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High clouds and seeing


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Hi all

My question today is how high clouds affect seeing. On ‘Clear outside’, it’ll show conditions as red (bad) when all other factors other than high cloud coverage are good. When looking up just with my eyes, the sky looks clear and I can see stars etc. If I’m not mistaken, high clouds transmit a lot of visible light, so do they have a noticeable effect when looking through a scope? Is the effect just greater turbulence and atmospheric wobble or does it decrease apparent brightness? Thanks all!

tldr; Is it a waste of time taking the scope out if there are high clouds?

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High cloud can of course come in thicknesses varying from almost nothing up to so thick that you can't see anything.

Apart from that, the type of target will also make a difference. The moon is often perfectly observable through thin cloud, and also brighter double stars and clusters are possibilities. At the other end, diffuse nebulae and galaxies will, by their nature, tend to lose their (insubstantial) boundaries when viewed through mist and easily become invisible.

Did you specifically mean to ask about "seeing"? That tends to be affected more by wind shear. Transparency is the measure of light transmission. In very thin, high cloud, even if pervasive, observing may still be good. Here's an example from Metcheck:

image.png.6563811cfa9d432160d32047855dd445.png

image.png.4481f5c26d1379ff594452c00601c79a.png

Here, Transparency and seeing are both forecast to be good. ("AstroIndex" is a proprietary, experimental combined score, which often seems to be affected by any forecast cloud)

2 hours ago, sorrimen said:

Is it a waste of time taking the scope out if there are high clouds?

I asked the same question myself when I was starting out. It depends how much effort is involved for you to set up, and how disappointed you are if the effort is wasted. What I have certainly learned in the last two or three years is that the forecasts in the UK are not reliable in "normal" conditions. So if you stay indoors when high cloud is forecast, quite often you will end up kicking yourself. I now tend to set up if it looks half decent AND the forecasts are correctly telling me what I can see out of the window in the late afternoon.

 

 

 

 

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1 hour ago, Zermelo said:

High cloud can of course come in thicknesses varying from almost nothing up to so thick that you can't see anything.

Apart from that, the type of target will also make a difference. The moon is often perfectly observable through thin cloud, and also brighter double stars and clusters are possibilities. At the other end, diffuse nebulae and galaxies will, by their nature, tend to lose their (insubstantial) boundaries when viewed through mist and easily become invisible.

Did you specifically mean to ask about "seeing"? That tends to be affected more by wind shear. Transparency is the measure of light transmission. In very thin, high cloud, even if pervasive, observing may still be good. Here's an example from Metcheck:

image.png.6563811cfa9d432160d32047855dd445.png

image.png.4481f5c26d1379ff594452c00601c79a.png

Here, Transparency and seeing are both forecast to be good. ("AstroIndex" is a proprietary, experimental combined score, which often seems to be affected by any forecast cloud)

I asked the same question myself when I was starting out. It depends how much effort is involved for you to set up, and how disappointed you are if the effort is wasted. What I have certainly learned in the last two or three years is that the forecasts in the UK are not reliable in "normal" conditions. So if you stay indoors when high cloud is forecast, quite often you will end up kicking yourself. I now tend to set up if it looks half decent AND the forecasts are correctly telling me what I can see out of the window in the late afternoon.

 

 

 

 

Thanks so much for this speedy response. This is super detailed and exactly what I needed, plus a bit of a lesson in seeing vs transparency etc. Given that it’s just an 8” dob a few minute walk away and I’m still fairly new I may as well just get out when it looks relatively clear, so I’ll follow your advice. 

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Lingering high haze and thin cloud can indicate good seeing (but poor transparency) as it shows that wind speeds are low up there in the atmosphere, so would be a good time to try some lunar and planetary observing.

Yesterday evening noticed that jet contrails in the sky were hanging around for a long time and taking ages to diffuse, this is another good sign that the atmosphere is stable and seeing will be good. Didn’t get a chance to get the scope out though! 

Edited by CraigT82
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  • 4 weeks later...

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