74 replies to this topic

[GeorgeLiv]

I rarely come here in summer months. But when I do, I find it alarming in how simplistic some amateurs can be, misunderstanding SQM-based LP maps & searching for some holy-grail dark site, yet won't consider the occasional & special circumstances that arise (or arrive) to darken any light-polluted town.

 

The title of this thread could also be: Meteorology 101 = Skyglow 101.

 

It might be a bold statement to say that Canadians experience less light-pollution (LP) than Americans. Logically they should have more LP because of abundant hydro electricity & relatively cheap rates..and lots of snow. Despite constant hints here on Cloudynights of lower sky-glow over Canada, no-one has bothered to ask why? Is it due to weaker lighting or sparser populations? Is it because of the spectrum of the lighting? Is it due to less smoke or less humidity?

 

 

Looking at the above illustration you should be aware that people located at A have shallower & colder air than people at B. Regarding sky-glow, two significant parameters between north or south of that polar jet stream are: The difference in height up to the Tropopause (the start of the Stratosphere) is typically 4 kilometers; and the typical "air-mass" between A & B (when forest-fires are not raging) are vastly different. Ok so the quick answer is: It's the cold Canadian air, specifically the shallower, drier and cleaner "Polar Continental" air-mass created throughout most Boreal regions. In winter months this includes most of the prairies down to the Dakotas and Montana. "Air-mass" you say?

 

 

This is basic "air-mass" meteorology. Air-masses directly influence how much sky-glow is created & sensed over one location. This should also explain the observed increases in sky-glow last few years (or decades).

 

Air-masses are uniform bodies of air, characterized as having specific properties of temperature, moisture and particulate contaminants. They tend to be created over "source" regions. Distinct sets of properties distinguish one air-mass from another, and obviously, as air-masses move, they produce the changes in the observed weather (cloudcover, rain/snow, temperature & air quality) as well as in the amount of sky-glow.

 

Slide player with some basic info on air-masses.

Slide player with fronts & air masses.

 

There are only two fundamental sources for air masses: continental (c) and maritime (m), formed over large bodies of land or water, respectively. Four distinct thermal regimes are also designated indicating the importance of latitude. These are: (A) for arctic, (P) for polar, (T) for tropical & (E) equatorial.

 

We combine the two letters in the above to get the various air-masses - such as: cT (continental tropical) and cP (continental polar). For your info, the major differences between these two "continental" air masses is the temperature and their heights. The USA is never under the influence of equatorial air-masses and rarely by arctic in winter. Air-masses are always moving. The steering mechanism is the Jet-Stream (there are two: Polar & Sub-tropical). Theoretically an astute US astronomer can take advantage of the circumstances when clean cold air sweeps out the now more dominant "tropical" air-masses.

 

Good video for air-mass sources & their general characteristics.

Excellent basic polar jet info.

 

There aren't many good 3D diagrams of the structure of the atmosphere...

 

 

...but the following one is good. Although an exaggeration, you can plainly see the significant difference in height between latitude 30°north & latitude 60°north.

 

 

However, the diagram can't show the particles above those latitudes. This is the soupy mixture you're trying to observe or image through. Next post I'll show the difference in particles and their scattering processes.

[Herchel]

We are being  screwed with high power rates and  carbon tax ,, can't afford proper heat and froze all winter ,, BUT  we have amazing clear winter skies here when it is clear the stars are blazing bright.

 

I am now in the A zone

Edited by Herchel, 19 June 2024 - 10:24 AM.

[RichA]

I rarely come here in summer months. But when I do, I find it alarming in how simplistic some amateurs can be, misunderstanding SQM-based LP maps & searching for some holy-grail dark site, yet won't consider the occasional & special circumstances that arise (or arrive) to darken any light-polluted town.

 

The title of this thread could also be: Meteorology 101 = Skyglow 101.

 

It might be a bold statement to say that Canadians experience less light-pollution (LP) than Americans. Logically they should have more LP because of abundant hydro electricity & relatively cheap rates..and lots of snow. Despite constant hints here on Cloudynights of lower sky-glow over Canada, no-one has bothered to ask why? Is it due to weaker lighting or sparser populations? Is it because of the spectrum of the lighting? Is it due to less smoke or less humidity?

 

Jet_streams.jpg

 

Looking at the above illustration you should be aware that people located at A have shallower & colder air than people at B. Regarding sky-glow, two significant parameters between north or south of that polar jet stream are: The difference in height up to the Tropopause (the start of the Stratosphere) is typically 4 kilometers; and the typical "air-mass" between A & B (when forest-fires are not raging) are vastly different. Ok so the quick answer is: It's the cold Canadian air, specifically the shallower, drier and cleaner "Polar Continental" air-mass created throughout most Boreal regions. In winter months this includes most of the prairies down to the Dakotas and Montana. "Air-mass" you say?

 

Tropopause-height.jpg

 

This is basic "air-mass" meteorology. Air-masses directly influence how much sky-glow is created & sensed over one location. This should also explain the observed increases in sky-glow last few years (or decades).

 

Air-masses are uniform bodies of air, characterized as having specific properties of temperature, moisture and particulate contaminants. They tend to be created over "source" regions. Distinct sets of properties distinguish one air-mass from another, and obviously, as air-masses move, they produce the changes in the observed weather (cloudcover, rain/snow, temperature & air quality) as well as in the amount of sky-glow.

 

Slide player with some basic info on air-masses.

Slide player with fronts & air masses.

 

There are only two fundamental sources for air masses: continental (c) and maritime (m), formed over large bodies of land or water, respectively. Four distinct thermal regimes are also designated indicating the importance of latitude. These are: (A) for arctic, (P) for polar, (T) for tropical & (E) equatorial.

 

We combine the two letters in the above to get the various air-masses - such as: cT (continental tropical) and cP (continental polar). For your info, the major differences between these two "continental" air masses is the temperature and their heights. The USA is never under the influence of equatorial air-masses and rarely by arctic in winter. Air-masses are always moving. The steering mechanism is the Jet-Stream (there are two: Polar & Sub-tropical). Theoretically an astute US astronomer can take advantage of the circumstances when clean cold air sweeps out the now more dominant "tropical" air-masses.

 

Good video for air-mass sources & their general characteristics.

Excellent basic polar jet info.

 

There aren't many good 3D diagrams of the structure of the atmosphere...

 

102500-004-FD431E88.jpg

 

...but the following one is good. Although an exaggeration, you can plainly see the significant difference in height between latitude 30°north & latitude 60°north.

 

3D-frontal-diagram.jpg

 

However, the diagram can't show the particles above those latitudes. This is the soupy mixture you're trying to observe or image through. Next post I'll show the difference in particles and their scattering processes.

Come  to Toronto and compare it with any city  with the same overall water vapour level.  I  think you'll find the light pollution keeps up with most urban areas world-wide, except freak shows like Las Vegas or Tokyo.  Summertime, mag 3 is a stretch.

[Herchel]

Born & raised in toronto's east end by lake ontario , light pollution was not too bad until they changed the street lights ,, downtown sucked.

 

Maybe changed after i moved 2018

Edited by Herchel, 18 June 2024 - 05:58 PM.

[deSitter]

Come  to Toronto and compare it with any city  with the same overall water vapour level.  I  think you'll find the light pollution keeps up with most urban areas world-wide, except freak shows like Las Vegas or Tokyo.  Summertime, mag 3 is a stretch.

I can get 4 here regularly (4 miles from central Atlanta) since the LED streetlights replaced the wretched yellow sodium lights. Sometimes I think I can just make out some dimmer stars, e.g. Little Dipper when it's standing straight up. I have a real dark sky site (Bortle 3) only 90 miles east. I think it is the dense tree cover.

 

-drl

[Diana N]

I rarely come here in summer months. But when I do, I find it alarming in how simplistic some amateurs can be, misunderstanding SQM-based LP maps & searching for some holy-grail dark site, yet won't consider the occasional & special circumstances that arise (or arrive) to darken any light-polluted town.

 

The title of this thread could also be: Meteorology 101 = Skyglow 101.

 

It might be a bold statement to say that Canadians experience less light-pollution (LP) than Americans. Logically they should have more LP because of abundant hydro electricity & relatively cheap rates..and lots of snow. Despite constant hints here on Cloudynights of lower sky-glow over Canada, no-one has bothered to ask why? Is it due to weaker lighting or sparser populations? Is it because of the spectrum of the lighting? Is it due to less smoke or less humidity?

 

Jet_streams.jpg

 

Looking at the above illustration you should be aware that people located at A have shallower & colder air than people at B. Regarding sky-glow, two significant parameters between north or south of that polar jet stream are: The difference in height up to the Tropopause (the start of the Stratosphere) is typically 4 kilometers; and the typical "air-mass" between A & B (when forest-fires are not raging) are vastly different. Ok so the quick answer is: It's the cold Canadian air, specifically the shallower, drier and cleaner "Polar Continental" air-mass created throughout most Boreal regions. In winter months this includes most of the prairies down to the Dakotas and Montana. "Air-mass" you say?

 

Tropopause-height.jpg

 

This is basic "air-mass" meteorology. Air-masses directly influence how much sky-glow is created & sensed over one location. This should also explain the observed increases in sky-glow last few years (or decades).

 

Air-masses are uniform bodies of air, characterized as having specific properties of temperature, moisture and particulate contaminants. They tend to be created over "source" regions. Distinct sets of properties distinguish one air-mass from another, and obviously, as air-masses move, they produce the changes in the observed weather (cloudcover, rain/snow, temperature & air quality) as well as in the amount of sky-glow.

 

Slide player with some basic info on air-masses.

Slide player with fronts & air masses.

 

There are only two fundamental sources for air masses: continental (c) and maritime (m), formed over large bodies of land or water, respectively. Four distinct thermal regimes are also designated indicating the importance of latitude. These are: (A) for arctic, (P) for polar, (T) for tropical & (E) equatorial.

 

We combine the two letters in the above to get the various air-masses - such as: cT (continental tropical) and cP (continental polar). For your info, the major differences between these two "continental" air masses is the temperature and their heights. The USA is never under the influence of equatorial air-masses and rarely by arctic in winter. Air-masses are always moving. The steering mechanism is the Jet-Stream (there are two: Polar & Sub-tropical). Theoretically an astute US astronomer can take advantage of the circumstances when clean cold air sweeps out the now more dominant "tropical" air-masses.

 

Good video for air-mass sources & their general characteristics.

Excellent basic polar jet info.

 

There aren't many good 3D diagrams of the structure of the atmosphere...

 

102500-004-FD431E88.jpg

 

...but the following one is good. Although an exaggeration, you can plainly see the significant difference in height between latitude 30°north & latitude 60°north.

 

3D-frontal-diagram.jpg

 

However, the diagram can't show the particles above those latitudes. This is the soupy mixture you're trying to observe or image through. Next post I'll show the difference in particles and their scattering processes.

This is an awesome post!  Thank you for supplying us with all this information (especially the wonderful diagrams)!

[CharLakeAstro]

Great post. To add, 90% or more of the Canadian population live within the "B" zone of the top diagram, myself included.

From the bottom image, I live somewhere around the "Ferrel Cell" zone... which explains the rarity of good "seeing" in my area.

[Sebastian_Sajaroff]

Lucky guys!
My area is anything but favourable to astronomy.

[PEterW]

I use the Aeosol (aerosol Optical depth) map in the Windy app to help understand how much muck is likely about. Living in the UK, our weather can come from pretty much anywhere, with sources of crud similarly - including occasional Saharan dust.

Have to say that when I’ve been in Canada I have always been stuck by the blueness of the sky (when it wasn’t cloudy of course). We tend to get lots of (likely contrail seeded) haze if we get a stable weather system… yuk.

Peter

[Fabricius]

But it's not all gold that glitters.

Canada also has one of the most light polluted areas in the world.

 

Is it a UFO? Aurora Borealis? No, it's the greenhouse area near Leamington and Kingsville, emitting an extraordinary amount of light.

Worse than Toronto or Montreal.

 

l am sorry to say that those growers have copied the business model of growers in my country (i.e. lighten up greenhouses with 1000 watt lamps, do not shield the lights, wait and see if anyone complains, then shield as less as possible,  if the electricity costs become unbearable swap the lamps for leds).

 

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[Sebastian_Sajaroff]

But it's not all gold that glitters.

Canada also has one of the most light polluted areas in the world.

 

Is it a UFO? Aurora Borealis? No, it's the greenhouse area near Leamington and Kingsville, emitting an extraordinary amount of light.

Worse than Toronto or Montreal.

 

l am sorry to say that those growers have copied the business model of growers in my country (i.e. lighten up greenhouses with 1000 watt lamps, do not shield the lights, wait and see if anyone complains, then shield as less as possible,  if the electricity costs become unbearable swap the lamps for leds).

Worse than Montreal ? That must be something !

[columbidae]

Can't have skyglow if you're constantly overcast, right?

[rjacks]

Population density?

[mountain monk]

GeorgeLiv,

 

Thank you for that extraordinary post, one of the most instructive I’ve seen n CN. I am blessed with dark skies.

 

Dark skies.

 

Jack

[GeorgeLiv]

There's a lot more. At the end, you should understand that because the stars and LP sources are both fixed in the short term, any sky assessment in an astronomical pursuit (SQM, transparency, Bortle, limiting magnitude) is ultimately an assessment of the air-mass above you.

 

Air masses contribute to our psyche. It's a multi-sensory process; we see it and feel it. But we also breathe it and sometimes we can even taste it. Notice the short & disappearing contrails at top image (on right of cT label.) The bottom image shows a setting sun in a smoke-laden maritime tropical air-mass.

 

 

The European continent is not a source of any air-mass (except maybe for parts of Kazakhstan north of the Caspian Sea), but is influenced by what's shown in the following diagram.

 

 

The North American continent is both a source and influenced by drastically differing air-masses, hence the great frequency of tornadoes.

 

 

Once created - from solar heating or massive heat-loss during long nights, air-masses start to move. When they collide they create what we call frontal boundaries which can evolve into low pressure systems. Thereafter air masses become "modified", losing or gaining moisture, cooling or heating from either land, open lakes, oceans or insolation.

 

 

Even though the diagram above is simplistic, that's how most low pressure systems look like. Remember that the blue cP air-masses are about 4 kilometers shallower than adjoining mT air masses. In the real world, and in the last 3 decades since global warming started, systems can become quite complex  Can you figure any of the following out?...

 

 

Meteorologists do try to make sense of the mess. And here's what they come up with (not the same dates as above)...

 

 

It's a graphical representation of the fronts and the systems that move along for every 6 or 12 hours. As an Astronomer, what you need to pick out is the cP air-mass giving you minimal LP. I highlighted the slot of cP in blue. For good to very good conditions, the barometric pressure should be above 1020 millibars (or 102.0 kilopascals, or > than 30.15 inches).

 

How do you find that clean continental polar air? You need the fronts from here or from here, but also the temperatures from here. You should find an abrupt change in air temperature. As cP air-masses become modified in summer, (open lakes, warm land), it's rather difficult to pick them out. Then you'll also need I.R. or visible satellite loops.

 

But there's more.

Edited by GeorgeLiv, 20 June 2024 - 06:36 PM.

[tcifani]

Meteorologists do try to make sense of the mess. And here's what they come up with (not the same dates as above)...

 

 

 

It's a graphical representation of the fronts and the systems that move along for every 6 or 12 hours. As an Astronomer, what you need to pick out is the cP air-mass giving you minimal LP. I highlighted the slot of cP in blue. For good to very good conditions, the barometric pressure should be above 1020 millibars (or 102.0 kilopascals, or > than 30.15 inches).

 

How do you find that clean continental polar air? You need the fronts from here or from here, but also the temperatures from here. You should find an abrupt change in air temperature. As cP air-masses become modified in summer, (open lakes, warm land), it's rather difficult to pick them out. Then you'll also need I.R. or visible satellite loops.

 

But there's more.

This is fantastic info. For those of us who are not as savvy reading weather maps, which website would you recommend for looking at air masses, fronts, temperatures and so forth?

 

I live in the southeast US, not right on the Atlantic Coast but probably close enough to receive those tropical marine air masses.

 

This screen shot is from Wunderground Wondermaps. Not sure if this is a reputable website or maybe you can recommend better, but I like the graphics.

 

Thanks,

Tony

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[GeorgeLiv]

Virtually all of the diagrams I've posted, including all illustrations you're likely to find online, are an approximation. Our atmosphere is a dynamic fluid, changing by the minute! Some important points...

 

In one Earth year, the entire structure of the atmosphere shifts south then north, corresponding to the seasons. Near the summer solstice, distinctions for fronts and air-masses are lost. Air-masses for North America quickly lose their characteristic properties due to heating of the land and unfrozen lakes via longer periods of sunlight. And moisture from the Pacific in the SW USA invades northward.

 

 

The illustration above shows the expected temperatures relative to the Jet-Stream for tomorrow, June 21st. I'm NOT sure if it's showing the Polar-Jet or the Subtropical-Jet, but gauging by the lack of temperature variance, I believe it's the Subtropical-Jet. The Polar-Jet stream weakens in summer and sometimes disappears, returning quite bold & distinct by October/November.

 

Current I.R. loop of western North America...

 

 

With Global-Warming, the Intertropical-Convergence-Zone (that messy zone in the south in the loop) has expanded, although it does return to normal at times. This shifts northwards the Ferrel & Hadley cells! Spring starts earlier, warm-moist and hot-dry air-masses show themselves earlier. Monsoonal flow in the southwest starts in June instead of August (loop above). And pretty much everywhere, Fall starts much later.

 

It means that cities like Toronto & Montreal (latitudes of 43.7° & 45.5° respectively) are now experiencing what Americans at about latitude 38° once experienced. In fact many years ago I thought Toronto had changed, rarely getting snow on the ground by the 1990s. Both above mentioned cities get the weather that Washington D.C. once did (lat 38.9°). It's much rarer for clear crisp continental-Polar air to invade, to linger, to clear the night-sky of that awful soupy mixture.

 

The frequency of mT and cT air-masses has increased in the populated south of Canada, so that what any Northern tier American experiences, we also experience. It reached 35° C for two days here in Montreal (June19-20) which was unheard of two decades ago. Milky bright nights are the norm now in summer, eagerly awaiting for September, when the nights finally return to "normal".

 

For Tony tcifani, start here: https://www.weather.gov/ and click on clickable area for your area. Links should be available for everything you need. But the fronts and frontal boundaries are mapped on the National maps. Or you can pursue this. I don't believe regional maps for fronts are generated. And it looks like you're already in tropical circulation, with trade winds. Brace yourself for an interesting Hurricane season.

 

I particularly like to use the IR & Visible satellite imagery found here. The daytime visible loops will show fair-weather puffy clouds as well as smoke plumes.

Edited by GeorgeLiv, 21 June 2024 - 05:12 PM.

[GeorgeLiv]

l am sorry to say that those growers have copied the business model of growers in my country (i.e. lighten up greenhouses with 1000 watt lamps, do not shield the lights, wait and see if anyone complains, then shield as less as possible,  if the electricity costs become unbearable swap the lamps for leds).

I was wondering if you're near these greenhouses, can you take a night shot post it here? Are they lighting their "crop" for 24 hours? No off period? Pot?
 

Edited by GeorgeLiv, 20 June 2024 - 08:25 PM.

[Tony Flanders]

It might be a bold statement to say that Canadians experience less light-pollution (LP) than Americans.

It may or may not be bold, but frankly, I don't have a clue what that statement is supposed to mean, much less why you believe it's true.

Obviously Canada has less grand total aggregate skyglow than the U.S. because its population is vastly smaller. But somehow I doubt that's what you mean. Could you repeat the quoted statement above in a way that it can actually be supported or refuted by data?

Incidentally, I am generally only interested in the skyglow on nights of typical good transparency -- which is to say, nights when the Canadian airmass extends over my area (U.S. Northeast).

Edited by Tony Flanders, 20 June 2024 - 08:55 PM.

[GeorgeLiv]

It may or may not be bold, but frankly, I don't have a clue what that statement is supposed to mean, much less why you believe it's true.

Obviously Canada has less grand total aggregate skyglow than the U.S. because its population is vastly smaller. But somehow I doubt that's what you mean. Could you repeat the quoted statement above in a way that it can actually be supported or refuted by data?

Incidentally, I am generally only interested in the skyglow on nights of typical good transparency -- which is to say, nights when the Canadian airmass extends over my area (U.S. Northeast).

Pardon my ambiguity.

 

From my travels, my experience is that as Canadians, we definitely have more outdoor lighting than our southern counterparts per capita, (except for Chicago prior to 2018).

 

But the frequency of clear, dry, cool (or cold) and shallower air over Canada is much greater than in the USA. Well, for some reason I thought this point would be obvious to all seasoned astronomers.
 

Edited by GeorgeLiv, 20 June 2024 - 09:56 PM.

[GeorgeLiv]

I use the Aeosol (aerosol Optical depth) map in the Windy app to help understand how much muck is likely about. Living in the UK, our weather can come from pretty much anywhere, with sources of crud similarly - including occasional Saharan dust.

Peter

Hi, Could you post that specific link for the Aerosol map you're referring to?

[CharLakeAstro]

Leamington greenhouses primarily grow produce for human consumption, tomatoes, cucumbers and the like. They use pink and purple lights which is said to enhance taste.

 

Some interesting sidebar backstory...

 

The old Heinz factory  in Leamington was ditched by the parent company, with the intention to sell ketchup in Canada but produced elsewhere. French's ketchup took advantage of the anti-Heinz sentiment and the Heinz boycott,  and began producing Ketchup in a new facility in Ontario, with a bold "proudly made in Canada" marketing campaign. Not only is their ketchup made in Canada, but the tomatoes are from Leamington too.

 

It proved very successful, and Heinz market share in Canada has cratered. I and many others have not purchased Heinz since they left. Ironically, Heinz later pivoted and began producing Heinz ketchup in Quebec,  but the tomatos from Leamington now goto French's. The damage was done and Heinz has not yet recovered the lost marketshare which their accountants had gleefully abandoned in the pursuit of higher margins. Seems ketchup in their eye must sting... 

 

I played a minor role in this, with the French's new production as a client. IMO French's ketchup tastes better too... maybe I am bia$ed 

 

The cards are still in play...

https://www.clubhous...-fresh tomatoes.

 

 

I was wondering if you're near these greenhouses, can you take a night shot post it here? Are they lighting their "crop" for 24 hours? No off period? Pot?
 

 

Edited by CharLakeAstro, 20 June 2024 - 10:28 PM.

[RichA]

Born & raised in toronto's east end by lake ontario , light pollution was not too bad until they changed the street lights ,, downtown sucked.

 

Maybe changed after i moved 2018

I was in Amherst N.Y. and looked at Toronto across the lake.  Two sheets of pure white light hovered above the city.  In 1970, you could see the Milky Way from the suburbs...

[GeorgeLiv]

High-pressure systems with low humidties are a blessing for any Astronomer. But did you know that cooler & dry air results in lower sky-glow than hot & dry air?

 

Besides the height to the start of the Stratosphere, what else is different above human figures A & B?

 

 

We know that A is cold (or cool) while B is hot. Let's say that both have the same Relative Humidity (RH), which is typically measured inside a Stevenson screen, four feet above the ground, never at cloud-heights. Clouds in the sky do indeed form when the RH approaches 100%.

 

 

The above illustration shows the vapor-pressure of water (or vapour-pressure for UK readers) at 1 atmospheric pressure. The curved line is the Saturation vapor-pressure, or the MAXIMUM possible (left scale) for the temperatures shown at bottom. Adding more water results in condensation (clouds, fog or mist). The actual maximum possible mass of H₂0 that can be dissolved in 1 kg of air is shown on the right.

 

Green Xs: For similar sized parcels of air, near ~33% RH, the parcel at 0°C actually has about a tenth of the water content as that for 30°C.

 

Red Xs: For a parcel of air, completely saturated at -10°C (RH =100%), if you heat this same parcel to +15°C, the RH drops to ~15%!

 

Most of the particles above figures A or B are the same! For a dry & dust-free atmosphere, up-to the typically dry Stratosphere, over 99% of the particles involved in scattering light are the air-molecules nitrogen (N₂) and oxygen (O₂). The rest are mainly argon (Ar), carbon dioxide (CO₂) and water (H₂0) in the gaseous state (water vapor). All these molecules have sizes that are on the order of a thousand times smaller than any wavelength of visible light. Water vapor is called a vapor because it can exist in equilibrium with its liquid and solid phases. As a gas, water is also a thousand times smaller than any wavelength of light. Ozone is found in the Stratosphere). All these molecules scatter light by the classic Rayleigh scattering law.

 

 

If the air remains very dry, and particles are added into the air (from high winds, smoke or industry) then the scattering process becomes "Mie scattering". But this is rare. In cold air-masses simple lifting processes easily forms clouds. Low stratus clouds are common formations in cP air-masses.

 

Typically there are many "condensation" nuclei in air. The deeper the air, the greater their numbers. These condensation nuclei will enlarge the water+pollutant particle for non-selective scattering. Some nuclei have such an affinity to bond with water that condensation can occur at only 80% RH. The shaded blue area below the Saturation vapor-pressure line shows this effect.

 

The capacity of air to hold water in the gaseous state, and by way of the laws of probability, suspended water droplets, decreases with the lowered temperature. It's high with hot air, low with cold air.

Edited by GeorgeLiv, 21 June 2024 - 11:08 PM.

[Tony Flanders]

The frequency of clear, dry, cool (or cold) and shallower air over Canada is much greater than in the USA. Well, for some reason I thought this point would be obvious to all seasoned astronomers.

It seems to me that skyglow is pretty much of a red herring in this thread; in fact the main postings have barely mentioned it.

The proper title for the thread so far would be "Why Canadians Have Better Transparency than Americans."

The relationship between transparency and skyglow is interesting and complex, and probably deserves its own thread. However, poor transparency is a menace for deep-sky observers even at sites with zero artificial skyglow.