Astronomy, Photography and Weather

General Category => Weather & Atmospherics => Topic started by: Big Dipper on October 10, 2009, 07:49:44 pm



Title: Interpreting Infrared Satellite Image Data
Post by: Big Dipper on October 10, 2009, 07:49:44 pm
As someone who has got somewhat disillusioned with the way that different weather forecasting sites seem to interpret the same satellite data, I've come to rely very much on the Sat24.com animation site which is updated every 15 minutes.

However, I'm wondering whether I'm interpreting the data there correctly - or more likely, placing too much credence on the shades of white/grey that I see.

Last night the image showed a dark grey area moving towards Oxford which I regarded as the sign of a reasonably transparent sky. However, even with the darkest area on the satellite image over me, the sky outside remained pretty much overcast until about an hour or so before sunrise.

So using the IR satellite image snapped just now, if I was in Plymouth right now, I should forgo watching the X-Factor and set up now. But if I was in Manchester, forget it!

And if I was up in Thurso, what should I be doing now?

I appreciate that the animated data can change quite rapidly so thought I'd choose a snapshot situation to see what the rest of you would do at the time of the snapshot only.

Thanks in advance for what is no doubt a bit of a 'noob' question.

Edit: Apologies - here is the image that I meant to post:
(http://www.sat24.com/image.ashx?country=gb&type=slide&time=&index=4&sat=)


Title: Re: Interpreting Infrared Satellite Image Data
Post by: martinastro on October 10, 2009, 08:27:20 pm
Hi Andy, I'm glad you brought this up. The reason is because it was the IR view. High cloud tops near the top of the atmosphere such as a cirrus, cirrostratus etc show up as white on the images, the brighter the white the higher the cloud tops (and colder). Low, and even mid level cloud, can have little or no signature on IR. That's why the approaching dark area was in fact cloud and not a large clear sector. I hope this helps.

http://www.weatherzone.com.au/help/article.jsp?id=75#


Title: Re: Interpreting Infrared Satellite Image Data
Post by: Big Dipper on October 10, 2009, 10:07:35 pm
Martin, many thanks for your reply and a thousand apologies! In my haste to go and watch a certain programme (mentioned in my post) I totally forgot to post the satellite image that I was making reference to!

I have now amended my original post to show the image I was using as an example and will check out the link that you've kindly supplied shortly.


Title: Re: Interpreting Infrared Satellite Image Data
Post by: martinastro on October 10, 2009, 11:18:29 pm
No problem Andy, and thanks for the image. If I saw that image I would think it would be a poor night for observing from the abundance of high level cloud from the weak warm front and the suggestions of mid or low level cloud (darker areas). A few gaps may well open up but it wouldn't be a good night all round.

The link is a basic one but I'm sure there's much better information out there.

I think Sat 24 is the best around. Don't trust Metcheck though - in my opinion it's rubbish, I wouldn't even look at their website.  :)

Hope this helps.


Title: Re: Interpreting Infrared Satellite Image Data
Post by: Big Dipper on October 11, 2009, 12:16:34 am
Thanks Martin. You've been very helpful and Sat24's animations are certainly my main source for checking on current conditions (I also find the differing swirls fascinating to watch)!

BTW Metcheck has been down for several weeks recently until this week - not sure why.

Going back to my original comment on how data is interpreted, the prize this evening has to go to ITV's Teletext for my area which informed me of the following:

"A largely cloudy but mostly dry night, with little in the way of cloud."

You can't beat a foolproof forecast like that, covering all of the angles!  ;D


Title: Re: Interpreting Infrared Satellite Image Data
Post by: martinastro on October 11, 2009, 12:35:46 am
LOL...what a great forecast from ITV  ;D


Title: Re: Interpreting Infrared Satellite Image Data
Post by: Big Dipper on October 11, 2009, 12:39:30 am
As they would probably say in Meteorological circles, the guy responsible for that forecast needs a bit more precipitation in his whisky!  ;)


Title: Re: Interpreting Infrared Satellite Image Data
Post by: martinastro on October 11, 2009, 03:17:24 pm
Nice one Andy  ;D

Here's another link....

http://profhorn.meteor.wisc.edu/wxwise/museum/a3/a3example1.html

I'm trying to find some more at the moment.


Title: Re: Interpreting Infrared Satellite Image Data
Post by: Roman White on November 07, 2009, 07:19:22 pm
Well, here is an example of IR satellite image that is... hmm... a bit incorrect

As one can suppose, it should be clear during all today's evening at my area (red dot on the map)
(http://i35.tinypic.com/2s9ay5l.jpg)
Instead I observe 100% Stratus clouds all day long.  ;)


Title: Re: Interpreting Infrared Satellite Image Data
Post by: martinastro on November 07, 2009, 08:12:35 pm
From the image alone I would have expected high level cloud although I would have thought it would be high level cirrus or cirrostratus. IR images only detect the ice rich high level clouds, I wonder what the vis sat image showed out of interest?.


Title: Re: Interpreting Infrared Satellite Image Data
Post by: brianb on November 07, 2009, 08:20:08 pm
Shallow stratus doesn't show up on infrared sat photos because there is no temperature contrast. Fog banks "vanish" too.


Title: Re: Interpreting Infrared Satellite Image Data
Post by: Roman White on November 07, 2009, 09:50:45 pm
I wonder what the vis sat image showed out of interest?.
The last one at 1300UTC showed a layer of Stratus above Poltava area, but large gaps in ~50km to the west & south.

P.S. Wow, just a short time before this thin Stratus layer started to dissipate and now it seems that a completely overcast sky turns into something better (?)... err... although transparency is horrible, only a few bright stars near zenith are visible.  :-\


Title: Re: Interpreting Infrared Satellite Image Data
Post by: Roman White on November 08, 2009, 11:12:11 am
Here is the answer for your question Martin:

(http://i36.tinypic.com/24axuuh.jpg)
As you see above, there is a clear gap over W Russia and a Stratus layer over Ukraine, which both have very small difference in contrast. Therefore it becomes almost invisible at night.  :-\ >:(


Title: Re: Interpreting Infrared Satellite Image Data
Post by: martinastro on November 09, 2009, 06:34:47 pm
Thanks Roman, I like what you done with both the vis and IR sat images there for comparison - very interesting!