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Clouds and Seeing - Fact or Urban Legend?

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#1 KTAZ

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Posted 05 August 2020 - 09:46 AM

For many years I have read reports from folks saying that some of the best seeing they have had comes between passing clouds on partly cloudy nights. I’ve also read reports that very high and thin clouds can steady planetary seeing. I’ve never experienced this myself since I am generally too lazy to set up on a cloudy night and wait for an opening to observe or shoot through.

 

Is this a myth? Has anybody had this experience? Is there a scientific (meteorological) explanation for it?


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#2 Jim Davis

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Posted 05 August 2020 - 09:54 AM

I have found that true on the high thin clouds. I don't think they cause good seeing, but the clouds are formed at times of good seeing. A couple of years ago at the Cherry Springs Star Party, we had a week of good, clear nights. Saturday night turned hazy, and everyone went to bed. I took a look at the planets, and it was one of the best nights of seeing I experienced in Pennsylvania. A group of us spent hours checking out Jupiter and Saturn, while everyone else were snoring.


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#3 Asbytec

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Posted 05 August 2020 - 10:00 AM

I've tended to notice normal seeing deteriorates as clouds approach the field of view, then improves back to normal as they leave the area. Not necessarily some of the best seeing between clouds, just better than in the clouds. To my way of thinking, it makes some sense. Aircraft usually hit some turbulent air passing through cumulus clouds.

As for high thin clouds, I might agree some better seeing conditions often exist with a high thin overcast. I've done some lunar and planetary observing during those observing conditions. This makes sense, as well, as stratus clouds might suggest smooth air. Often at altitude.
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#4 Galaxyhunter

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Posted 05 August 2020 - 12:28 PM

When I started this hobby, I was told that "Seeing & Transparency" are basically tie together in the opposite direction.  Example,  in the Summer with the humidity & haze, the air is actually steadier, leading to better Seeing, but because of the haze & humidity, the Transparency is down.  Now in the Winter when the air is crisp & clean,  the Transparency is high but you get a lot more star "Twinkle" so the Seeing is lower.

 

I have found this to be somewhat true even though I have not documented this.  Now this is from the conditions of the upper Midwest, other locations could have different outcomes. YMMV



#5 Jim Davis

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Posted 05 August 2020 - 01:10 PM

In Pennsylvania, we typically have bad seeing year round. Mostly due to the way the jet stream runs across the state. It is a rare day when we get even "good" seeing. A very good night is very rare.



#6 Tony Flanders

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Posted 05 August 2020 - 01:31 PM

I've tended to notice normal seeing deteriorates as clouds approach the field of view, then improves back to normal as they leave the area. Not necessarily some of the best seeing between clouds, just better than in the clouds. To my way of thinking, it makes some sense. Aircraft usually hit some turbulent air passing through cumulus clouds.


Different kinds of clouds form for different reasons. Cumulus clouds form at the top of thermal uplifts, almost always during the daytime. It's that thermal uplift that causes turbulence. Stepping back from the details, the patchy, clumpy nature of cumulus clouds implies some kind of inhomogeneity, and inhomogeneity is what causes bad seeing.

A priori, one would expect clouds that form in uniform sheets that stretch over large areas of land -- such as altocumulus -- to be a sign of homogeneous atmospheric conditions. So it would make sense for them to be associated with good seeing. Those are, in fact, the kinds of clouds where I have experienced good seeing.

But over-simplified science like what I've done above, together with snippets of anecdotal data, are not a substitute for systematic data.


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#7 Rutilus

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Posted 06 August 2020 - 07:51 AM

I've tended to notice normal seeing deteriorates as clouds approach the field of view, then improves back to normal as they leave the area. Not necessarily some of the best seeing between clouds, just better than in the clouds. To my way of thinking, it makes some sense. Aircraft usually hit some turbulent air passing through cumulus clouds.
 

This is what I have also experienced. 


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#8 rhetfield

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Posted 06 August 2020 - 08:40 AM

I had good seeing on the planets between clouds earlier this week as the rain clouds broke up.  Could see extra bands and the red spot (actually red for a change) along with very crisp festoons on Jupiter at both 65x and 144x.  Better than usual banding on Saturn also and a bit easier to see the cassini division.  Also saw a 2nd moon on Saturn (rare in my skies - especially considering how close the full moon was).  Was able to push Saturn to 216x.  Some of the best atmosphere all year.


Edited by rhetfield, 06 August 2020 - 08:41 AM.



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