RE: [ATM] Stone observatory folly

[Mark Holm <mdholm@telerama.com>]

One source I read said that low altitude (two to three meters) bad 
seeing at night is actually do to cooling rather than warmth from the 
ground.  On a clear night, anything with a fairly high area to heat 
capacity ratio (grass, leaves, sand, low heat capacity observatory 
buildings, etc.) cools pretty quickly by radiation.  The cold surfaces 
in turn cool the nearby air.  Since the surfaces are all relatively near 
the ground, and since cooled air falls, you end up with an inversion 
layer of cold air hugging the ground.  This is one reason hill or 
mountain tops are frequently good observing sites.  Cold air flows off 
of them into the valleys.

The greatest part of the bad seeing comes from the interface between the 
cold inversion layer and the warmer air over it.  If the interface would 
stay nice and smooth, it wouldn't be so bad, but even a hint of breeze, 
or some convection from warm, high heat capacity objects on the ground 
puts the interface into uneven motion.   Cold air has a higher 
refractive index than warm, so the uneven interface between the two 
works just like a sheet of wavy glass.  Perhaps a more apt comparison 
would be the surface of a swimming pool.  When the water is still, you 
can see through the surface fairly clearly.  Make some waves, and the 
visibility plummets.

Now the beauty of it is that the inversion layer only has a few hours to 
form each night, so it usually can't get too thick.  A few meters can 
get you above it.  A sensitive electronic temperature gage mounted on a 
pole can be used to profile the temperature of the first few meters of 
air to see if such an inversion layer exists at your site and how thick 
it is.  Some of the more electronically inclined list members ought to 
be able to describe a fairly simple and not too expensive way to rig up 
such a beast.  I am guessing you probably want temperature resolution on 
the order of 0.25C in the range of typical night time temperatures.  
Absolute accuracy isn't a big concern so long as differential readings 
can be stable over a reasonable period, such as one night.

Of course, you might want to consider your location too.  If your 
proposed observing site is in the middle of a large basin (Los Angeles 
for example.), you may be commonly stuck under a much thicker inversion 
layer that doesn't dissipate and reform every 24 hours.  Two or three 
meters might not help much in those circumstances.

-- 
Mark Holm
mdholm@telerama.com


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