said, the original light pollution atlas is not very useful even when there is snow on the ground. For example, if the snow albedo is 0.6, the orignal light pollution atlas increases the light pollution by a factor of 4 everywhere (= 1.3 light zones). In reality, the effect of snow is more like the blue curve, which is a factor of 1.7 at 30km (= 0.5 light zones) and smaller for greater distnces. If, on the other hand, you do take into account the snow albedo, then the satellite data shoud correctly recover the blue and purple curves.

-Dave

(For a point source, the blue and purple curves would go up to the orange and red at very close distances. The source here has a finite size.)


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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3870600 - 06/17/10 02:20 PM Attachment (111 downloads)

Here I compare Tony's Sky Quality Meter data in the Boston area with the light pollution atlas using the 1996/1997 data and the 2001 data. Tony's data is from his blog entry entitled the Ground Truth for the Light Pollution Atlas.

The x-axis in the figure is the distance from Cambridge Common and the y-axis is the brightness in magnitudes per square acrsec (bigger means darker). The black line is the actual measurements taken by Tony with a Sky Quality Meter (SQM-L). The red and the blue lines are from my calculations of the atlas using the 1996/1997 data (which is contaminated with snow cover) and the new 2001 data.

As Tony mentioned above, the new atlas based on the 2001 data is in better agreement with real observations. Nevertheless, the 2001 atlas still has a tendency to over-estimate the brightness (i.e. the blue line is too low compared to the black line). Also note the jumps and plateaus in the real data which are not in the atlases (see Tony's blog).

The discrepency could be caused by a variety of different things: 1) the day Tony took measurements could have been especially clear of aerosols. Therefore, if we change the aerosol parameter in the model, we could reproduce the data, 2) other parameters in the model just need to be "tuned", 3) the assumed relationship between the satellite brightness and the light sources needs to be modified or 4) something is more fundamentally wrong with the model.

-Dave


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Tony Flanders
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Reged: 05/18/06

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Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3870743 - 06/17/10 03:52 PM

Quote:

The discrepancy could be caused by a variety of different things: 1) the day Tony took measurements could have been especially clear of aerosols ... 2) other parameters in the model just need to be "tuned", 3) the assumed relationship between the satellite brightness and the light sources needs to be modified or 4) something is more fundamentally wrong with the model.




Yet another possibility -- something is fundamentally wrong with the SQM-L. Remember, this is not a sophisticated scientific instrument. It's a clever, cheap gizmo that has been tuned to some semblance of scientific accuracy by seat-of-the-pants methods. I think that two different SQM-L measurements are comparable to each other, but you can't necessarily expect them to agree with results from scientific istruments. Among other issues, the SQM-L is well-known not to have the same spectral response as a CCD fitted with a V-band filter.


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Phillip Creed
Idiot Seeking Village
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Reged: 07/25/06

Loc: Canton, OH
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3871094 - 06/17/10 07:10 PM

Dave,

First off,



That's a lot of hard work that went into this. This is a long-overdue refinement of the light pollution map. Regardless of where your new model still has lingering issues, one thing's certain, at least IMO--it's more accurate than the previous one.

I frequent many sites in Ohio and noticed that a lot them in the "yellow" zones were actually quite dark. The areas around Mohican State Park and, in NW Ohio, the area around Harrison Lake always seemed like they were darker than their "yellow zone" would indicate based on observations I'd had there.

Many sites in SE Ohio ("green" on the old map) were good enough to rival some "blue" zone sites in western PA that I'd previously used. According to the new data, some of these sites, notably the Zaleski State Forest / Lake Hope State Park area are borderline blue/green, and I've found this matches previous observations of sky darkness.

Further refinement, as you've already pointed out, is necessary. But a job well done, nonetheless. The 2001 data and accompanying image much more closely resemble sky conditions I've seen in the areas that I've frequented.

One thing I did note was that Cherry Springs, according to the 2001 data, has gone from "blue" to "dark gray". I'd have to heartily concur. I use a "dark gray" site frequently, Calhoun County Park, WV, and on nights of similar transparency, they're very close.

Clear Skies,
Phil


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George N
Carpal Tunnel
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Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3872177 - 06/18/10 12:32 PM

The new map looks more accurate to what I’ve experienced living in rural NY, with a camp in the central Adirondacks, plus frequent trips to Cherry Springs and once-a-year to Stellafane.

Snow is bad news for observational astronomy! I’ve been getting about .3 to .5 brighter SQM readings with snow cover, and qualitatively the sky just looks brighter – not to mention the annoying light from the ground while trying to observe.


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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: George N]
      #3873391 - 06/19/10 02:02 AM

I'm posting because I have discovered a relatively minor problem with the 2001 DSMP satellite data used to make the new atlas. Apparently, in some locations the light sources in the 2001 data are displaced from where they should be. The maximum displacement is about 4 to 5 km in the east/west direction and 2 to 3 km in the north/south direction. I have determined the origin of this problem is the DSMP satellite data online because there are also independent population and land-use data in the same location as the DSMP satellite data. These are on the same projection/grid and there is no horizontal displacement in these datasets. I dicovered this problem by overlaying the maps in google earth and zooming in on small towns.

I think I might be able to come up with an algorithm to fix the problem.

This doesn't affect the what we have talking about above, and you would never notice it if you didn't zoom in on google maps. Nevertheless I'm sorry I didn't check things out more closely before I posted.

-Dave


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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3875110 - 06/20/10 05:21 AM

I've come up with an algorithm to fix the displacement errors in the 2001 light data. The new data is now available on the webpage. The fix is not perfect but it's definitely better than before. Most people will not notice, unless you overlay the maps in google earth and zoom way in.

The adjustment is not uniform across the map and some area have no adjustment. The biggest adjustment is about 5 pixels (the size of a pixel is 1/120 degrees). I'll add more detail on the adjustment on the web site soon.

-Dave


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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3876804 - 06/21/10 02:17 AM

You can read about the small position errors in the 2001 light data and how I corrected for them here. None of this effects the brightness of the lights, etc.

The corrections are small (I thought about reposting the figures in this thread, but I can't see a difference after I apply the corrections). You will only notice the improvement when you compare the light pollution atlas to small towns when zoomed in in Google Earth or something similar.

-Dave


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csa/montana
Den Mama
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Reged: 05/14/05

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Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3877174 - 06/21/10 10:32 AM

Dave, thanks for all the work you have put into this! It is greatly appreciated!

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DaveL
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Reged: 09/30/09

Loc: Madison, WI, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: csa/montana]
      #3879058 - 06/22/10 05:40 AM

Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. The image should be semi-transparent so that you can see the info from google maps underneath (I'm hoping all web browsers show it the same way).

-Dave


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George N
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Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3879770 - 06/22/10 01:49 PM

Quote:

Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. ...-Dave




It works great Dave, and at least in my area of NY/PA it makes more sense than the old map.


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Matt Lindsey
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3891603 - 06/28/10 07:48 PM

Thank you Dave for this valuable tool! I used to observe around Wildcat Mountain S.P. in Wisconsin and the skies seemed much darker than a green zone. Indeed, it's actually in the GRAY! Likewise, my dark observing sites in Maryland just got darker.

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DaveL
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Matt Lindsey]
      #3892118 - 06/29/10 12:14 AM

Your welcome!

I usually observe in southwest Wisconsin (although not at Wildcat Mountain unless I'm camping there too). It turns out that all my green dark sky sites are actually blue and my blue dark sky sites are gray. These new colors seem to make more sense to me when I compare my sites to ones further south.

-Dave


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mountain monk
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #3895592 - 06/30/10 05:55 PM

Thanks for all great work. Glad to see that my neck of the woods--NW Wyoming/Jackson Hole is getting darker!

Dark skies.

mm


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mountain monk
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: mountain monk]
      #3895650 - 06/30/10 06:29 PM

Really thanks for your work. I was in green, now I am in blue and gray is 15 minutes away. Black is 30-40 minutes. You made my day.

Dark skies.

mm


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Starman1
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: mountain monk]
      #3923479 - 07/15/10 03:23 PM

The issue of sky brightness' lack of correlation with the satellite light maps is well-known.
In the southwest, my neck of the woods, light pollution seems to relate extremely well to humidity in the lower atmosphere. We have the greatest amount of dust in the lower atmosphere when it is dry and windy, yet the brightest skies are when the humidity near the ground creeps up.
So a site that is in, for instance, a red zone, might get a reading of 16.8 when the humidity is high, and a reading of 17.5 when the humidity is low.
This idea corresponds well to what I see at my home--Polaris is an averted vision object when the sky is very hazy and humidity is 80% or more, while magnitude 4 stars are easily visible when the humidity is near zero.

It's possible it is not solely humidity but the interaction between airborne water vapor and smog or dust. But I have taken measurements at my home for over 5 years and the transparency of the sky varies right along with the humidity. Air SHOULD be just as transparent with water vapor in it, but it's not.

That factor seems to make any sort of absolute light pollution map unlikely unless you have annual averages for each site correlated to a color map.


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Dan G
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3924133 - 07/15/10 09:20 PM

Great work. Thank you.

Dan in NY


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Tony Flanders
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3924721 - 07/16/10 07:22 AM

Quote:

The issue of sky brightness' lack of correlation with the satellite light maps is well-known ... That factor seems to make any sort of absolute light pollution map unlikely unless you have annual averages for each site correlated to a color map.




Oh sure, but that's a separate issue. Obviously you're going to get significant variation from one night to another, from one time of night to another, and (in places with deciduous trees and/or snow) from one season to another.

But one would hope that on average, a red zone in one part of the world would be roughly -- vaguely -- equivalent to a red zone in another part of the world.

However, it now seems pretty clear that this is not true. In the original Light Pollution Atlas, a red zone in a band stretching from New England and the Maritimes across the Upper Midwest seems to be on average roughly equivalent to an orange zone in a band stretching from CA to the Old South.


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Sarkikos
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3925266 - 07/16/10 12:57 PM

So... any idea if and when Clear Sky Chart will be updated with the new values?

Mike


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Sarkikos
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3925280 - 07/16/10 01:02 PM

I have noticed that at my dark sky site, often the sky is more transparent when it is very dewy at groundlevel, and less transparent when it is not so dewy. It seems as if when it is less dewy, the moisture is still hovering above in the atmosphere, but when the ground is very dewy, the moisture has settled down and the sky has cleared. So for transparent skies, heavy dew can be a good thing. Am I understanding this correctly?

Mike


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Sarkikos
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3925317 - 07/16/10 01:20 PM

My dark sky site on the Eastern Shore of Maryland is a yellow zone according to the Clear Sky Chart. However, I have noticed that at the dark site, the Milky Way has a definite bulge into Ophiuchus. In fact, on relatively good nights I can follow the MW down to the vicinity of Cebalrai and up to Sabik. According to CSC, in a yellow site there are "some dark lanes in milkyway but no bulge into Ophiuchcus," in a green site "Milkyway shows much dark lane structure with beginnings of faint bulge into Ophiuchus," while in a blue zone the "Milky way shows bulge into Ophiuchus." There is definitely something wrong here.

Mike


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Tony Flanders
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3925355 - 07/16/10 01:40 PM

Quote:

I have noticed that at my dark sky site, often the sky is more transparent when it is very dewy at groundlevel, and less transparent when it is not so dewy.




I suspect you have that precisely backward; it's the good transparency that causes the dew.

Dew forms when radiative cooling cools surfaces below the ambient air temperature. A surface at the same temperature as the air can get wet from fog, but it can't form dew, properly speaking.

Water vapor in all its forms is quite opaque to infrared, which is why there's so much interest in putting infrared scopes in Antarctica and atop high, dry mountains. So haze is a very good inhibitor of radiative cooling. Conversely, very clear nights maximize radiant cooling and promote dew formation.


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Sarkikos
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3926424 - 07/16/10 09:39 PM

So my observation is correct, that transparency does correlate with dewing, but my understanding of the causal relationship is incorrect? I just assumed that dew does not spontaneously generate as in the old beliefs about flies and meat, or mice and wheat, that there must be some reason behind it, and that it had something to do with the transparency of the sky.

Mike


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Sarkikos
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3927203 - 07/17/10 10:12 AM

Don,

Quote:

In the southwest, my neck of the woods, light pollution seems to relate extremely well to humidity in the lower atmosphere. We have the greatest amount of dust in the lower atmosphere when it is dry and windy, yet the brightest skies are when the humidity near the ground creeps up.
So a site that is in, for instance, a red zone, might get a reading of 16.8 when the humidity is high, and a reading of 17.5 when the humidity is low.




In Maryland, I have the exact opposite experience. At my dark zone site, when the "humidity near the ground creeps up" the sky tends to be more transparent. Now, when you say "humidity near the ground creeps up," do you mean dewing? When we have heavy dew - and a prediction of high humidity - the skies are usually more transparent. For instance, when we have heavy dew there are galaxies that I can see in my 10" Newt that I cannot see when we do not experience as much dew. As long as I keep the optics free from dewing, I will see dimmer objects on a dewy night. At least, so far, that is my experience.

Mike


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Starman1
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3927253 - 07/17/10 10:43 AM

No, I don't mean dew. I find the clearest skies then, too.
But I mean the troposphere as "near the ground" (as opposed to stratosphere)--especially the first 10,000 feet of it.

In my neck of the woods we get a saturated atmospheric haze that causes the sky to appear silver. The humidity is very high, and the temperature is above dewpoint. It's like New Orleans in the summer, when you can see about 2 blocks down the street before the air becomes opaque.

We have this situation in LA from the ocean in about 5 to 8 miles. At the beach the air becomes opaque. Further inland, the air is less opaque but the number of stars in the sky reduces to a mere handful.

Yet, when the Moon comes up, the air appears clear, yet the transparency of the first 4,000 to 6,000 feet of air is very poor.

If the temperature drops below dewpoint, a lot of this moisture drops to the ground and the air becomes more transparent. But it still never reaches the transparency of the Fall, when hot, dry, air blows off the desert and gives us 100 degree temperatures with 10 percent humidity.
At that time, there is a lot of dust in the air, but the skies are superlatively clear. Terrible seeing conditions, but great transparency.

I guess what I'm saying is that there is a state between high humidity and fog. And that is a near-continuous state near my home. It makes the skies brighter, by about a full magnitude, than they are when the air is dry, because the light of the city causes the water vapor haze to reflect and scatter the city lights.

The key here is that you can still see everything as if it was clear, and looking up shows reduced transparency, but not clouds or fog. When you are above it, and looking down into the city, you can see this reduced transparency below you. And it's not smog, because the parts of the city with the worst smog never seem to get this and the air appears more transparent there.

So, it's not fog per se, but high humidity haze. I grew up with that in the midwest, where the summer skies were so filled with moisture (near 100% humidity all the time) that visibility for pilots of small planes was reduced from 20 miles to sometimes less than 5.

What do I call that except "humidity near the ground"?

Now I've been keeping weather records here for many years. The upper atmosphere (10,000 to 50,000 feet) has become a LOT more cloudy in recent years. It's in the form of cirrus or undefined cloud types, but it has reduced transparency at my high altitude site (8350'/2550m) a good portion of the year. Since the national weather service says the stratosphere has been getting drier over the last decade, the phenomenon may be a local one, but southern California definitely has more high clouds now than it did in the '90s, which, in turn, had more clouds than the '80s.
A lot of it is due to jet flights over the area, which has steadily risen over the same two decades. And some of it may be due to higher temperatures causing more evaporation from surface water features.

Whatever the causes, between the high altitude transparency reduction and the low-altitude poor transparency, viewing has become more challenging for the SoCal viewer.


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half meter
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #3932110 - 07/19/10 09:09 PM

Sure, here in Michigan, the most transparent nights have low humidity. In the dead of winter, with the temps below 0°F, the air is extremely dry and it can be very hard to even identify the Big Dipper because all the surrounding stars seem almost as bright. And in the summer, when heavy dew appears an hour after sundown, it's a sign that transparency will be good.

However, if ground fog appears some time in the evening, then the prediction is for great seeing (and lousy transparency.) It must have to do with the rate of temperature decline; the appearance of fog instead of (or in addition to) dew means the generally large day/night temperature swings experienced in Michigan will not be as large. This has a doubly good effect: large optics have a chance to equalize in temperature to the surrounding air, and the air temperature itself is more or less stable.

One of the special treats we sometimes enjoy during a string of clear nights in the Midwest is to get a night of fog (meaning great seeing), and get the next night drenched in dew (for great transparency), or vice versa. We plan our observing targets accordingly!


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Sarkikos
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: half meter]
      #3932125 - 07/19/10 09:22 PM

I always bring a draw-bag full of towels to a dark site. A couple big ones to drape over my table, little ones to cover the atlases and books, and still more to wipe off my gear before I load it back into the mini-van. I also bring tarps to lay the OTA, finders and other stuff on the ground while I cover them with towels. After awhile, the towels have absorbed almost all the dew. A mini hair-dryer hooked to my Orion power supply also helps. All this despite the fact that I have dew shields and dew-busters to keep the optics dry all night long.

...now back to the thread...


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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Sarkikos]
      #3932417 - 07/19/10 11:35 PM

Quote:

If the temperature drops below dewpoint, a lot of this moisture drops to the ground and the air becomes more transparent.




Normally, if the air temperature drops to (or below) the dewpoint, mist, fog or cloud forms. But if the air remains clear, or the fog/cloud isn't precipitating, how does the moisture "drop to the ground"? Dew forms 'spontaneously', if you will, when the air is cooled upon contact with a surface at or below the dew point.

If there is no wind to stir up the air, a temperature inversion develops, becoming deeper as the night progresses. With continued radiative cooling a ground fog may form, especially in low-lying areas where denser, cooler air spills into the hollows, nooks and crannies in the landscape.

But the relative humidity is not lowered as the dew 'leaves the air' and condenses on the ground. The continued cooling serves to keep the air at high (at least very near 100%) humidity. Moreover, this process occurs within say, tens or perhaps hundreds of feet of the surface--hardly enough to materially affect transparency.

It must be said that of perhaps equal importance as relative humidity is absolute humidity. Continued cooling will remove some water from the air as dew, but it would take a good 10C (near 20F) of cooling to make much of a difference. But the matter is moot, for the same reason pointed out above; this is occuring in the very lowest level of the atmosphere, quite near the ground. Above the temperature inversion there is practically no change at all.

As already noted by our perspicacious Tony F., the formation of dew is more an indicator of an already transparent airmass, not a cause. It could seem that transparency is improving because of the dewing, but for those who suffer some degree of light pollution, it's probably more likely that it's the extinguishing of city lights later at night which contributes to the improvement.


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Starman1
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: GlennLeDrew]
      #3932447 - 07/20/10 12:00 AM

I observe at high altitude, where a 30-40 degree swing from 3pm to 10pm is common. Shortly after dark, when the temperature is falling rapidly, relative humidity reaches 80-90 percent and dew forms on everything--especially anything metal facing the sky.
What happens then is that the atmospheric layer containing all the moisture drops, as it cools, below the altitude where I am. The humidity can drop to 10 percent and the dew on everything evaporates quickly. The dry air is considerably more transparent and the night gets darker. The effects of light scatter in the higher levels of the atmosphere definitely reduces. And the band of the atmosphere between the horizon and 45 degrees up definitely grows more transparent. After the dew is gone, it's not uncommon to see sparks everywhere. If it's not the reduction in atmospheric water vapor that increases the transparency--especially between the horizon and 45 degrees--then I'm at a loss to explain it, because it happens in directions where there are no cities and no light pollution.
Going down in the morning, you drive into fog--the fog that settles in the valleys and the fog that indicates still air.

I've seen this cycle repeat over lots of years. Enough to see a strong correlation between low humidity and clarity.


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Tony Flanders
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: GlennLeDrew]
      #3933009 - 07/20/10 10:30 AM

Quote:

It could seem that transparency is improving because of the dewing, but for those who suffer some degree of light pollution, it's probably more likely that it's the extinguishing of city lights later at night which contributes to the improvement.




Perhaps. But you should remember that Don is observing near the California coast, which has its own very special weather patterns. Both on a seasonal basis and a daily basis, hot weather inland draws cool, moist air from the Pacific over the coast and the adjoining hills/mountains, while (relatively) cool weather inland pushes the dry interior air back out toward the ocean. So to some extent, the hotter it is in the desert, the cooler it is in the big coastal cities.

There are all kinds of variations on the underlying theme, but the basic pattern is for haze/fog to form in the afternoon, continuing on through evening, and then dissipating as the air flow reverses late at night.


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GlennLeDrew
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #3934028 - 07/20/10 07:06 PM

Don (and Tony),
I wasn't aware of your high elevation status--perhaps because of inattention on my part. My comments were more intended for the majority of folks who happen to crowd together onto the lowlands. You 'mountain men' certainly can have rather different weather conditions.


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George N
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: GlennLeDrew]
      #3934285 - 07/20/10 09:29 PM

Where I live in rural south/central NY 90% of the people live in the Susquehanna River valley. I live and observe (home and several other places) about 1,200 feet higher. It’s not unusual for the river valley to fog up, while it remains clear up where I live or observe. When that happens it gets noticeably darker, with a change of .5 or more in SQM readings. The fog holds the light down in the valley where most of the lights are. I think the same effect happens at both Stellafane and Cherry Springs in PA, where the observing areas are higher than the population centers (so to speak – Carter Camp PA near Cherry Springs has a population of 6 I believe, the other two towns consist of a few square block area).

….but anyway…. From my experience the new ‘revised, snow-free’ LP maps are more accurate in the Northeast, at least in a general sense.


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Re: New Light Pollution Atlas w/o Snow Cover new [Re: George N]
      #5036743 - 01/25/12 09:06 AM

Does anyone happen to know the magnitude breakdown for the colors that Dave is using in his revised image? He multiple shades of green now, whereas the clear sky chart's light pollution map only utilized a single shade.

I've contacted Dave directly but he hasn't responded to me.


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Starman1
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: zeldaboy101]
      #5037032 - 01/25/12 12:07 PM

Look at the bottom of this page:
http://cleardarksky.com/lp/MtPinoslp.html?Mn=light%20pollution
There are some issues with the correspondence of SQM magnitude readings and the color map.
And there are some issues with the accuracy of the data for 2012.
For instance, my home is in a red area on the map, which should be an SQM reading of 18.38 to 19.50 yet the very best reading I've ever gotten at my house is 17.88. By those standards, I should be in a white zone.
So, either my area has gotten worse and the map is dated or there is not a correlation between the color zones and SQM readings shown on the linked page.
I think the inaccuracy is in the numerical correspondence of color zone to SQM reading.
I regularly observe in a couple blue zones, gray zone, and green zone, yet none of the sites produces the SQM readings that correspond to the chart.
I get 21.3-21.5 in my blue zones, 20.8-21.0 in my green zone, and 21.7-21.8 in my gray zone.

The point I am making is this: the color zones on the LP map can be important as "relative" differences, i.e. a blue zone is darker than a green zone. But they are NOT pinned down to absolute differences, i.e.neither the SQM reading nor the NELM magnitude corresponds exactly to what is seen at those zones.

Using the color zones as a way to find a darker site is a valid use of the tool. But anticipating a particular SQM reading at that site will lead you to anticipate darker skies than you may actually find.

Plus, any site that is not a gray or black zone will have light "bubbles" in some directions in the sky, making the site different than a mere zenithal reading would imply. For instance, Cottonwood Springs campground in Joshua Tree National Park typically yields 21.3-21.6 zenithal readings on the SQM, is in a "blue" zone, yet has a light dome to the SW (Palm Springs and Indio) yielding a sky brightness of about magnitude 19 at a 45 degree altitude (an object "disaappears" as it sinks into that light dome), yet also produces a magnitude 21.7 reading ar 45 degrees altitude in the NE! And this type of localized light pollution reading is common at sites with light domes in one direction or another.

So take the color maps with a grain of salt. They are very good RELATIVE predictors and that is about all.


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George N
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #5037687 - 01/25/12 06:31 PM

Quote:

.......For instance, my home is in a red area on the map, which should be an SQM reading of 18.38 to 19.50 yet the very best reading I've ever gotten at my house is 17.88. .......
So take the color maps with a grain of salt. They are very good RELATIVE predictors and that is about all.




My SQM results are consistent with yours.

As far as I can tell the LP maps (new or old) for the USA do not adjust for terrain shadowing. A mountain could block one’s view of a light dome. Also, at two of my observing locations a very high percentage of the population and lights are in river valleys that fog up often. That results in a considerable improvement in the LP.

I think the on-line LP maps are a great asset as a “first estimate” of where to go to find darker skies. Beyond that, observers need to use their eyeballs or an SQM (or maybe even a DSLR) to find the best observing location.

Smart astro home buyers always check potential purchases on a clear dark night before buying.


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Tony Flanders
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #5038538 - 01/26/12 08:11 AM

Quote:

For instance, my home is in a red area on the map, which should be an SQM reading of 18.38 to 19.50 yet the very best reading I've ever gotten at my house is 17.88.




Odd! My current favorite local spot, deep inside the white zone according to the original Atlas, and still comfortably inside at according to the 2001 map, runs around SQM = 18.5 on a typical good summer night, and at least 18.0 even when there's snow on the ground.

My favorite mid-suburban site, in the red zone according to both maps, runs around SQM = 19.2.

I would say that in the 2001, snow-correct maps, my readings correlate decently well with the predictions.

I wonder if Los Angeles's famous air pollution is the culprit in your case. Or perhaps you're in a local pocket of light pollution too small for the method to pick up, or your area has really changed a lot in the last decade. You would know it if the last of these is the answer.

It's also possible that L.A. registered inappropriately dark because of said air pollution, which holds the light down toward the ground and lets less of it reach the satellites.


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Starman1
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Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #5038709 - 01/26/12 10:06 AM

Tony,

I live about 5 miles from the ocean as the crow flies, in West LA. That is in the "coastal marine" climatic zone--we're covered by "marine layer" fog about half of the nights in a year.

If the satellites measured the brightness of LA's lights during the season of the marine layer, the brightness of LA's skies would have been underestimated. At Mt. Pinos (about 100 miles away from LA and 8350'), I measured a 21.89 on the SQM during one particularly dense marine layer event, even though that site is typically 21.3-21.5.

But on the nights when the skies are very clear, the relative humidity is still quite high in LA and the skies are not really very transparent. The skies, in those conditions, glow orange at night--especially in the direction of the center of the city. I get typical SQM readings around 17.5-17.7.

[one night, doing a house call in Hollywood, the sky never got darker than mid twilight and only first magnitude objects were visible at all! His night sky was blue.]

On nights with a slight haze, I get readings of magnitude 16.5-16.9 on the sky brightness. The entire sky has a bright silver sheen that obscures even first magnitude stars until they're well off the horizon, and Polaris is only seen with averted vision.

In the summer, during an inversion layer event, when the smog level is at its highest, the air is usually quite dry and I get readings in the mid 17s for the SQM.

So the factor that seems to influence the readings more than any other is relative humidity/water vapor haze in the air.

But, more to the point, the SQM readings on the Clear Sky Chart don't match any of my observing sites, either, whether desert or mountain.

So either all the light pollution zone colors are wrong in SoCal(a possibility) or the SQM readings don't match the color zones (more likely).


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Re: New Light Pollution Atlas w/o Snow Cover new [Re: George N]
      #5045221 - 01/29/12 11:54 PM

I've also noticed that the color zones in the map are optimistic. My Catskills imaging location is in the blue zone (on both the old and new maps), so the SQM reading should be 21.69 or better.

The only time I ever get 21.7+ on the SQM-L is when it's murky, and some of the distant high altitude light pollution gets absorbed by the murk (just guessing). When it's cloudy, the SQM-L can read 22.00+

On non-murky clear nights the average SQM-L reading is 21.4 to 21.5, which is right for a green zone. In the spring, when no milky way is around at all, the SQM-L might read 21.65 on rare occasions. When the milky way is overhead in the summer, the SQM-L has problems, of course.


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Illinois
Carpal Tunnel


Reged: 12/18/06

Loc: near Dixon, Illinois USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: DaveL]
      #5046439 - 01/30/12 06:00 PM

Quote:

Sure, no problem!

I've now added a link to the new atlas as an overlay in Google Maps. You can also access this from the main webpage. The image should be semi-transparent so that you can see the info from google maps underneath (I'm hoping all web browsers show it the same way).

-Dave




My parents house near Dixon in Illinois is GREEN. I believe that is possible when the night is perfect clear I see milky way very easy and one time I woke up at 3 in the morning in late summer that I can see M33 up high in the sky easily with my eyes is might be blue zone. Its rare and seem little darker is at the between 2 and 4 in the morning! Thanks....great job!


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Tony Flanders
Postmaster
*****

Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: George N]
      #5049549 - 02/01/12 10:24 AM

Quote:

As far as I can tell the LP maps (new or old) for the USA do not adjust for terrain shadowing. A mountain could block one’s view of a light dome.




Yes, that fact is stated very clearly in the original Light Pollution Atlas.


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zeldaboy101
Carpal Tunnel
*****

Reged: 03/13/04

Loc: Maryland
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Tony Flanders]
      #5065461 - 02/10/12 01:22 PM

Thanks for the input guys, looks like i'll need to make it clear that these are only relative estimates.

I'm attending the Towson University GIS (mapping software) Conference in March and will be making a map for their map competition. It'll use a light pollution image that's cut out to only show the mid atlantic states (which is the focus of the conference) and will show things like state outlines, a couple major cities labeled, and the main topic I decide on.

It's most likely going to be something like "darkest state and national parks in the mid atlantic that are worthy of IDA Dark Sky Park designations." The only one currently is cherry springs but spruce knob is just as dark and there's some other really dark areas that may have state or national parks with particularly dark skies. It might even be worth nothing parks that might want to add a night sky program sorta like a couple mid west parks have!

As far as I can tell, Dave L doesn't list any kind of copyright to his image, and maybe he can't since it's his manipulation of government supplied data originally? I've contacted him directly before but haven't gotten a response. I plan on giving him credit if I can use his image without issues, otherwise i'd have to use the older LP map.


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vsteblina
sage


Reged: 11/05/07

Loc: Wenatchee, Washington
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Starman1]
      #5070377 - 02/13/12 12:30 PM

Quote:



I live about 5 miles from the ocean as the crow flies, in West LA. That is in the "coastal marine" climatic zone--we're covered by "marine layer" fog about half of the nights in a year.





In Wenatchee, we get high humidity in the winter and it absolutely destroys the skies. In summer, with humidity around 10% the skies are much better.

My cabin at 3000 feet is many times above the winter fog and when the lights are trapped below it is totally darik.

So it is a two edged sword. In town, winter observing is a pretty bad, but less than 30 minutes away at the meadow EVEN with high humidity, but zero lights the skies reamin very dark.

Snow on the ground and high humidity.....forget it.


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audioaficionado
professor emeritus


Reged: 05/24/12

Loc: Medford, Orygun, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: vsteblina]
      #5265276 - 06/10/12 08:04 PM

Thanx for the links and maps. Looks like I'd have to drive 110 miles East on the Winnemucca highway in Southern Oregon to get to the nearest black area. Otherwise there are some nice gray areas a lot closer to my home in the orange-red area of West Medford, Oregon. I've noticed when it does rarely snow everything is a lot brighter at night. That Google maps overlay is awesome.

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audioaficionado
professor emeritus


Reged: 05/24/12

Loc: Medford, Orygun, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: audioaficionado]
      #5272945 - 06/15/12 02:24 PM

"New 2001 light pollution map shown on the original Alber's equal-area conic map projection. (This map also has an expanded color scale.) "
https://sites.google.com/site/3davel/home/light-pollution/more/image-1

This map overlaid in a Google Earth map would be an awesome master map to find that special dark sky location with the roads that go there.


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cam1936
sage


Reged: 08/01/08

Loc: Alberta, Canada
Re: New Light Pollution Atlas w/o Snow Cover new [Re: audioaficionado]
      #5275793 - 06/17/12 03:16 PM

Quote:

"New 2001 light pollution map shown on the original Alber's equal-area conic map projection. (This map also has an expanded color scale.) "
https://sites.google.com/site/3davel/home/light-pollution/more/image-1

This map overlaid in a Google Earth map would be an awesome master map to find that special dark sky location with the roads that go there.




I'd rather have it extend further north! Does anyone know of a link to the 2001 data that goes further north than the 50th?


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Phillip Creed
Idiot Seeking Village
*****

Reged: 07/25/06

Loc: Canton, OH
Re: New Light Pollution Atlas w/o Snow Cover new [Re: cam1936]
      #5299974 - 07/02/12 10:54 PM

One issue I've had with this site recently is that it won't display the map without either Google Chrome or IE-9 warning the site's content is not secure. Hopefully this gets fixed soon.

Clear Skies,
Phil


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audioaficionado
professor emeritus


Reged: 05/24/12

Loc: Medford, Orygun, USA
Re: New Light Pollution Atlas w/o Snow Cover new [Re: Phillip Creed]
      #5300061 - 07/03/12 12:31 AM

I haven't had that problem with Firefox.

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