[AZ-Observing] Re: North country windstorm
- From: Brian Skiff <Brian.Skiff@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Tue, 24 Jun 2003 15:06:14 -0700 (MST)
>> Have you often had nights where the telescopic seeing is the "reverse" o
>> the naked eye view?
Plenty, notably in winter after a storm passage (Monday night was
almost the same synoptic situation, just no snow!). The naked-eye sky
will show only "normal" twinkling, yet the star images in the telescope
are 20, 30, even 50 arcsec(!). It happens when the wind has switched
to the northeast, but aloft it's still coming down the back side of the
trough from the N-NW. Other times, sure, the sky is pulsating wildly,
and the images are merely "bad" rather than piss-poor.
Certainly plenty of occasions, too, where it's very windy like you
saw a couple nights ago, but well ahead of a frontal passage---and the
seeing is subarcsec as estimated from close double stars. And again,
nearly calm and the seeing is lousy, usually from local cold air
drainage. There's also the case that Tom Polakis and I remember from
when we were at Las Campanas in Chile, where the wind was howling yet
seeing was terrific. The wind business is a matter of lack of vertical
turbulence (unlike during a trough passage, when vertical motions are
strong), and the uniformity of temperatures---as long as it's isothermal,
wind is not a detriment to seeing.
Alan MacRobert had some nice articles about all this (and "fast" and
"slow" seeing) in S&T about ten years ago.
\Brian
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