What conditions produce good 'seeing'

Rokketboy (Jared) · #1 22-04-2009, 08:07 PM

Hello all,
I was just wondering if there are any particular weather/atmospheric conditions produce particularly good or particularly bad seeing conditions.
If its windy, are seeing conditions poor?
If its been a particularly hot day, does that make for bad seeing?
Any thoughts/facts would be great.
Cheers

matt · #2 22-04-2009, 08:45 PM

OK....I'll kick this off...but I'll only get the list started...others can/will chime in with the extra bits and pieces.

First of all, we're talking about seeing, as opposed to transparency. Seeing is about the prevailing atmospheric conditions as opposed to cloud/smog/fog/dew etc which is transparency.

So...for good seeing, it usually accompanies a high pressure system and airflow at varying altitudes moving in pretty much the same direction at comparatively low speeds.

Also, study the effects of laminar airflow over even terrain, rather than being on the receiving end of airflow which is 'falling' on you after rising over a nearby range.

kinetic (Steve) · #3 22-04-2009, 08:59 PM

Quote:

Originally Posted by Rokketboy

Hello all,
I was just wondering if there are any particular weather/atmospheric conditions produce particularly good or particularly bad seeing conditions.
If its windy, are seeing conditions poor?
If its been a particularly hot day, does that make for bad seeing?
Any thoughts/facts would be great.
Cheers

I'm certainly no expert on weather but
I think it varies if you are inland or coastal.
From my experience here in Adelaide the best nights
I have ever had follow this sort of pattern:

Usually a stable High has just passed here
The jet trails of planes last for an hour or so before dispersing.
Sometimes there is very high thin cirrus.
The high voltage powerlines buzz with a slight dew.

It's time to open the dome and do an all-nighter!

Steve

Robh (Rob) · #4 22-04-2009, 10:45 PM

Jared,

Check out the article "Beating the Seeing" from Sky & Telescope.
For your questions, specifically page 5. Ignore the USA weather maps.
http://www.skyandtelescope.com/howto...tml?page=1&c=y

Regards, Rob

Wavytone · #5 22-04-2009, 11:13 PM

If its windy, are seeing conditions poor?

>> yes, the more the wind the worse the seeing. In any more than a light breeze, say 5 knots max, don't bother setting up.

A slight breeze helps keep the dew off; dead calm = dew.

If its been a particularly hot day, does that make for bad seeing?

>> Yes, because objects on the ground exposed to sun have soaked up a lot of heat and will be a source of thermals all night as they cool down.

There is a huge variability in microclimate too, according to exactly whre you are and what soil type surrounds you.

For example a lush well-gardened or farmed area with black soils that have been well watered are terrible for dew - council ovals, for example, and being dark the soil will radiate heat all night and the temperature drop will be relatively large. Setting up a scope in this kind of terrain you will be fighting dew all night (SCT's, refractors and maks) and having problems with cooling your optics down to the air temp.

An area that is typically light, sandy soil or mostly sandstone will not hold water on the surface (and is unlikely to be watered), and being light coloured, does not radiate heat back to space at night so it will not be so cold.

After spending 20 years observing in the Blue Mountains near Mt Banks at 1050 metres all year round, it was always noticeable that in the sandstone territory at night I never saw a frost and dew was not a problem, yet the tar road (black) could have ice on it at night. Similarly Leura oval (looks like a reasonably attractive place to observe) was a lousy place for dew because it had black soil and was amply watered to keep the grass green.

ngcles · #6 22-04-2009, 11:52 PM

Hi Rocketboy,

That article at S&T is a good one and summaries the main points well. High-altitude Jet-stream winds are almost always bad news. You can get a map that plots them for Australia here:

http://weather.wvec.com/auto/wvec/gl...JetStream.html

avoid them where possible. A nice, big, slow-moving, stable high-pressure system with widely-spaced isobars is almost always good for good seeing, but generally won't bring good transparency. Low-pressure systems with closely spaced isobars usually mean trouble in paradise.

Transparency, obviously enough is how see-through the atmosphere is. In Sydney at least, the two (transparency & seeing) almost never go hand-in-hand.

Altitude coupled with a laminar air-flow helps too. It's no accident that this is where some of the biggest and best observatories in the world are situated like Chile and Hawaii. They have laminar air-flow for 1000s of kms before it gets to them, high altitude and low humidity and cloud-levels.

Best,

Les D

Rokketboy (Jared) · #7 23-04-2009, 05:41 AM

Thanks for all the info guys, exactly what I was after!
Cheers.

multiweb (Marc) · #8 23-04-2009, 07:50 AM

Cool thread. Read with interest. Thanks for sharing resources and links

Robh (Rob) · #9 23-04-2009, 10:57 AM

Quote:

Originally Posted by ngcles

That article at S&T is a good one and summaries the main points well. High-altitude Jet-stream winds are almost always bad news. You can get a map that plots them for Australia here:

http://weather.wvec.com/auto/wvec/gl...JetStream.html

avoid them where possible. A nice, big, slow-moving, stable high-pressure system with widely-spaced isobars is almost always good for good seeing, but generally won't bring good transparency. Low-pressure systems with closely spaced isobars usually mean trouble in paradise.

Les. Thanks for the aussie jet-stream site.
Regards, Rob.

iceman (Mike) · #10 23-04-2009, 11:02 AM

There's a bunch of jetstream weather resources at:
http://www.iceinspace.com.au/?weather

Some of the best seeing i've had is in very dewey or very foggy conditions.

Robh (Rob) · #11 23-04-2009, 11:09 AM

Quote:

Originally Posted by iceman

There's a bunch of jetstream weather resources at:
http://www.iceinspace.com.au/?weather

Some of the best seeing i've had is in very dewey or very foggy conditions.

Thanks Mike. Didn't realise.
Regards, Rob.

mozzie (Peter) · #12 23-04-2009, 08:55 PM

after a good thunder storm the seeng is usually fantastic have a look next time
mozzie

gregbradley · #13 23-04-2009, 09:29 PM

For my area 150kms west of Sydney I find winter skies are more stable than summer, after a thunderstorm as mentioned can be steady and seeing improves as the night goes on and can peak perhaps around 2am.

Windy is bad. Apart from that no real patterns noticed.

Greg.