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General Astronomy >> Light Pollution

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Starman1
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Reged: 06/24/03

Loc: Los Angeles
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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Reged: 05/05/04

Loc: Great Lakes
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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GlennLeDrew
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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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Reged: 06/24/03

Loc: Los Angeles
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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Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
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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zeldaboy101
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Loc: Maryland
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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Reged: 06/24/03

Loc: Los Angeles
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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Reged: 05/19/06

Loc: Binghamton & Indian Lake NY
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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s58y
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Reged: 12/12/04

Loc: Eastern NY
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
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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
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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
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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
scholastic sledgehammer


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
scholastic sledgehammer


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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