Quote:
Originally Posted by Stonius
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
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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.
