Which has more effect? Elevation or laminar airflow?

[Stonius (Markus)]

My understanding is that flat, featureless plains give rise to laminar airflow, but that elevation puts the observer above the earth's 'boundary layer' as the land releases heat back to the atmosphere.


I'm told the ideal situation is a single hill in the middle of a large, flat plain.


The thing is, large featureless plains by definition tend to be running a bit short on hills. And hills tend to bring their friends along in the form of mountain ranges. In the rare situation where you actually have the desired conditions, (like with the extinct volcanoes here in Victoria), people have a bizarre tendency to want to own land, so they're typically not open to the public. Of course, the biggest flat plain with no boundary layer would be at the beach with light on-shore winds, but we all know what salty sea-air does to sensitive equipment.



So, it seems most people have to pick one or the other. Laminar flow or elevation. Any ideas which has more effect?

[Rerouter (Ryan)]

Laminar airflow helps more, but you can plan your mountains, as the air is still laminar until it hits mountains, so if you know the air is coming over the ocean and your on the start of the ridge, your good

[gary]

Originally Posted by Stonius

I'm told the ideal situation is a single hill in the middle of a large, flat plain.

Essentially that is what the Big Island of Hawaii is.

A 4,207 metre high peak rising out of the vast plain of the Pacific Ocean.
The surrounding ocean has the advantage that, on average, it is roughly
the same temperature.

The Big Island is essentially a volcano rising from the ocean floor.

When measured from the ocean floor, that is from base to peak,
Mauna Kea measures 10,210 meters.
By comparison, Everest rates a second at 8,848 meters.

Unlike Everest which is abrupt, craggy and jagged, Mauna Kea
forms a uniformly rising hump, over which laminar air flow passes.

Mauna Kea has an additional advantage that you can drive to the top.
The only catch is that you require a 4x4 in low gear to come down.
The air is so thin and provides so little cooling that constant braking
in a conventional vehicle runs the risk the brakes will catch on fire.
Now and then someone makes the mistake and a burnt out vehicle
with occupants standing by looking embarrassed provides another photo op.

[DarkArts]

Originally Posted by Stonius

Of course, the biggest flat plain with no boundary layer would be at the beach with light on-shore winds, but we all know what salty sea-air does to sensitive equipment.

Damian Peach (who I place in the category of astro demi-god) conducts many of his planetary campaigns on Barbados - he says it's because of the laminar air flow and proportion of clear nights. He has also cited stable jet stream as a factor for good astro. Given the highest peak on Barbados is 336m, DP's altitude was clearly less than that (and if I'm guessing his locations accurately, they were <200m elevation and only ~5km (or less) from the shore (Barbados is only about 10km across).

Don't forget that good laminar flow also depends to an extent on what's down-flow, as the turbulence boundary can extend quite some distance up-flow of the obstruction - the single peak in the middle of nowhere is ideal because there's no down-flow obstruction either.

So, IMHO, (at least) moderate distance from the shore, just enough elevation to avoid boundary effects and laminar air flow are the key factors. After that, being higher and drier gives additional benefits.

Of course, for mere mortals, the most important factor is an absence of neighbours with bright exterior lights pointed at your observing position.

[Stonius (Markus)]

I guess for me, I live in an apartment. I have to travel to observe anyway, so I can pick my sites. I spent yesterday driving around west of Melbourne and found one spot on the plains and another at elevation. I'm hoping they will do.

I can imagine using the one on the plains when there is more wind about because I figure the wind shaking the telescope will negate any advantage from elevation.