Is there a guide to atmosphereic density by elevation?

[Stonius]

For example, we know that having less atmosphere between you and the stars will enable higher resolution views (seeing dependent, of course).


But under the same conditions, how much better will the seeing be at, say, 300m compared to sea level? Or even 700m? These are elevations that are typically achievable in Australia, but I guess overseas, maybe people can be observing at up to 3,000m?



I figure (apart from the typical weather fluctuations) that the atmospheric pressure is higher, the closer to sea level you are. So, is there a way of measuring how much better off you are at elevation? What is the half-way point, where there is just as much atmosphere below you as above?


Any smart meteorological types out there?


Markus

[Merlin66]

Yeah!
Atmospheric absorption varies with the zenith angle.
I’m not at home to check my sources but if you Google you’ll find it.
I’m sure seeing will also be affected by the altitude of the observer, but I can’t remember seeing comparative data.

[Stonius]

Oh yes, I was aware that the lower the altitude, the more atmosphere you're looking through. I'm looking for a similar correlation only to do with altitude alone.


So if zenith at sea-level is 1 atmosphere thickness, how many atmosphere thicknesses is it at 1000m?


Curious. Of course, there is more to it than that - bad layers in the atmosphere and mixing of air currents add complication to the model, but obviously it makes a difference since professional observatories tend to be on top of mountains.


Markus

[JA]

Hi Markus,

An elevation of 1000m would reduce the air density by about 5%. If you want some serious reduction ~50% you'd need the likes of K2 / Everest.
https://www.researchgate.net/figure/Change-of-air-density-to-altitude_fig1_326209923

Best
JA

[mura_gadi]

Its fairly easy to see from the top of a good hill just how much pollen, dust or wood smoke is in the valleys and depressions depending on the season. That layer can run to 30mtrs+ and removes a lot of detail just to the naked eye.

Getting height above that ground fog of particles achieves a lot, its about getting above or away from the airborne pollutants, not just having less atmosphere above you.

Don't forget the hPa/millibar, that changes the density of the air above you as well.

[astro744]

See atmospheric extinction at https://asterism.org/resources/atmospheric-extinction-and-refraction/#:~:text=Atmospheric%20extinction%2 0is%20the%20reduction,to%20one%27s% 20line%20of%20sight.

Plenty other resources on the topic if that's what your after.

[Crater101]

For what it's worth...I live rural at an elevation of 855m. However, I do see a lot more cloud than many, and I also live not too far from a coal mining area, where a lot of the residents (mostly the older ones) still have coal burners. Depending on the wind direction, that can sometimes add a faint layer of haze, particularly in the winter months.
While the elevation does help - the days tend to be cooler, and there's a lot less heat haze to deal with in the evenings - the overall geography still has to be taken into consideration.

[sheeny]

G'Day Markus.


I've attached an Excel spreadsheet that should let you calculate what you want to know. It's overkill for your needs, but I had it handy. Used to do these calculations all the time for work, for furnaces and pneumatic conveying, ventilation, and dust extraction systems, because Oberon (where I live and worked) is +1100m.


Typically here at +1100m, on average the air is 7°C cooler than sea level due solely to altitude. So standard atmospheric density at sea level is 1.2 kg/m3 at 21°C. where in Oberon is it 1.08 kg/m3 @14°C.


Not only is the air less dense at altitude, but you aren't looking through the thickest part of it! The densest kilometre isn't in the way in Oberon, for example. This is worth at least 1 magnitude in naked eye visibility. When I first returned to Oberon, I noted that I could regularly see M7 stars or better with the naked eye, where at sea level M6 is considered the limit in a dark site.


For more information (if you need it) search for the US Standard Atmosphere. It may have NASA or NOAA in the title as well. It should give you a table of atmospheric temperatures, pressures and densities all way up through the stratosphere. I have it in one of my Fan Engineering texts that are buried somewhere...:rolleyes:


Al.

Al.

[sheeny]

https://en.wikipedia.org/wiki/U.S._Standard_Atmosphere
https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html
https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/us-standard-atmosphere-1976/us-standard-atmosphere_st76-1562_noaa.pdf

[Stonius]

Hey Al, wow, that's a pretty definitive answer. Thank you!


If I'm reading your excel right, the 50% point is at about 6,000m elevation.


At a more achievable 1,000m, the air is at about 80% density. At 3,000m, the air is at about 70%.


But as others have pointed out it's the other factors that are important too.


Thanks!

[sheeny]

No worries, mate.


Al.

[gary]

Hi Markus,

Turbulent airflow, or more precisely, the lack thereof, is everything.
Which is to say, seeing dominates because it dictates the minimum
angular resolution that observations can be made at.

Observatories are sited at places such as Mauna Kea (4200m) and
Paranal (2600m) not simply because they are at high elevations but
because the seeing at these particular locales is exceptional.

Obviously with more elevation the less atmosphere you are looking
through and potentially the less layers of atmosphere with different
temperatures and differing flow rates which affect seeing.

But not all places at high elevation necessarily have ideal seeing.

For example, the Big Island of Hawaii is essentially the world's largest
mountain. Though Manua Kea rises 4200m above sea level, it extends
another 6000m below sea level.

It's profile is very smooth. Unlike Everest where the ascent at times
is vertical, Mauna Kea rises steadily and relatively gently. 40km of steady
driving from the coast and you reach the top.

Then there is the location of the Big Island itself. In the middle of
the Pacific, far away from any other sizeable land mass. The surrounding
Pacific Ocean is of course "flat" and has relatively stable temperatures.
Most importantly, the airflow over Mauna Kea is relatively laminar.
Its smooth shape allows the winds to gently pass over it. This makes
the seeing there exceptional.

By comparison, I have also been fortunate enough to have seen most of the
world's 8000m+ peaks. After a arduous multi-day hike, it required
a break in the weather early one morning, when the air was stiller, to get
a view of Kangchenjunga, in northern India, world's third highest peak.
I've seen snow drifts off the side of Everest in Nepal
where I would estimate the wind speeds were probably in excess of 80km/h.
I've seen the peaks and valleys surrounding K2 and Nanga Parbat, in Pakistan,
extending as far as you can see. Each valley may be having totally different
weather from the neighboring one. The greater Himalayas span 2000km
and there is nothing to promote laminar airflow about them. As a result,
the seeing is not consistent as sites with lower elevations, such as
Mauna Kea and Paranal. Seasonal monsoons are also another downside to
the Indian Ocean facing sides of the Himalayas.

Below, snapshot of Mauna Kea taken from the twin mountain, Mauna Loa.
Note its gentle profile that extends beyond the image another 40km to the
coast and then continues 6000m below sea level. This point from where
the photo was taken is between Mauna Kea and Mauna Loa and is a saddle
and a highway runs through it called the Saddle Road.
The observatories are at the peak of Mauna Kea.
Mauna Loa is home to the world's premier CO2 monitoring station
from where the famed Keeling curve originated.

[Rod-AR127]

I'm learning heaps here. Thanks for all the replies and the OP original post.

[Stonius]

Thanks Gary,


I have a book called 'Planetary Astronomy' by Christophe Pellier which goes into some detail. He mentions laminar air-flow and layer mixing. I can't remember him saying much about the thickness of the atmosphere itself, but I guess that's because by itself it doesn't have much impact compared to the other factors. After reading it I spent a lot of time flying around my local area in Virtual Reality Google Earth looking for the ideal spot, where the laminar air from the plains meets more or less the first hill. It's not easy to find a spot like that which is open to the public at night, but where you're also not likely to be disturbed.


City's are a heat-sink though. Getting out of the city is half the battle - it's a bubble of bad air. For planetary it doesn't have to be a dark sky site, it just needs good seeing.


Cheers


Markus

[sheeny]

As Gary points out, altitude (IMO) is of more value with respect to limiting magnitude than resolution due to seeing (and occasionally it'll get you above low level cloud or fog).


In my experience, the average seeing in Oberon is probably a bit worse than Sydney due to the higher average wind speed over the great dividing range. The GDR acts like the top surface of an aeroplane wing or the bottom surface of a venturi - in general the wind speed over the mountains is higher than either side of it. But when there's no wind... seeing can be excellent, provided the jet stream also plays well.


Al.

[Boozlefoot]

[QUOTE=sheeny;1569018]The GDR acts like the top surface of an aeroplane wing or the bottom surface of a venturi - in general the wind speed over the mountains is higher than either side of it. But when there's no wind... seeing can be excellent, provided the jet stream also plays well. QUOTE


Agreed Al, much the same case here at Windeyer. Easterlies are great for a short time, but the upper turbulence can be very noticeable for short periods.

Also, the NOAA site is a wealth of information for all of us; not only including this topic. Just used it's calculator to determine today's solar noon so I can layout a Southern shadow. Definitely it's worth a look for other handy networks and data. :question:

[Merlin66]

I use Sun Calc for meridian timing and solar positioning.
https://www.suncalc.org/#/-38.1873,144.7089,16/2020.12.04/11:04/1/3