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Eastern U.S. Summertime Haze Trends

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#1 Phillip Creed

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Posted 08 May 2013 - 06:01 PM

As someone who's been sentenced--ERRRRRR--used to living in NE Ohio all these years, the elements certainly make things challenging for stargazers. The colder months are dreadfully cloudy due to influence of Lake Erie.

The warmer months? They're not too bad. But with warmer temps comes another enemy for deep-sky observers, and that's haze.

(NOTE--by "haze", I'm referring to aerosols in an otherwise clear sky. Cirrus is often confused with haze (sometimes called "high haze" or "high-level haze", but that's a cloud issue, not an issue with the transparency of the unsaturated air itself)

Up until about 2009, I lost more nights for DSO work due to haze instead of clouds in the summer months. Recent EPA regulations (as well as attempted regs that were then thrown out in court, then modified, thrown out again--it's...complicated), combined with low natural gas prices, have cut coal consumption and sulfur dioxide emissions from power plants have fallen by more than half since 2005.

That's key because sulfates are hygroscopic. Under moist environments, they are very efficient at scattering light. This is the misty summertime haze one sees on humid days in the Eastern U.S. that washes out the blueness of an otherwise-clear sky and renders a good portion of DSO's off-limits, even from dark skies.

Since 2010, I've noticed a significant drop in the number of nights in NE Ohio that have been compromised by haze. I've actually had a few nights with 70F dewpoints that were suitable for DSO work; that NEVER happened even 5 years ago.

I don't know if the other parts of the Eastern U.S. have noticed the difference. In Ohio, where a lot of this pollution is from, the reduction in our number of hazy nights is not surprising. I'm just curious how others in the Eastern U.S. perceive haze trends to be going.

The only hard data I could find regarding haze in the Eastern U.S. is from Goddard Space Flight Center:

http://aeronet.gsfc..../GSFC_500.html?

The key stat is the Aerosol Optical Thickness (often labeled with the Greek Letter "tau") @ 500-nm, which corresponds closely to the 510-nm peak of human night vision. Converting AOT to magnitudes/air-mass is simple--for any given wavelength, multiply AOT by 1.09. So if your AOT @ 500-nm is 0.20, you're losing about 0.22-mag/air-mass to aerosol, in addition to the "built-in" ~0.15-mag/air-mass you'll lose due to Rayleigh Scattering and ozone adsorption.

According to this data, AOT @ 500-nm in the summertime (Jun-Aug) at GSFC ran about 0.40-0.45 through about 2006, then has hovered below 0.3 the last few years. Not exactly Desert Southwest transparency (AOT ~ 0.1 or less on most nights in that part of the country), but the trend seems to be going in the right direction.

This is just data from one point in Maryland. For all you observers elsewhere in the Eastern U.S., what do you think?

Clear Skies,
Phil

#2 mountain monk

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Posted 08 May 2013 - 07:03 PM

You have my sympathy. Out West the problem is smoke. I lost more nights to smoke last year than to clouds.

Dark skies.

Jack

#3 amicus sidera

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Posted 08 May 2013 - 07:40 PM

This is a very worthy line of inquiry! I wouldn't doubt that the dimunition of emissions from coal-fired plants has reduced haze levels, especially in the lower regions of the atmosphere; any sulfate-laden volcanic aerosols which reach the area in question are almost always located much higher, usually at stratospheric levels. Subtracting known volcanic aerosols from the equation would, of course, be necessary to obtain accurate readings.
Fred

#4 David Pavlich

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Posted 08 May 2013 - 09:40 PM

I'm from Vienna, OH, about 10 miles north of Youngstown. At least you get frontal passages that clear the air for a while. Seeing is rarely top notch because of said frontal passages and the Jet. The Gulf south has great seeing in the late spring and summer because of the stable atmosphere. HOWEVER, the constant haze due to lack of frontal passages means the transparency is the pitts. No matter where you live unless it's West Texas/New Mexico/Arizona, there's going to be compromise.

David

#5 Phillip Creed

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Posted 09 May 2013 - 12:15 AM

This is a very worthy line of inquiry! I wouldn't doubt that the dimunition of emissions from coal-fired plants has reduced haze levels, especially in the lower regions of the atmosphere; any sulfate-laden volcanic aerosols which reach the area in question are almost always located much higher, usually at stratospheric levels. Subtracting known volcanic aerosols from the equation would, of course, be necessary to obtain accurate readings.
Fred


Fred,

Volcanic emissions typically only account for a few hundredths of AOT. Only when there's a major eruption (e.g. Pinatubo in 1991) will volcanic aerosols amount to a 0.10-AOT "spike".

Clear Skies,
Phil

#6 amicus sidera

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Posted 09 May 2013 - 06:33 AM

Thanks for the clarification, Phil! I didn't realize that volcanic aerosols contributed so little to the AOT.

Fred

#7 BrooksObs

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Posted 09 May 2013 - 08:22 AM

Speaking as someone with a very long observing record from a single location I can definitely say that skies in the northeastern U.S. have become decidedly less transparent over the past twenty years relative to those experienced from the 1950's through the first half of the 80's.

Whereas cold/cool front passages were often followed by several consecutive days of pristinely deep blue skies, especially during autumn and winter, many years ago this has not been the case now in a long time in New York State. We are fortunate these days to get a single day perfectly clear following frontal passages and only on very rare occasions, two. Generally, within 24 hours after a front has come by the sky has grown noticeably whitish and a bright and obvious aureole surrounds the Sun. At night one is still capable of observing, but the sky is clearly less transparent than it should be normally.

The change in the situation first came to note in my observing record books around the time of the First Gulf War, when skies were observed to be increasingly milkier in appearance. While I cannot list any specific local meteorological or other atmospheric data, the situation is very obvious from a visual standpoint to the experienced observer. Intensely blue skies have become such a rarity in the East that people will actually comment about the blue of the sky when one happens to occur.

I would venture that at least a percentage of the phenomenon is directly related to jet air traffic/contrails. Certainly, the purest skies I've witnessed in a long time were during the three or so days following 9-11 when virtually all commercial air traffic was suspended. Noteworthy is that there was a scientific study of atmospheric clarity conducted in the western U.S. at this same time which likewise showed a dramatic decline in the atmospheric scattering.

Certainly, there are additional factors, such as the impact of urban sprawl and a distinct alteration in the weather patterns themselves, but I think most observers either underrate or are simply unaware of the long-term effects of air traffic on sky clarity over the decades.

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#8 Phillip Creed

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Posted 10 May 2013 - 03:45 PM

Industrial pollution, even that carried from upwind sources in the Ohio Valley, is much less in the Northeast vs. the Ohio Valley. So if your air is less compromised by industrial emissions, other factors (e.g. jet contrails) might make a larger portion of your aerosol extinction. I'm guessing the Northeast corridor would have *significantly* more air traffic than the Midwest.

I haven't noticed any change in the number of razor-sharp sapphire blue skies here in Ohio. Maybe it's because, historically, our skies around here was so compromised by industrial pollution that we never really *had* any days/nights with crystal-clear skies.

We used to have days around here where it was so hazy, the sky was orange all day long and you could look up at the sun and not instantly blink. Other days would have a washed out, milky haze that severely restricted visibility of distant hilltops.

Nowadays, haze in Ohio has dropped significantly in severity, duration and frequency. The "orange sun at noon" days that regularly happened when the temps breached 90F are gone.

The EPA's Regional Haze Rule calls for the restoration of "natural visibility" by (just in the nick-of-time) 2064. For most of the Eastern U.S., that would be about 11 deciviews of haze on the 10th percentile of visibility days (i.e. the sticky, hot days of July). Standard visual range, if only impeded by Rayleigh scattering, is about 243 miles, or 390 km. Visibility is calculated as:

VR = 243 miles / [e^(dv/10)], where dv = deciviews.

If you run the math, it means that under natural conditions, even a "bad" air day--one of those hot, sticky, air-that-you-can-wear days--in the Eastern U.S. should have about 80-mile visibility.

If your 10th-percentile visibility days (not your worst days; just the rough average of your worst and your 20% worst) are running ~80 miles visual range, the rough equivalent would be having the same sky transparency as Flagstaff, AZ. It's pretty mind-blowing compared to modern conditions, but the night sky's degradation from light pollution vs. its potential is analogous.

True, you can still have haze from forest fire smoke and volcanoes, but the story's the same--air pollution has a *significant* impact on stargazing and most of the visual degradation is *not* natural.

Clear(er) Skies,
Phil

#9 Tony Flanders

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Posted 11 May 2013 - 03:48 AM

Speaking as someone with a very long observing record from a single location I can definitely say that skies in the northeastern U.S. have become decidedly less transparent over the past twenty years relative to those experienced from the 1950's through the first half of the 80's.


Possibly. But anecdotal data counts for nothing in a case like this; I would love to have some actual data.

Intensely blue skies have become such a rarity in the East that people will actually comment about the blue of the sky when one happens to occur.


That's been true for a long time. I first lived in California (for a few months) in 1973, and I vividly remember on coming back East the difference between the routinely blue skies of California and the very rarely deep blue skies of the East. Certainly back then (and still now) I would comment on those rare days as "this sky reminds me of California."

I would venture that at least a percentage of the phenomenon is directly related to jet air traffic/contrails.


Jet traffic has certainly had a major effect on sky quality; no doubt about that. There was a night last week -- I forget which -- when you could actually watch the contrails diffusing across the upper atmosphere into a high-level haze.

But my gut feeling is that this is bimodal; once the upper atmosphere reaches a certain humidity level, the effect is huge, but much of the time the effect is small or none.

If there's a common, low-grade degradation, I'd be more inclined to blame pollutants in the lower atmosphere -- either from industrial activity, coal-burning power plants, or cars.

There has also been a significant increase in forest fires, and a big fire in the West can easily degrade transparency thousands of miles downwind. This effect is seasonal, of course.

I know there have been studies on Mt. Washington in New Hampshire that detected a long-term decrease in visibility and traced this to coal-burning plants in the Midwest. It might be worth looking those up.

#10 star drop

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Posted 11 May 2013 - 07:53 AM


Jet traffic has certainly had a major effect on sky quality; no doubt about that. There was a night last week -- I forget which -- when you could actually watch the contrails diffusing across the upper atmosphere into a high-level haze.

But my gut feeling is that this is bimodal; once the upper atmosphere reaches a certain humidity level, the effect is huge, but much of the time the effect is small or none.

If the effect is small or none how does that explain the fact that last week observers in Cherry Springs, Pa less than fifty miles straight line distant and only 200 feet more elevation had five good nights of observing and I had one night plus a few hours on another night? The contrails smudged up both day and night. There were no fires since we are in the New York State DEC enforced ($500 first offense minimum fine) no burning season due our waiting until everything greens up. Being under a flight lane is depressing to say the least.

#11 BrooksObs

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Posted 11 May 2013 - 12:02 PM

Speaking as someone with a very long observing record from a single location I can definitely say that skies in the northeastern U.S. have become decidedly less transparent over the past twenty years relative to those experienced from the 1950's through the first half of the 80's.


Possibly. But anecdotal data counts for nothing in a case like this; I would love to have some actual data.



Unfortunately, this is the naive outlook of so many of those lacking knowledge of the basis of the available meteorological data. The fact is that one will not find documentation that addresses atmospheric conditions over regions as might directly relate to visibility in the sense we are talking about here. There may be a few localized studies, but they do not address the big picture. The Mt. Washington study, for example, with its data taken at a relatively high altitude above sea level, can hardly be considered as directly comparable with the low-level soupy conditions that previal in the actually populated areas of the region, especially given that the inversion layer in the northeast is typically within just a couple of thousand feet of sea level during any of the warmer months. Nor does that location find itself near, or under, heavily travelled air traffic lanes.

I have a close friend who is a professional meteorologist and likewise longtime amateur astronomer of some note. On several occasions I have approached him asking if he could locate any existing long-term tabulated data in the literature that would relate to sky clarity evolution, or evening cloudiness. After his searches he reported that apparently no such accurate data is to be found, other than short-term studies. In spite of the supposed "number of clear hours/days" in the course of a year we often see quoted in solar eclipse predictions and such, these values turn out to be based largely on rather arbitrary measurements subject to the whims of the original reporting observer. They are typically not scientific measurements of any sort whatever. Likewise, they generally refer only to daylight hours and not conditions occurring after nightfall. Further, most are made by airport personnel who are concerned with air navigation and safety within a few dozen miles of a particular field, not in conditions over a region as they might affect astronomical observation. A sky covered with thin cirrus will generally be reported as "clear", the same will be true when slightly hazy conditions prevail as long as some of the brightest stars are visible. This even further removes these data from any value in addressing the question at hand.

The fact remains that only reasonably accurate and believable data will come from the observational records/logbooks likely to originate with long-term dedicated observers, those in a position to have recorded meaningful data over decades. Like it or not, these data represent the only reasonably reliable information directly addressing possible changes in the atmospheric situation long-term.

BrookObs

#12 Tony Flanders

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Posted 11 May 2013 - 03:45 PM

On several occasions I have approached him asking if he could locate any existing long-term tabulated data in the literature that would relate to sky clarity evolution, or evening cloudiness.


The people who do track this, of course, are professional observatories. But there are no professional observatories working in optical wavelengths in the Easter U.S. -- for obvious reasons.

I once embarked on a project of trying to measure daytime transparency by taking photos with a digital camera and measuring the saturation of blue. Maybe I'll give it a whirl again.

#13 amicus sidera

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Posted 11 May 2013 - 11:58 PM

Anecdotal data is better evidence than no evidence at all. I've observed the same phenomenon over the last fifty years here in New Jersey that BrookObs has at his location in the Northeast.

I do not think that there is any question that increased jet aircraft traffic plays a major role in all this. Reducing the number of such flights to pre-1980 levels would likely help matters immensely. Of course, the 15% of Americans regularly use this wasteful form of transportation would of course cry out mightily should such restrictions be imposed, and since they are generally of the monied class nothing of the sort would be attempted; instead, blame will undoubtedly be assigned elsewhere.

As for the haze-and-pollution producing coal-fired power plants, these could be replaced handily by reactors using thorium, which are orders of magnitude safer than those designed to use uranium; however, as with air travel, chances of this occurring are slim to none.

#14 Tony Flanders

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Posted 12 May 2013 - 03:59 AM

Anecdotal data is better evidence than no evidence at all.


True -- especially if it's scrupulously documented. What one would like is a logbook kept every night, clear or cloudy, noting the conditions on that night and the time the observation was made. This would still be subjective, but it would now be real data.

A logbook noting only nights spent observing is subject to all kinds of selection effects. And memory alone is notoriously unreliable.

BrookObs is right that Percentage of Possible Sunshine, a datum recorded by many weather stations, is inevitably subjective to some extent. Nonetheless, there are plenty of times when it's indisputably sunny and plenty of times when it's indisputably not sunny. And there's no particular reason to think that the subjective evaluations have drifted systematically over time.

The fact that it's been made by different observers over time is probably a plus overall, since individual people's subjective judgments most definitely do drift systematically as they get older.

So while it's nearly worthless for tracking transparency, Percentage of Possible Sunshine does seem valuable for tracking cloudiness.

#15 Qwickdraw

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Posted 12 May 2013 - 07:12 AM

The change in the situation first came to note in my observing record books around the time of the First Gulf War, when skies were observed to be increasingly milkier in appearance.
BrooksObs


Maybe just coincidently but I recall hundreds of oil wells were set fire to in Iraq at that time.

#16 amicus sidera

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Posted 12 May 2013 - 08:05 AM

Anecdotal data is better evidence than no evidence at all.


True -- especially if it's scrupulously documented. What one would like is a logbook kept every night, clear or cloudy, noting the conditions on that night and the time the observation was made. This would still be subjective, but it would now be real data.

A logbook noting only nights spent observing is subject to all kinds of selection effects. And memory alone is notoriously unreliable.


I agree 100%, Tony... any hope which amateurs might hold out for changing this situation would of needs have to be grounded in solid data which could not be realistically challenged.

Fred

#17 Phillip Creed

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Posted 12 May 2013 - 08:17 AM

Anecdotal data is better evidence than no evidence at all.


True -- especially if it's scrupulously documented. What one would like is a logbook kept every night, clear or cloudy, noting the conditions on that night and the time the observation was made. This would still be subjective, but it would now be real data.

A logbook noting only nights spent observing is subject to all kinds of selection effects. And memory alone is notoriously unreliable.

BrookObs is right that Percentage of Possible Sunshine, a datum recorded by many weather stations, is inevitably subjective to some extent. Nonetheless, there are plenty of times when it's indisputably sunny and plenty of times when it's indisputably not sunny. And there's no particular reason to think that the subjective evaluations have drifted systematically over time.

The fact that it's been made by different observers over time is probably a plus overall, since individual people's subjective judgments most definitely do drift systematically as they get older.

So while it's nearly worthless for tracking transparency, Percentage of Possible Sunshine does seem valuable for tracking cloudiness.


Tony,

Even there, % possible sunshine has one key problem--it doesn't factor in diurnal variation in cloud cover. Some places have a massive difference in daytime (usually higher) vs. nighttime cloudiness, while others have only a small diurnal variation.

Clear(er) Skies,
Phil

#18 Tony Flanders

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Posted 12 May 2013 - 09:59 AM

Even there, % possible sunshine has one key problem--it doesn't factor in diurnal variation in cloud cover. Some places have a massive difference in daytime (usually higher) vs. nighttime cloudiness, while others have only a small diurnal variation.


Sure. And it's seasonal too; fair-weather cumulus are primarily a summertime phenomenon.

However, it's a whole lot better than nothing. And if there are long-term trends in cloudiness, you'd expect them to be reflected both in daytime and nighttime cloudiness.

I'm too lazy to download the data for typical East-coast cities and analyze it right now. Any volunteers?

#19 amicus sidera

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Posted 12 May 2013 - 10:33 AM

I'm similarly afflicted by a spell of laziness, Tony :grin:; just in case it abates, where might the pertinent data be found?

Fred

#20 Phillip Creed

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Posted 12 May 2013 - 10:41 AM

Here's a great place to start:

http://gdata1.sci.gs...ODIS_MONTHLY_L3

It takes a little bit of a learning curve, but it's a very powerful tool. This takes satellite data since 2000 for multiple parameters.

I set it for the full available range, March 2000 - April 2013.

I've attached an image generated from this site for nighttime cloud cover over the U.S., from March 2000 - April 2013.

Acknowledgement: “Analyses and visualizations used in this [study/paper/presentation] were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC."

Clear Skies,
Phil

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#21 Phillip Creed

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Posted 12 May 2013 - 10:45 AM

Here's the nighttime cloud cover over the Great Lakes, Mid-Atlantic and Northeast states over the same time interval.

Acknowledgement: “Analyses and visualizations used in this [study/paper/presentation] were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC."

Clear Skies,
Phil

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#22 Phillip Creed

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Posted 12 May 2013 - 10:47 AM

As far as an hour-by-hour cloud climatology, this is the only source I could find:

http://www.diurnal.m...mates.org/TEXT/

Hope this helps.

Clear(er) Skies,
Phil

#23 amicus sidera

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Posted 14 May 2013 - 05:34 PM

Many thanks for the links and excellent maps, Phil! I intend to look into this in depth as soon as possible.

Fred

#24 Phillip Creed

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Posted 14 May 2013 - 06:18 PM

For reference, here's the nighttime sky cover over western and central Europe for the same 2000-2013 time frame.

Acknowledgement: “Analyses and visualizations used in this [study/paper/presentation] were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC."

Clear Skies,
Phil

Attached Files








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