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'a Simple Guide To Understanding Skew-T Diagrams


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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

Well I've put up the last 3 diagrams, the last one being the actual skew-T for 06Z this morning for the London area. So you now have all the basic information on what each line means. I'll come back tomorrow and try to explain things again on the actual skew-T. Then we can investigate the actual use of this type of diagram to help forecasting, shower and thunderstorm clouds, their intensity, other things like layered clouds at high level, contrails, wind information down the side and what that can tell us; will it be foggy, what the maximum temperature may be. Lots and lots of things to look at, BUT SLOWLY please. Remember its for those who are not sure of these things. Those of you with that knowledge please be patient.

regards

John

Edited by johnholmes
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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

tks Paul

but its not finished yet!

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

Have added some more bits to have a look at, including a 'real' case from today. Please do feel free to make comment, good or bad. All I would ask is that those of you who understand the topic, only comment to me if you find a mistake.

many thanks

John

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Posted
  • Location: Hoyland, Barnsley
  • Weather Preferences: Thunderstorms.. Heavy snow.
  • Location: Hoyland, Barnsley

Thanks for the tutorial john. Finding it a bit complicated but i got the rough idea. ill keep working at it.

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi Demon

tks for that; don't hesitate to ask if you are a bit lost, it is might complicated.

regards

John

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Posted
  • Location: Hoyland, Barnsley
  • Weather Preferences: Thunderstorms.. Heavy snow.
  • Location: Hoyland, Barnsley

You've given all the info needed, just a case of sitting down and letting it sink in. Its a great forcasting tool!

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

the example for today is not a good one, and reading through it I've not explained it all that well. So I will do another one when it looks a good example. Once we have done that then I can give some ideas on other things which can be forecast from the skew-T diagram.

regards

John

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

another attempt to try and make it easier for those not yet up to speed on skew-T's

post-847-1115412407.jpg

Edited by johnholmes
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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

Got a free evening, so when the 12Z run of skew-T's are in I'll have a look at the computer predict and show how its forecasting for tomorrow and maybe Tuesday also.

regards

John

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

coming up later this evening, honest!

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

a bit late but never mind.

To set the scene; GFS has high pressure to the west and low pressure over Denmark - if only mid January!.

NW winds for most, 5-10mph generally but nearer 15 mph in the exposed northeast.

GFS rainfall for 12Z suggests showery outbreaks, generally slight and for most parts, other than some areas of the south west and along parts of the south coast which should stay dry..

Afternoon temperatures from only 8-10C in the east to 12-13 C in the west/sw and perhaps 15C in a few sheltered spots along the south coast. Net Wx temps for 15Z are much the same; these are perhaps a bit underdone for the east as many places saw around 12-14C this afternoon(Sunday). Dewpoints should be about the same sort of value as today(the airmass is not changing, other than just tending to warm up a little, especially in the west).

So taking the 12Z skew-T predict for east Lothian, as being the most representative for the eastern side, what do we have.

see below:

using 12-14C temperature and dewpoint of around 3-4C gives tops of 20000ft or so. This for the east and northeast with showers less likely the further south west you are. So showers would seem likely to be less heavy with less chance of thunder in these areas.

post-847-1115586427.jpg

regards

John

ps sorry about the mistake!

Edited by johnholmes
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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

and to Adi

herewith the 06Z run with the 12Z skew-T for the London area.

post-847-1115644560.jpg

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Posted
  • Location: Chichester, West Sussex
  • Location: Chichester, West Sussex

Cheers John

So were in that skew-t would you look to see if there would be instability and showers or thunder storms

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks
Cheers John

So were in that skew-t would you look to see if there would be instability and showers or thunder storms

<{POST_SNAPBACK}>

quick check is look on the right hand side and it gives figures for CAPE and LI;

I believe, although this part of forecasting has developed since my time, that for thunderstorms you need a CAPE of over 300, better still 1000, and 1000-2500 could give severe, even intense storm activity

for LI -1 to -4 may give a moderate thunderstorm with some hail; <-4 for severe storms and large hail and <-10 for sev to intense storms with prob of tornado/large hail.

I tend to still rely on my understanding of how much potential instability there is in an ascent; a simple guide is that the greater the area between the ELR and the SALR which goes from the cloud base the more likely severe storms are. There is a figure we used but I cannot remember it and I believe its the basis for both CAPE and LI

CAPE= Convective Available Potential Energy and LI=Lifted Index, which is itself a pretty crude measure of CAPE. Hope that helps Adi and any others. Please PM for any extra and I will try and help.

regards

John

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Posted
  • Location: Chichester, West Sussex
  • Location: Chichester, West Sussex

Cheers John

Its is starting to make sense now

Regards

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Posted
  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

It seems to be coming together quite well.

The significance of wind speeds increasing with height and changes of wind direction with height still needs to be explanined. Perhaps calculating the cloud base and affects of surface heating are not quite clear as well yet. Just when I think I am getting the hang of it though I see something like this.

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Posted
  • Location: Chichester, West Sussex
  • Location: Chichester, West Sussex

Yeah I have used the skew –t’s to look at wind direction for a few years now as you can see if there is any chance of shear and the wind speed

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi B and A

It will all come together including talking about the winds as well - patience boys. I've only got one brain, that hurts enough as it is. More on cloud bases and tops very soon, and not just convective cloud. The winds in a little while.

regards

John

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

B again

Try not to look at that type of skew-T for a while. Its pretty complicated so lets stick with the relatively simple one we can all get off Net Wx for a while

regards

John

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Posted
  • Location: Hoyland, Barnsley
  • Weather Preferences: Thunderstorms.. Heavy snow.
  • Location: Hoyland, Barnsley

Good work john all just clicked in to place. Keep it up

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Posted
  • Location: just south of Doncaster, Sth Yorks
  • Location: just south of Doncaster, Sth Yorks

hi

Brickfielder made this comment

' Perhaps calculating the cloud base and affects of surface heating are not quite clear as well yet. '

the cloud base is, in convective situations, a function of the surface dewpoint and the temperature. Usually, unless there is a change of airmass, the dewpoint changes by no more than 2-3C from early morning to mid afternoon, maybe 4-5C sometimes. This may seem quite a lot but is small when compared to what the temperature can change by. On a nice clear morning with the convective cloud developing as the temperature rises, a change of 10-15C is not unusual.

As an example my ASW(Automatic Weather Station) today showed a rise of temperature of 10C, whilst the dewpoint rose from 1C to less than 4C, so less than 3C change for it.

As a rough rule of thumb, the larger the difference between dewpoint and temperature, then the higher the cloud base will be.

Its just a function of how moist the air is.

Another 'rule of thumb' that I used to use was that the actual cloud base was usually about 25mb LOWER than the condensation level. So if the condensation level was, say, 925mb, then the cloud base would be around 25mb lower, that is at 900mb.

The condensation level is that, on the skew-T, where the dewpoint line and the DALR meet. Remember that the dewpoint line follows the HMR(Humidity Mixing Ratio) lines. These are the PURPLE pecked lines going, at an angle, from left to right, on the Net Wx skew-T. The temperature line is the BLUE one going left to right, in BLUE, on the skew-T. However, just to confuse the issue, the line you must follow, is the DALR(Dry Adiabatic Lapse Rate) line which is RED and goes from right to left on the skew-T.Have a look at the last diagram I put up to show this.

There is a scientific explanation of why this is so, but for this series of tutorials it is not necessary to know it.

Hope that helps B, and for others.

regards

John

Edited by johnholmes
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