Weather Info

[technofetishism (Nick)]

Is there going to be clouds? Always a worthwhile question before one bothers setting up the gear, never fun to then have to take it down as you have been clouded out. So whats one to do but check the weather.

A few of the sites I personally find useful (and there's probably others I dont know about but wish I did)

The Bureau of Meteorology Is a pretty useful choice with its various radars and satellite images.

Brisbane Storm Chasers Good satellite imagery, longer and higher resolution that whats available from BOM for all of Australia.

Skippy Sky More details and map types for things like transparency, seeing, windspeed, clouds etc.

7Timer! Gives a little 3 day forecast of things us astronomers care about

Any other useful sites people can think of for those of us out there who dont know them yet?

[sil (Steve)]

The Space Weather site (available via BoM site) contains daily Solar and Geophysical activity.

[Aidan (Aidan)]

Thanks... Hadn't seen these

All I need is a program to chase away clouds and I'll be set. Haven't had a clear night in about 2 weeks.

Aidan

[DavidU (Dave)]

http://realtime2.bsch.au.com/vis_sat...=&start=&stop=

[pgc hunter]

I use Skippysky, BSCH and Weatherzone for my forecasting. Skippysky is actually pretty accurate with forecasting cloud 24-48 hours out. BSCh hi-res satpic loop is excellent for estimating what's going to happen in the next few hours.

[Shamrocker (Aidan)]

Probably a silly question but can anyone tell me how to read the seeing index on skippy sky! In Brisbane tonight it seems to me to be quite hazy and the seeing is poor. If I was 'in the know' how would I gather this from Skippy Sky?

[anthonywr (Anthony)]

SkippySky
and
http://www.cawcr.gov.au/staff/ljr/pr.../forcloops.htm

[Astro_Bot]

Originally Posted by Shamrocker

Probably a silly question but can anyone tell me how to read the seeing index on skippy sky! In Brisbane tonight it seems to me to be quite hazy and the seeing is poor.

Seeing was bloody awful tonight (at least where I was, looking west over the city!).

The SkippySky seeing map reads like isobars on a weather chart - use the colours and lines to determine what the boundary values are, e.g. the boundary between yellow and green is '4' and the yellow zone would be seeing between 3 and 4.

The scale of seeing used is 1-10, and is probably the modern Standard Scale, which is (from here):

The version given by the editors of Sky & Telescope as A Scale of Seeing (with clarifications adopted from Sidgwick, p.467) is representative of these modern versions of the Standard Scale:

1 — Airy disk and diffraction rings are blurred together; star image is about twice the diameter of the third diffraction ring (if the ring could be seen); star image 13 arcseconds (13") in diameter.
2 — Airy disk and diffraction rings are blurred together; image occasionally twice the diameter of the third diffraction ring (13").
3 — Airy disk and diffraction rings are blurred together; image about the same diameter as the third ring (6.7"); the central Airy diffraction disk is not visible, but the image is brighter at the center.
4 — The central Airy disk is often visible; short arcs of diffraction rings briefly seen on brighter stars.
5 — Airy disk always visible but is mobile; short arcs of the diffraction rings visible about half the time.
6 — Airy disk always visible though not sharply defined; short arcs of the diffraction rings continuously visible.
7 — Airy disk sometimes sharply defined, and distinct from rings; diffraction rings seen as long arcs or complete circles.
8 — Airy disk always sharply defined; innermost ring seen continuously but always in motion.
9 — Airy disk and innermost diffraction ring are stationary; the outer rings are momentarily stationary.
10 — The complete diffraction pattern is stationary.

If I was 'in the know' how would I gather this from Skippy Sky?

It seems to be a rough guide, but, like the weather forecast, not perfect. You can try running through the seeing map at different times to get an idea of how seeing conditions might change over time.

You can do the same with transparency.

Another resource given above, 7Timer!, uses (AFAICS) the Full Width at Half Maximum (FWHM) measure of seeing, which is described here.

[Shiraz (Ray)]

I use the BOM IR(day and night) satellite for short term cloud prediction.

At my imaging site, the fine resolution seeing seems to be best correlated with the 200hPa jetstream being under about 50kts and the best predictor for this is hidden away in the BOM site - if the jetstream is dark green or worse, the seeing will generally be poor.
http://www.bom.gov.au/australia/char...t=Refresh+View

The older seeing scales were based on moderate refractors and do not really capture what happens with a larger reflector. The definitions on http://www.damianpeach.com/seeingscale.htm are probably a lot more use for these telescopes. FWIW, I have never seen seeing corresponding to the "excellent" video.

[sil (Steve)]

Shiraz, thanks for that info!


As a beginner I don't really bother with weather forecasts, I look outside and if I can see stars I'll setup the scope. However I did a lot of preparations for the transit of venus, including using a number of long range forecasts online and ended up just following BoM and 7Timer for and fortnight before the event. I found 7Timer during those 2 weeks had the more accurate forecast when compared to the weather on the day, particularly the evening before it was bang on (clear, some clouds) while the BoM was miles off (overcast and storms).

Also its hard to know what the data size is they use for predictions. I found, by changing my lat/long location, that the forecast for me was identical for over 50km around (I was trying to find a location I could get to where the chance of rain for the transit was less than from home). Then when it did change it changed dramatically. So some services seem to have an average forecast for a broad region if there are only a small number of weather stations. You might have to look at the forecasts from a service around your area and see for yourself how wide the area covers (if you're right near a boundary in the forecasts you might be better averaging the two).

I also contacted observatories leading up to the transit and asked if they had a particular weather service they relied on. They all used BoM but said its only as a rough guide. It's generally okay to use for the two days before and not reliable for longer than that. The best forecast is still wait until the observing time and pop your head outside and judge for yourself

Out of interest, has anybody ever found any home weather stations producing a more reliable forecast (or any that really cater nicely to astronomers?).

[Colin_Fraser]

Originally Posted by sil

So some services seem to have an average forecast for a broad region if there are only a small number of weather stations.

Have a look deep into BOM site and you will find a listing of all weather stations in Australia.
Now do a count and you will see they have closed down hundreds in Victoria in the last few years.
Ballarat for instance had about 5 stations scattered around the area. Now there is only ONE and that is 6km from the city.
Porepunkah's station is 75km away. Thats why you have difficulty figuring out what the weather is, or going to be.

[Phil Hart]

Originally Posted by technofetishism

Is there going to be clouds? Always a worthwhile question before one bothers setting up the gear, never fun to then have to take it down as you have been clouded out. So whats one to do but check the weather.

A few of the sites I personally find useful (and there's probably others I dont know about but wish I did)

The Bureau of Meteorology Is a pretty useful choice with its various radars and satellite images.

Brisbane Storm Chasers Good satellite imagery, longer and higher resolution that whats available from BOM for all of Australia.

Skippy Sky More details and map types for things like transparency, seeing, windspeed, clouds etc.

7Timer! Gives a little 3 day forecast of things us astronomers care about

Any other useful sites people can think of for those of us out there who dont know them yet?

SkippySky, 7Timer and just about every other free weather site on the web use the same US weather model data. Check out this post for a whole lot more info:

http://philhart.com/content/cloud-fo...an-astronomers

I've been meaning to update that post with this site: yr.no
As far I know, it's the only (free) site that shows European Weather model data, which probably has the edge over the US model.

Best bet is to check Synthetic Forecast Loops (linked on my page and above in this thread) from Australian BoM, US model via SkippySky and European model via yr.no.

If they all agree then you can have high confidence in forecast. If they all say something different then you know you are taking a punt.

Phil

[alexV]

Or one big fan.

Originally Posted by Aidan

Thanks... Hadn't seen these

All I need is a program to chase away clouds and I'll be set. Haven't had a clear night in about 2 weeks.

Aidan

[sil (Steve)]

The Clear Sky Chart site covers Canada and USA.

[pgc hunter]

I would ignore both Skippysky and 7timer's seeing predictions.

Skippysky's seeing predictions seem to be tied to the jetstream and ignore all other factors. Almost always it displays Pickering 8 or more for Melbourne (with 1 being terrible and 10 being perfect on this scale).... which is obviously bulldust. I've gone out on many a night when skippysky predicted excellent seeing, only to be met with mush and mud.

However, I will say Skippysky is very useful for predicting cloud 24-48 hours out.

7timer on the other hand always tends to predict the worst possible seeing at the same times during every 24 hour cycle and always throws up a pattern of the worst seeing on the scale during the night, and better during the day regardless of time of year or jetstream conditions... atleast for Melbourne anyway. Again, this is far from accurate.


Gauging the severity of star twinkling by the naked eye is one age old method, and while twinkling stars suggest poor seeing, it is not fail-safe. One evening I saw some of the worst twinkling and baulked at the idea of getting the scope out, but a few hours later I was enjoying one of the top 5 sessions of my life.

The only sure-fire way of figuring out the seeing is getting out there with the scope...

[technofetishism (Nick)]

yeah, i never bother with seeing predictions i just use it for cloud

[Wavytone]

What I do...

1. Learn to read the BOM synoptic map and especially the animated time sequences, with respect to understanding:
- the prevailing wind patten over Australia blows from west to east; hence clouds will move that way;
- the wind circulation around a high (anticlockwise) or low (clockwise), and what that means for clouds;
- how the wind strength varies according to the spacing of the isobars (bunched together = very windy, far apart = light winds, the centre of a high = calm)
- what a front means, in terms of wind and cloud.

2. Watch the satellite photos on the BOM site to identify the clear sky vs clouds, bright white = higher clouds, the darker grey fuzz (often hard to spot) = low-level stuff, and the cloud that marks a front.
Use the animated photo sequences to figure which direction and how fast they're moving, so you can have some idea whether there will be a cloud or clearing over you when you want to observe.

Doing the above will enable you to figure whether or not there is a reasonable chance 1-3 days before observing night.

3. Find an online webcam that shows the daytime sky at a location either near, or west of your site. Use this to check if the sky looks clear before you set out. This alone will eliminate many wasted trips.

Failing this, find a vantage point that gives you a view of the sky over the observing site. In my case, i live in Sydney and my observing site is a 2 hour drive west, in the blue mountains. A short walk up the road before sunset gives a distant view of the mountains on the horizon, and I can easily see if it's worth going, or not.

4. Watch and learn the local terrain effects associated with common weather patterns. For example, for Sydney:
- a low off the coast will direct a southerly onto the coast, extending inland, and a southerly off the sea carries moisture = solid low-level cloud which will extend inland 100-200km. However if really desperate, driving inland far enough will eventually find holes in the cloud;
- warm sunny day with a NE sea-breeze on the coast in spring = coastal cumulus clouds or thunderstorms in the afternoon, but clear west of the mountains;
- while on the surface you may have a NE sea-breeze, 300 metres higher up the wind is a westerly (!)

5. If you are under a solid cloud deck in late afternoon, watch out for a deep red sunset. I then check the synoptic map - if the cloud is associated with a cold front, the deep red sunset = clear sky behind the front. So hop in the car and go.

6. Fronts and thunderstorms = clouded out, but if the satellite photo shows there is clear air behind them this will often be exceptionally transparent and can provide good seeing.

7. The microclimate around a particular site:
- black rich soil typical of farms or urban areas - especially ovals and parks = heavy dew, problems with poor seeing due to thermals from the ground as the soil cools, locally cold at night and possibly frost in winter.
- pale sandy soil or sandstone (best) = minimal dew and not so cold
- on a wide open plain, in a depression or valley = fog is likely on a calm winter night, or in summer poor seeing due to thermals from the land.
- on a hill or mountain that stands at least 30 metres above the general terrain = less fog
- high ridge or mountain with a 300 metre cliff = almost no fog, ever (the cool moist air will slide off into the valley).
- a high site with cliffs on the western side = poor seeing due to thermals, the late afternoon sun will heat the cliff face and it will take many hours before it cools down. This applies winter and summer.

Lastly, if you must use an app on your iDevice, the only ones worth using are the ones that use the BOM data directly such as Oz Weather (this accesses the readings from the Automatic Weather Stations every 10 minutes). The apps that use data from sources such as Wunderground or Yahoo are all rubbish, as is the Seabreeze website.

[andrew_d_cool (Andrew)]

Hi,

>Skippysky's seeing predictions seem to be tied to the jetstream and >ignore all other factors. Almost always it displays Pickering 8 or more

I don't know where you've picked up the idea that SkippySky's Seeing uses the Pickering Scale.

I suggest that you read the online Help file, for which a link is provided on every SkippySky page.

>be tied to the jetstream and >ignore all other factors.

Seeing as calculated by professional astronomers involves a temperature function derived from temperatures taken at many levels through the atmosphere. Basically, changes in temperature between adjacent air layers cause turbulence, which in turn causes poor Seeing. The temperature profile can be matched to a vertical profile of Seeing.

Now given that you don't pay for SkippySky, I can only afford the bandwidth to download wind vector data at ground level and at the Tropopause, which if you had to pick two levels of atmospheric winds that
strongly influence Seeing, those two would be very high on your list.

SkippySky uses a weighted algorithm to assign the relative contribution of wind at those two levels to poor Seeing, and scales the result from 0..10,
a scale that simply suggests bad Seeing to better Seeing.

That algorithm has been adjusted "down" twice to make it harder to achieve higher Seeing indicies.

It is NOT in any way linked to arcsec resolution, or the Pickering Scale, or any other *******'s Scale.

Remember too that the GFS model operates at 0.25 degrees resolution. You cannot and must not expect it to predict the precise conditions over the clothesline in your backyard. (how many times have I typed that...?)

Think of trends in the data, and not absolutes.

Kind Regards,

Andrew
www.skippysky.com.au

Originally Posted by pgc hunter

I would ignore both Skippysky and 7timer's seeing predictions.

Skippysky's seeing predictions seem to be tied to the jetstream and ignore all other factors. Almost always it displays Pickering 8 or more for Melbourne (with 1 being terrible and 10 being perfect on this scale).... which is obviously bulldust. I've gone out on many a night when skippysky predicted excellent seeing, only to be met with mush and mud.

However, I will say Skippysky is very useful for predicting cloud 24-48 hours out.

7timer on the other hand always tends to predict the worst possible seeing at the same times during every 24 hour cycle and always throws up a pattern of the worst seeing on the scale during the night, and better during the day regardless of time of year or jetstream conditions... atleast for Melbourne anyway. Again, this is far from accurate.


Gauging the severity of star twinkling by the naked eye is one age old method, and while twinkling stars suggest poor seeing, it is not fail-safe. One evening I saw some of the worst twinkling and baulked at the idea of getting the scope out, but a few hours later I was enjoying one of the top 5 sessions of my life.

The only sure-fire way of figuring out the seeing is getting out there with the scope...