What weather makes for poor seeing conditions?

[MarkVIIIMarc]

Last night I went out to try my new lens and was surprised how poor my views of Jupiter were from not just that eyepiece but my other ones also.  In the weeks I had owned my scope it was the first time the sky was really different.  In hindsight I am thinking I am thinking I should have expected the seeing conditions to have been terrible given the weather. 

Earlier it rained.  At one time a few drops of rain fell and I moved me and my equipment under cover.  It has been cold and very windy the last few days and I was sneaking peaks inbetween clouds just to try out the eyepiece.  The clouds were moving at quite a clip so even though mybe 4/5 of the sky was cloudy a minute of pacing gave be a break to see Jupiter.  To my eyes though the sky above the clouds looked dark and a good number of stars were visible when the openenings presented themselves. 

What general weather makes for poor viewing?  Big temperature changes leading to thermal rises?  Wind in general?

Thanks in advance.

[Dave In Vermont]

Take a look at the 2 maps I've posted below. Do you see the wavy-lines? Those are called 'isobars.' They indicate the barometric-pressure along that line is the same. The next line to these also have the same pressure along it. They only differ from the other by 1 percent of the next. So 1 line may be 30.1" barometric-pressure, while the one next to it is 30.2" and so forth. But when these lines are closer together, this shows rapidly changing isobars. This indicates a disturbance in the weather will happen quickly - and this can be read as rain, high-winds, and even a tornado. So the closer the isobars are to each other, the atmosphere is changing quickly - at ground-level, or higher up in the atmosphere.

And this is one way to figure the 'seeing' we will have.

Still skies -

Dave

 

[Putaendo Patrick]

What weather makes for poor seeing conditions?  English weather, as I remember!

[acey]

"Seeing" refers to stability of the air, "transparency" refers to clarity. Hazy, stable air has good seeing and poor transparency; clear, turbulent air will have poor seeing, good transparency. For planetary viewing, high-power double-star splitting etc, you want good seeing. For DSOs (galaxies and nebulae) you want good transparency.

When a rain front passes you can have cold, clear air behind - people say the rain has "cleaned the air", though really the air has just moved. This clear air can have excellent transparency, ideal for DSO viewing, but can also be highly turbulent, with poor seeing. It sounds like this is what you experienced.

On breezy nights you'll have no problem from mists (or dewing on optics) - all good for DSOs. Completely calm nights can be beautifully clear, but those can also be nights when mists and haze form. Those stable conditions can be great for planetary viewing.

I've had some of my best DSO sessions in the immediate aftermath of heavy rain - or in breaks between downpours. A raincover for your scope can be a handy thing. But that's only when there's a front passing. So it's a good idea to have a barometer.

[Stu]

Above answer is an excellent explanation. The only thing I would add is that I've recently become aware of the impact that the Jetstream has on seeing conditions for planetary viewing. When the high speed winds are overhead, conditions tend to be bad, will comparatively little detail visible. When the Jetstream shifts, as it has done a few times recently, the views can be dramatically better.

http://www.ukweatherforecast.co.uk/jetstream-forecast/

[Colinlp]

1 hour ago, Stu said:

Above answer is an excellent explanation. The only thing I would add is that I've recently become aware of the impact that the Jetstream has on seeing conditions for planetary viewing. When the high speed winds are overhead, conditions tend to be bad, will comparatively little detail visible. When the Jetstream shifts, as it has done a few times recently, the views can be dramatically better.

http://www.ukweatherforecast.co.uk/jetstream-forecast/

The effect of the jetstream was something we were told about in university whilst doing a visual project.The "Model Data" section of https://www.wunderground.com/wundermap/ is my favourite resource, the 200mb, 300mb and the shear graphs seem to be the most important

[Linda]

At least, I have noticed that you can't predict from the number of stars how the seeing is. I have experienced a lot of bad seeing this winter, also when a lot of stars were visible. But I've written in my log often that there was no point in using high magnification, while the scope was cooled down properly. Luckily, at the double Jupiter moon transit, the seeing was pretty good again, so it is not the scope itself.

[Stu]

44 minutes ago, Colinlp said:

The effect of the jetstream was something we were told about in university whilst doing a visual project.The "Model Data" section of https://www.wunderground.com/wundermap/ is my favourite resource, the 200mb, 300mb and the shear graphs seem to be the most important

Great link Colin, many thanks!

[bigrich]

Thanks for this link l to will use it thank you again Richard

[Peter Drew]

I have noticed when observing through gaps in the clouds that the oncoming clouds seem to carry a bow wave of bad turbulence ahead of them causing an impact on the image just before they arrive.

[laudropb]

Lots of good explanations and answers in this post, but in the end predictions are fraught with difficulties and it is often a case of getting out and having a look for yourself.

[acey]

3 hours ago, Stu said:

Above answer is an excellent explanation. The only thing I would add is that I've recently become aware of the impact that the Jetstream has on seeing conditions for planetary viewing. When the high speed winds are overhead, conditions tend to be bad, will comparatively little detail visible. When the Jetstream shifts, as it has done a few times recently, the views can be dramatically better.

http://www.ukweatherforecast.co.uk/jetstream-forecast/

Good point. There are two distinct ways that air movement disrupts images. One is the tilting of wavefronts by turbulent air, causing the image to jiggle and blur: this is "seeing", and the effect comes mainly from low-altitude air movement (including convection from the ground). The other is curvature of wavefronts by air parcels of different density passing through the line of sight: this is scintillation, and it has a focussing or defocussing effect, making stars vary in brightness (and colour). This is due to high-altitude winds, so a jetstream is particularly significant. We tend to equate scintillation and seeing, thinking of both as "twinkling", but they are distinct. A site can have very good seeing but high scintillation: steady images with large brightness variation. This has been shown to be the case at observatories such as Mauna Kea. Scintillation has an aperture-dependent effect (lessening as aperture increases), which is why we notice twinkling mostly with naked eye viewing, much less so when looking through a telescope. With "seeing" it is of course the reverse. With a very large aperture telescope, where the Airy disc is very small, the image is a blur formed from the tiny disc moving rapidly. The diameter of this blob (in a perfect telescope) will be about 3 times the size of the Airy disc. So if an observatory has "seeing" of one arcsecond then the star images seen through the telescope would have a diameter of about 3 arcseconds.

Incidentally, scintillation can I think explain the very impressive magnitude limits claimed by some very experienced observers at the darkest sites. In extreme cases a star can be brightened momentarily by over a magnitude, so a person with a limit of 6 mag might get a fleeting glimpse of a star of above-atmosphere magnitude 7 or even 8. The person would be seeing the star briefly magnified and brightened by a passing parcel of high-altitude air. It's a moot point whether one should then declare the site to have a limiting magnitude of 7 or 8. This is why I think limiting magnitude should refer to stars visible for a sustained period, rather than glimpsed momentarily.

[Stu]

14 minutes ago, acey said:

Good point. There are two distinct ways that air movement disrupts images. One is the tilting of wavefronts by turbulent air, causing the image to jiggle and blur: this is "seeing", and the effect comes mainly from low-altitude air movement (including convection from the ground). The other is curvature of wavefronts by air parcels of different density passing through the line of sight: this is scintillation, and it has a focussing or defocussing effect, making stars vary in brightness (and colour). This is due to high-altitude winds, so a jetstream is particularly significant. We tend to equate scintillation and seeing, thinking of both as "twinkling", but they are distinct. A site can have very good seeing but high scintillation: steady images with large brightness variation. This has been shown to be the case at observatories such as Mauna Kea. Scintillation has an aperture-dependent effect (lessening as aperture increases), which is why we notice twinkling mostly with naked eye viewing, much less so when looking through a telescope. With "seeing" it is of course the reverse. With a very large aperture telescope, where the Airy disc is very small, the image is a blur formed from the tiny disc moving rapidly. The diameter of this blob (in a perfect telescope) will be about 3 times the size of the Airy disc. So if an observatory has "seeing" of one arcsecond then the star images seen through the telescope would have a diameter of about 3 arcseconds.

Incidentally, scintillation can I think explain the very impressive magnitude limits claimed by some very experienced observers at the darkest sites. In extreme cases a star can be brightened momentarily by over a magnitude, so a person with a limit of 6 mag might get a fleeting glimpse of a star of above-atmosphere magnitude 7 or even 8. The person would be seeing the star briefly magnified and brightened by a passing parcel of high-altitude air. It's a moot point whether one should then declare the site to have a limiting magnitude of 7 or 8. This is why I think limiting magnitude should refer to stars visible for a sustained period, rather than glimpsed momentarily.

Thanks Acey, interesting stuff.

Does your post imply that low level winds have more impact on seeing than the Jetsream? I did not necessarily think this was the case, having had recent experience of very good seeing when the Jetstream moved away.

Having said that, you do say it has a focusing/defocusing effect which may reflect what I have been seeing?

There are times when planets can appear as if they as under a stream of flowing water, the seeing is that bad, which again I assume if high level winds?

Any clarification welcomed.

cheers,

Stu

[acey]

2 hours ago, Stu said:

Thanks Acey, interesting stuff.

Does your post imply that low level winds have more impact on seeing than the Jetsream? I did not necessarily think this was the case, having had recent experience of very good seeing when the Jetstream moved away.

Having said that, you do say it has a focusing/defocusing effect which may reflect what I have been seeing?

There are times when planets can appear as if they as under a stream of flowing water, the seeing is that bad, which again I assume if high level winds?

Any clarification welcomed.

cheers,

Stu

I do think your observations are consistent with the findings on the relationship between jetstream and scintillation. Of course in practice we're looking through lots of air layers with lots going on, so on any given night we're probably seeing the effects of air movement at many levels, with some more significant than others.

[Colinlp]

I've noticed over the years that there definitely seems to be a strong corralation between the the jetstream and seeing, the caveat I see I think is when shear is high i.e. air moving upwards is high irrespective of the jetstream, so both need to be as low as possible. When the air is stiller seeing is better in both horrizontal and vertical planes, it's something I've been looking at for years now (excuse the pun, unintentional!). Interesting comment above about clouds having a bow wave of bad seeing in front of them, can't see as I've ever noticed but it makes perfect sense, I imagine it is all around them rather than a bow wave

[The Admiral]

5 hours ago, Peter Drew said:

I have noticed when observing through gaps in the clouds that the oncoming clouds seem to carry a bow wave of bad turbulence ahead of them causing an impact on the image just before they arrive.

Interesting observation. Don't forget that newly forming clouds comprise a column of updraught into the base of the cloud, with descending air around the periphery. It wouldn't be surprising if these didn't cause optical disturbance ahead of the cloud itself. If there is wind shear as well, it's possible for there to be upward moving air on the upshear side of a convective cloud in addition to its core, rather than downward moving air, in some cases sufficiently strong for gliders to gain lift outside the cloud.

Ian

[Ruud]

Here's a link to the Meteoblue Astronomical Seeing forecast for St Louis:

https://www.meteoblue.com/en/weather/forecast/seeing/st-louis_united-states-of-america_4407066

They take the jet stream into account as well as bad layers (top, bottom and temperature gradient).

For good seeing, the jet stream must be neither too fast nor too slow, and the bad layer must be neither too thick nor have a too big temperature gradient.

You also get a low, mid and high cloud forecast on Meteoblue, as well as forecasts for temperature and humidity.

[Putaendo Patrick]

Ruud - thanks for the meteoblue.com link! I've set ifor Putaendo, Chile and bookmarked it.

[rubecula]

Meteoblue- what an excellent site, thanks for the link Ruud