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Ground Level Boundary Layers/Obs. Site Selection

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#1 BarrySimon615

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Posted 15 July 2013 - 12:35 PM

Someone posted a comment about "ground level boundary layers" and it's influence on seeing. Those comments got me doing a bit of research and I found a PDF file that is interesting. I think it gives some detail about this topic although it does not specifically relate to astronomy and seeing. Certainly we know some basic things -

1) Do not set up and observe looking low across the top of a hot roof.

2) Do not set up and observe on hot concrete or asphalt.

However beyond that, can anyone explain and give further advice. Perhaps this PDF file that I found can be helpful -

http://www.envirocom...lution/Ch.3.PDF

Perhaps someone else can reinterpret this and rephrase it in a meaningful way for everyday utilization in selecting observing sites.

Barry Simon

#2 GlennLeDrew

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Posted 15 July 2013 - 03:11 PM

The areas where we can exert some influence on seeing involve local effects at or near ground level. In the main, avoid strong horizontal differences in surface characteristics. The more uniform the ground level properties over some area surrounding the observing site, the better the chance for good *local* seeing conditions.

#3 Mike B

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Posted 15 July 2013 - 04:25 PM

Some things i've picked up on over time concerning "localized seeing" effects:
1) *yes* to the hot pavement & viewing-over-rooftops aspect!
2) expand that idea to avoiding viewing over *ANY* radiating surfaces *or* elements, where possible- pavement, roofs, mechanical equipment which exhausts, attic vents & their heat-plumes,
3) avoiding (where & when possible) setting up on the leeward side of hills, buildings, or other turbulence-inducing topography... try to opt for the windward side!
4) sometimes you're stuck with warm pavement or rocky ground- scopes set up high on an EQ mount can avoid some of the effects- just by proximity. But Dobs take it on the chin here! :p
a) a reflective ground mat may help limit some of the radiating heat?
B) if a home patio, hose off the pavement with water, repeating as necessary with each evaporation,
c) shade the pavement in late afternoon,
d) i've reversed my Dob's rear-blower fan, so now it blows downward, away from the mirror, drawing cooler (hopefully) air from above- around the mirror & out the bottom... rather than blowing warm ground-air AT the mirror all nite! :foreheadslap:
e) set up a box-fan to gently blow across your scope all evening, as you view, ushering warmer ground-air away, and aimed so you & your body-heat are downwind of the scope (where feasible).

I hope this gets the juices flowing for MORE ideas to roll!
Good topic!
:waytogo:

#4 galexand

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Posted 23 July 2013 - 08:07 PM

In the summer, I set up on my covered front porch...I never get any dew, and if the seeing is good then the seeing is good. Had some great nights like that.

But in the winter, I set up in the same spot, and I guess my porch holds a little heat or something, because even after the telescope (a little 6" reflector) has had an hour to cool down, I still seem to get unconditionally poor seeing. One night on a whim I pointed a fan directly at the telescope, and I guess it changed the way the air was flowing enough, because suddenly I had good seeing...turned off the fan to be sure, and seeing reverted to crud. Don't really like sitting in front of a fan in the middle of winter though. :)

I think other than avoiding hot parking lots and so on, it's hard to know before you try...you just learn the conditions from experience at the sites you visit.

Once I set up on a tall parking garage, thinking I would have great horizons. Not sure if the air rising off the garage caused me trouble -- it was nothing compared to the nuissance of all the lights they put on top of garages!

#5 BPO

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Posted 24 July 2013 - 12:38 AM

It's a complex subject. Aside from the usual basics as outlined above (eg, avoid observing across rooftops, etc) it's probably not worth worrying about too much unless you're planning a professional observatory.

#6 Dwight J

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Posted 24 July 2013 - 03:06 AM

The best demonstration I have seen is when we have a fresh snowfall. Pavement, gravel, and concrete stay wet while the snow accumulates on the grass. A wood deck also had snow on it long before it started to cover the road. Plus one for covering the ground, I use a tarp, but not so much for improving the local seeing but to reduce dew and catch those dropped set screws. Avoid setting up in valleys or other low areas as cold air pours in at night that really messes up things. Dew is also more likely in low areas. Looking out over a large body of water (ponds, lakes) can also help as the surface water is essentially all the same temperature and the air above it less disturbed. I found luck to be the most essential ingredient.

#7 brianb11213

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Posted 24 July 2013 - 04:22 AM

Plus one for covering the ground, I use a tarp, but not so much for improving the local seeing but to reduce dew and catch those dropped set screws.

Yeah ... unfortunately the effect on seeing is at best marginal, the low level seeing issues are mostly caused by turbulence generated by heat sources which may be some distance away (hot roofs / tarmac / concrete / buildings, worst of all boilers, generating heat plumes that wreck seeing). High level seeing effects (jet stream) can only be alleviated by moving away from them, the movement required is several hundreds of miles & the pesky jet streams seem to follow you wherever you go. Tropical & polar areas may not have so much issues from jet stream but they do have severe storms & generally insufferable temperatures.

Avoid setting up in valleys or other low areas as cold air pours in at night that really messes up things. Dew is also more likely in low areas. Looking out over a large body of water (ponds, lakes) can also help as the surface water is essentially all the same temperature and the air above it less disturbed.

Yeah, but areas near ponds & lakes tend to have heavy dew & mosquito / midge infestations, whilst areas near the sea coast have salt spray and/or coastal fog. Forests would probably be great except for the trees blocking the view. There are no easy answers.

#8 azure1961p

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Posted 24 July 2013 - 06:42 AM

Asphalt isn't always bad and its easily dealt with...

Its always worse just after sunset but lessens its detrimental effects as the night progresses. Too asphalt at the end of a summers day is far worse than the cooler months. It is easily negated. During summer Id liberally but gently hose the area and it killed it for a good thirty minutes then the gentle douse again. Its really simple folks. After midnight I wonder if asphalt is they bad anymore. Prior though there is no doubt water cooling can produce excellent results. The upside (sometimes) is that observing over asphalt can also abate dew. Not always that's for sure but it can make the difference sometimes.

Its a quirky thing with pluses and minuses.

Brianb: yes the insufferable heatwaves in New England actually produce tropical climate seeing - but wow what a bother sometimes. Last week when we had a good heat wave it was wet and 80 at 10pm. Superb seeing but man what heat and humidity.



Pete

#9 Asbytec

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Posted 24 July 2013 - 07:17 AM

Pete, great point on asphalt and concrete. Wetting down your immediate surroundings is one thing, wetting down a city block is quite another. I observe on concrete regularly.

Still, concrete cools fairly rapidly, toward midnight it's cool to the touch. The problem seems to be working around materials or objects that retain or generate significant heat.

As far as seeing is concerned, though, local ground effects are dwarfed by higher level affects due to the angles (the proper term eludes me) and distances involved.

Glenn makes a great point about the temperature gradient being uniform.

#10 Mike B

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Posted 24 July 2013 - 11:57 AM

Still, concrete cools fairly rapidly...

I guess that depends on its thickness? In building thermal design, concrete/masonry is said to absorb+transmit heat at the rate of 1-inch per hour... so a 12" thick wall of brick or concrete, exposed continually to sunlight (ie. "insolation") will present a still cool face on the opposite (ie. interior) side for approx. 12 hours. Then, once the sun has set & the night grows cold, that stored up heat will reverse direction, bleeding outwardly toward the wall's surfaces- thereby warming the building's interior for many hours into the night. Again, at roughly 1-inch per hour.

In really hot weather, i'll hose the patio, even several times, to speed cooling... beyond that for several feet is lawn. But as has been stated, at some point the air you're looking thru will occur above warm pavement or rooftops. So there are limitations to what one can do here. :shrug:

However, it's definitely worth noting that the temperature gradient in the air above warm pavement is likely not linear, but geometric or exponential. So moving the optics & lightpath therewith upwardly, away from the warm ground, benefits far more than a linear function. For all the harrumphing EQ mounts get, for steady(er) hi-magnification views in warm weather, an EQ-slung Newt will have significant advantages over a ground-hugging Dobsonian-mounted Newt, having the optics elevated off the ground. I've seen this myself.
:ubetcha:

#11 Dwight J

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Posted 24 July 2013 - 02:04 PM

It is not just the surface of the asphalt that you have to worry about. Underneath that is packed gravel and sand adding to the thermal mass. It will radiate heat all night, even in the winter.

#12 Mike B

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Posted 24 July 2013 - 03:08 PM

Yeah, i wondered about that- but also wondered if the air encapsulated in the looser substrate beneath most paving might act as a buffer- going BOTH ways? I really don't know if, or to what extent that might be.

The ground itself would be equally bad in this regard, but if covered with landscaping materials (ie. lawn or field), said material effectively shades the ground all day.

It's during the really warmer weather that i notice this mostly... during which i have the option of deploying on grass; and it seems to make a difference in my Dob's planetary views.

#13 Kevdog

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Posted 24 July 2013 - 04:16 PM

I have it challenging where I live. I'm in Phoenix, AZ so thermal heat is a problem. I usually observe from my driveway as it's easy to get my scope from the garage and just set it up out there. But obviously I'm sitting on concrete, next to a big concrete block house as well. The rest of my property is covered with rock which radiates too, but not as bad as the concrete.

The best option I have is to take it to my pool area, which I filled in and covered with grass, but that's 150ft away from the garage. It's a hike with the C11. (It wasn't bad when I had the LT8 at only 45lbs).

It's not too bad in the winter, but in the summer the heat stays for a looong time. When it's 100F at 10pm the concrete isn't yet cooling off much!

Nice thing is I've never had to worry about dew!

#14 Jon Isaacs

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Posted 24 July 2013 - 09:37 PM

Barry:

In line with what Glenn said, I would add trees to the objects to be avoided. In my backyard, trees seem to give off heat and disturb the seeing.

Jon

#15 Asbytec

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Posted 26 July 2013 - 01:44 AM

Trees? Hummm...I used to observe on concrete right next to a grove of trees.

I guess it really doesn't matter if the air is boiling hot or frozen solid as long as its fairly uniform in density...and not moving across the line of sight with any velocity. The vertical component will change in density (like hot AZ air rising from the heated surface, but as long as the thermal layer is fairly flat it should be fine.






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