'a Simple Guide To Understanding Skew-T Diagrams

[johnholmes]

cheers Stuart,

glad it helps; its not easy, as I remember only too well on my first forecast course.

regards

John

[kold weather]

Hi John,good job by the way,I now full well its a very hard thing to get your head around,but once you do its one heck off a tool to use for short range forecasting.

[Adi F]

John I live right on the coast so apart from a decreased temp would that have any effect on the skew-t’s i.e. sea breeze?

Tell me if I am jumping too far ahead again, I need to learn to be patient.

You must be pulling your hair out with all our questions

[johnholmes]

John I live right on the coast so apart from a decreased temp would that have any effect on the skew-t’s i.e. sea breeze?

Tell me if I am jumping too far ahead again, I need to learn to be patient.

You must be pulling your hair out with all our questions

<{POST_SNAPBACK}>

hi Adi

you are right, it would decrease the surface temp, but little else, also inject some additional moisture at very low level. Its this combination that can sometimes set off 'convergence' thunderstorms a little way inland with the right wind and atmospheric conditions. Perhaps a topic to discuss a long way down the line!

regards

John

[Adi F]

hi Adi

Its this combination that can sometimes set off 'convergence' thunderstorms a little way inland with the right wind and atmospheric conditions. Perhaps a topic to discuss a long way down the line!

<{POST_SNAPBACK}>

Haha John I love the way you said “a long way down the line”.

I only asked because unlike those that live inland us people that live on the coast have too factor in the constant effects of day time sea breeze and night time land breeze.

I thought I was told years ago that the skew-t’s could predict whether the breeze affect would be strong or not (i.e. go a long way in land / out to sea or stay close to shore), I am probably wrong it was probably some other measurement.

John believe it or not I was in the Artillery and I actually did a basic course on meteorology, that course was based up at Larkhill and twice a day we sent a weather balloon up and we would track it (I think they still make soundings on Larkhill). And, yes we used to produce and look at these charts but I don’t remember them being called Skew-t, I thought they were called something else. The thing is I have always loved the weather but back then I was more interested in learning how to play war games, sending balloons up in the sky really was not important for me although I understood the relevance of it all. Now I only wished I concentrated and took it in.

[johnholmes]

hi Adi

Yes, when I was in Met we used what were called T-phi diagrams, pretty similar really. Whatever is used it will be a plot of temperature against height, usually. Please don't ask me to explain T-phi or any others. 99.9% on the forum would be asleep after the first two paragraphs, and I would certainly have to rack what little brains I have left to put some text together. I have various Met books but even there it is very very complicated. So lets just keep it as simple as we can.

As time goes by, assuming its wanted, we can discuss all manner of things which these aerological diagrams(!) help with. Max, Min temp, will there be fog overnight, what time will it lift at a certain place, will it snow or rain, will there be a thunderstorm will it be sunny. The list, if not endless, is pretty long. And then we can set about using the wind data also!!

Yes, that you saw at Larkhill was one of about 12 upper air stations we used to have, and two or three out on weather ships in the Atlantic, the Bay of Biscay, between Iceland and Greenland and off the far north of Norway. Now much of it is done by satellite sensors and with masses more data being sent down. Pretty mind boggling really.

Anyway I'm going to bed now.

Thanks for your tutorials on how to as well, when I get time I must try and get the various things you suggest to work for me.

regards

John

[the_white_stuff]

Good work JH, even though I find it hard to grasp I'm slowly learning it. I keep re-reading this thread and picking up new bits each time

[johnholmes]

hi

Just to try and make it really clear on the point I made in an earlier post about the differences in changes through a 24 hour cycle between the temperature and the dewpoint.

Below is the print out(sorry its not in colour - another problem I'm having!!), but it illustrates how on relatively clear nights with clear mornings just how much the temperature can change and how little the dewpoint does.

This is why meteorologists call the dewpoint a 'conservative' measure. As it only chnages markedly when there is a change of airmass.

I hope the diagram helps to iluustrate the daily changes for you, over the past three days.

regards

John

[Adi F]

Thanks John that’s an interesting diagram that dose certainly put it into perspective

[johnholmes]

hi skew-T readers

I've attempted to put all the items about ske-T diagrams in the one post. Eventually it will be made into a web page to go alongside the Net Wx skew-T forecast diagrams, and, I think, in the Net Wx Guides. That way they will always be readily available.

Look forward to any feed back, + or -.

regards

John

[Adi F]

Hi John that’s probably a very good idea having it as one post but I would imagine that it will be like writing a book if you are planning to cover everything that skew-t’s covers.

I for one will look forward to reading it.

[johnholmes]

hi

Not a brilliant example but it may help show how skew-T diagrams can be used to predict will there be/not be any showers, and how the prob of heavy showers can be shown to increase.

Max temps of around 16-18C with dewpoints, at the surface of 10-12C.

shows tops as per mb levels with each predict ascent.

the orange line shows the possible SALR curve from the condensation level.

regards

John

Edited May 19, 2005 by johnholmes

[Adi F]

Thanks for sharing that with us John

[johnholmes]

hi Adi

thanks for that, hope it helps some people, although as I said its not the best example.

regards

John

[johnholmes]

hi

With the probability of some storms this weekend I will attempt to do an additional tutorial showing what the skew-T's are suggesting may happen. Remember that it is the data provided by a skew-T that gives us CAPE and LI not the other way round.

One other feature not yet dealt with are the wind speeds and directions shown on the right hand side(rhs) of the plot. These give some idea of the changes occurring in the upper atmosphere. They tell us whether the air, at any particular point, is getting colder or warmer with height. We refer to winds as BACKING or VEERING, whether it is to do with winds at the surface or in the upper air.

Veering means changing in a CLOCKWISE direction and Backing means COUNTER-CLOCKWISE.

Winds that BACK with HEIGHT show that the atmosphere is getting colder with height. This may aid convective cloud tops to reach greater heights as time goes by.

If the winds VEER with HEIGHT it shows that the upper air is becoming less cold. Its a typical feature you can observe when a surface warm front is approaching that skew-T position. And is one of the aids a forecaster can use in predicting, without using any computer, to forecast the time of onset of rain

Backing or veering with height can also give additional information on the likely intensity of thunderstorms. In the SHORT term BOTH will aid storm intensification, as it gives the wind SHEAR often spoken of which is required for storms to intensify. Over a medium term VEERING means that the cloud heights will become more limited so this is not a good feature. As a rough rule of thumb, and it is only a rough rule, for 2-6 hours then backing or veering is good, in any longer term then VEERING is not a good sign for deep convection. So when you look at the skew-T forecasts remember that. Anything beyond the next 6 hours and veering is not good.

One other feature which the upper wind changes show is another item mentioned when discussing thunderstorm development. This is UPPER TROUGHS. As they approach they cause the upper winds to BACK and as they cross over the area they will VEER the upper winds. From what has been said before one can see that upper troughs just west of an area under discussion is another supporting fact for severe storm development.

I hope this quick run through of upper winds will help you understand a little more as the more experienced posters on the forum give their views on storms this weekend.

Please ask any questions or PM me and I will try to answer your query.

Good storm watching!

regards

John

[Adi F]

I have only just come across this but another great explanation.

Thanks John

[johnholmes]

I have only just come across this but another great explanation.

Thanks John

<{POST_SNAPBACK}>

thanks Adi, glad it helped

John

[snowflakey]

Just stumbled across this tutorial, looks like it's just what Im looking for, will post back when Ive studied it.

Thanks John :lol:

[johnholmes]

a pleasure mate and welcome to Net Wx. ask if you don't understand anything, be it skew-T or whatever, there is usually someone who can answer most questions on here.

regards

John

[Anvil Crawler]

Hi All being new here and this is my second post as im trying to understand skew T and have found this site helpful also which i came across a few days ago .I know a little about them but wonderd if anybody can help me on the bits im stuck on i have had a thread going here http://www.ukweatherworld.co.uk/forum/foru...=27202&posts=49 But i still need help as you can see i have some un answerd questions.Thankyou.

[Dave J]

Ozz,

I saw your question on UKww, but didn't want to answer it just incase Nick Verge told me my interpretation was odd again!! You know what he is like.

The CAPE, CIN, LFC, EL and other parameters are calculated by computer algorithms. So when you view a tephigram or Skew-T, it should be included within it or somewhere on it.

CAPE is the vertically integrated positive buoyancy of an air parcel experiencing adiabatic ascent, and is basically the area between the environmental curve and a line taken from the LFC (Level of free convection) along a saturated adiabat up to the EL (Equilibrium level). You can get many types of CAPE; SBCAPE (Surface based CAPE) MLCAPE (Mean layer CAPE) which all have their relative strengths and weaknesses for estimating instability, and it differs between forecasters which is used most in what situations.

CIN is convective inhibition and is the integrated negative buoyancy of an air parcel, and represents the amount of energy required to

lift an air parcel through a layer that is warmer than it.

LFC - the level of free convection is the height at which a parcel lifted adiabatically from the surface, becomes saturated at the LCL (Lifting condensation level) and then also becomes warmer than the surrounding layer of air as it is lifted up a saturated adiabat. (I.e. CAPE is avaliable)

The equilibrium level (EL) is simply where on a sounding, at a level above the level of free convection (LFC) where the temperature of a rising air parcel again equals the temperature of the environment. The height of the EL is the height at which thunderstorm updrafts no longer accelerate upward, though they can overshoot in certain circumstances. This leads to an approximate estimate of thunderstorm or convective cloud tops!

See attached diagram.

Edited February 10, 2006 by Dave J

[johnholmes]

thanks Dave for doing that, Ozzie if you have any other questions I'm happy to get pm's from you and try to help.

regards

John

[Anvil Crawler]

Thanks Dave and yes i know what you meen by Nick The main thing is that you understand skew T and know how to read them .John thanks for offer of pm but im afraid i can not yet as being a newbie A few more posts and i shall. Has anybody a Skew T they can post and i try and read the info back to see if i am correct.

Edited February 10, 2006 by Anvil Crawler

[johnholmes]

hi Av

don't be afraid to ask just because you are new makes no difference.

As to the skew-t that is in the learning module I did which you have been reading. I have tried to keep it as simple as possible, little mention of all the 'new' terms like CAPE etc etc, just simple explanations of what DALR, SALR HMR etc are and how to use the information for working out what the weather may do, from very little information other than a local temperature and dewpoint.

When(!!!) I get time I do intend to expand the teaching a lot more to cover many other things you can us e the skew-T for, max temp, fog point, layer cloud, advection of different air masses. The list is almost endless.

But enjoy what you can and please do ask questions.

regards

John

diagram 2 is the basic skew-t with the temperature and dewpoint line on it to show what the environment was in that place at that time.

j

[Dave J]

Maybe it would be a good idea to use a real convective situation and then try to make a forecast using a Skew-T from that day, and then compare this with what actually happened.

You could even use a "live" case study if the weather plays ball next week.

Edited February 10, 2006 by Dave J