Good BOM chart for identifying seeing conditions

[mental4astro (Alexander)]

Hi all,

Ol' Wavytone, with his thing for flying, has put me onto a BOM page that gives the data that's gathered by those big silver weather balloons they release everyday. The data that they gather in shooting up through the sky reveals an amazing amount of information that is helpful to us observing through our scopes. Bringing up this page can help you also decide if pulling out a scope is worthwhile if you intend to do high magnification observing.

The chart in question is their Aerological Diagrams. It brings up a map of Australia, and you just select the city/town you want. I am not across all the information that the chart shows, I'll leave that to others more informed than I, but I will note the information that is relevant to us. The chart shows various data entries versus altitude as the balloon rises. On the far right is a lot of 'arrows', these signify wind velocity and direction.

The most significant information is shown by the red line on the left hand side. It shows dew point against altitude. The more serrated the line is, particularly just below the 5,000m mark (noted in the chart as "5.0" as altitude is noted in 1000's), it means that there is an inversion layer (hot air trapped under cold), and this serration is the effective turbulence that's created as these two air masses pass over each other. The more drastic the serrations are, the greater the turbulence and the worse seeing conditions are. The smoother the line, the less, if any, turbulence, and the better the seeing. In an aircraft you notice this with the plane's dramatic and sometimes violent rise and fall and you poop your pants as you think you're about to fall out of the sky.

The serration is created by the dramatic rise and fall in temperature as the balloon rises travelling through the turbulence.

Here are two examples showing 'good' seeing and 'poor'. The first chart is from today for Sydney. It shows the red line without any significant wild variations in the dew point, just a single smooth steady line that shows the temperature is just consistently dropping as the balloon rises. All of this suggests seeing is good:



This second chart shows a significant 'serration' of the left side red line reaching the 5,000m mark. The means a lot of turbulence and poor seeing:



Being sceptical of anything Wavy tells me, , , I thought I'd put this info to the test on several nights. Low and behold! it worked! Those nights that saw the aerological diagram give a smooth, steady temperature drop gave fantastic seeing conditions. Those nights that saw the red line very serrated were nights of poor seeing conditions.

On those night now when I'm contemplating doing a sketch of the Moon or planets, or some other high magnification work, I consult this chart before setting up. It's saved me time and frustration knowing in advance if it would be worth the while or not. But if high magnification is not my main fodder for the night, I don't bother with the chart.

Wavy mentioned to me how this information is gathered. The balloons gather primarily temperature and humidity readings (maybe altitude?). These balloons have attached under them as part of their ballast, a silvered reflective catseye prism that reflects the radar signal from the airport. It is this that gives the BOM the balloons direction and velocity (altitude too?). Other variations on weather balloons sees the ballast carry a transponder rather than a reflector, and other instruments.

Hopefully others here will find this chart useful.

Mental.

[Tinderboxsky (Steve)]

Hi Mental,

Many thanks for the heads up. Excellent.

It is interesting to note the significant wind shifts associated with the serations in the example for poor seeing.

Cheers

Steve.

[mental4astro (Alexander)]

Ta, Steve,

Yes, I forgot to mention the wind direction again and its influence upon turbulence.

I've also made a couple of changes to the information I noted in my original post. I've had a PM by a more knowledgeable person than myself about these charts, and I'm very grateful for the enlightenment, and their integrity in notifying me first. Cheers mate, .

[Camknox (Cameron)]

What a resource! Thanks for sharing

[AstralTraveller (David)]

Yes, a very useful tool. Normally I would say that any clear night is a good night, after all we don't get enough of them, but there was a night recently that, despite it being clear, we packed up and went home. So I expect mostly I would consult these plots to see what sort of night it will be but perhaps I'll eventually feel I can use them to decide whether it's worth going out.

I found an explanation of the plots here. http://www.bom.gov.au/aviation/data/...ion/skew-t.pdf

[PeterM]

Great thread and really interesting thanks!
Bob (astronobob) sent me this storm chasers link that also explains using this data. What I am wondering is if this is how Skippy Sky's seeing data is generated, if not then what data is used to predict seeing and how accurate is it compared to the aerological data?
http://www.downunderchase.com/storminfo/stormguide/

[Paddy (Patrick)]

Thanks Alex - very useful. I wonder if these charts or any other give transparency information. So often in recent times I've set up the scope and the sky looks good only to be disappointed by how foggy it looks through the ep.

[Wavytone]

Hi Paddy,

Personal experience has been that the aerological charts are very helpful to pick a night with steady seeing (or conversely explain why the seeing is terrible) but not much help with regard to transparency. For that the satellite photos of the cloud layers are better, though note this relies on infrared cameras. High clouds are much colder than the land below, so show up well in infrared, but low cloud - especially clouds that are sitting on peaks or worse, ground-level fog - don't show up as they are basically the same temperature as the land.