To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


It is common for the air near the ground to cool off at night when the sky
is clear.  It is one of the effects of radiation cooling.  It will be worst
on just the sort of night an astronomer craves: dry air, clear skys.  The
cool air will flow downhill just as you describe.  Even if it doesn't move
much, it is almost impossible for the interface between the cool air and
the warm air over it to be completely still and even.  That unevenness will
cause refraction, i.e. poor seeing.

The cool air layer is often only a couple of meters thick, and there can be
considerable temperture change over the height of a man!  Getting higher up
by only a couple of meters can be a big help.

This is one case where a closed tube telescope might have an advantage. 
The tube, once it got reasonably equilibrated would trap a fairly stable
column of air of a couple meters length.  It might be enough to poke a
stable hole through the moving, unstable, cool air flowing near the ground.

Mark Holm
mdholm{at}telerama.com

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