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'a Simple Guide To Understanding Skew-T Diagrams


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#1 johnholmes

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Posted 21 April 2005 - 15:57

hi
This is the first page with an introduction to the tutorials about skew-T

Intro_to_skew_t__page_1.JPG

below is page 2 with diagram 1, which shows the skew-T diagram that you can find on Net Wx.


west_mids_Net_Wx_12z_26apr05.JPG

The next diagram will show the relationship with just two variables, the pressure heights and dry bulb temperatures.

herewith diagram 2
replace_daig_sunday.JPG


In the next diagram, diagram 3, we introduce the DALR, read the enclosed to find out what that means.

diagram_3.JPG
In the next diagram - diagram 4 we show and explain the ELR(ENVIRONMENTAL LAPSE) rate, and also what an INVERSION is

diagram_4_ELR.JPG


now to explain those two terms:
From this we can do a little experiment.
With a temperature of 20C follow the DALR up its line until it hits the ELR(Environmental Lapse Rate). At this point it cannot rise any further as it has the same temperature as the surrounding air. In fact for some way above it the ELR temperature continues to increase not decrease. When this happens we say it is an INVERSION. These are important to a forecaster trying to decide whether showers or thunderstorms may develop. With this sort of sounding then the answer is no. These inversions occur at various levels in the upper atmosphere.

So now to introduce yet another line on the master skew-T diagram. This is called the SALR(SATURATED ADIABATIC LAPSE RATE). So far we have assumed our parcel of air is dry. Now it becomes moist. There is no in between in the atmosphere, air is either dry or moist.
Condensation takes place as the air rises at its saturated rate, therefore latent heat is given out, and the moist air thus cools off more slowly than when it was dry. It cools at approximately 1.4C per 1,000ft; in other words at about half the rate it cooled when dry.

So in diagram 5 we have the Temperature lines in BLUE; Pressure lines in BLACK, DALR lines in RED, and the SALR lines in ORANGE

diagram 5-SALR.JPG
On the surface, many of us take weather readings, or read from an automatic weather station. This gives us something called the dewpoint and the relative humidity. Again no need to understand the physics of it, just accept that the dewpoint gives a measure of how moist the air is. One fact you must hold on to is that the higher the temperature then the more water vapour it can hold. So if the dry bulb temperature is 20C and the dewpoint is 5.0C, the relative humidity, say at 1000mb, is 36.8%. If the temperature was 7C, the relative humidity would be 87%. To get a relative humidity that value with a temperature of 20C, would need a dewpoint of 17.8C. So you see the warmer it is the more water vapour air can hold. This accounts for why it is so much wetter in the Tropics than the Arctic!

So we now need to introduce some means of measuring the humidity of the air in the atmosphere. This is done by something called the HUMIDITY MIXING RATIO(HMR) and is illustrated in the diagram below.

diagram 6 below

diagram_6_HMR.JPG

Yes I know its getting all very complicated but we only have one left to add. That is the dewpoint curve on the ELR. for that please see below.

Diagram 7 below


diagram_7_the_full_elr.JPG

Right that is all the theory, now we can look at what the original skew-t shows with all the lines on, I'll put this skew-T for 06Z for the London area on and try and explain all the lines again. Once you have had time to absorb these last three diagrams we can then start to look at 'live' ascents/skew-T diagrams to find out what we can about cloud development from them. In time we should get a good 'convection' day, then we can put our new knowledge to the test, along with any other that we may each have and see if our forecast is correct!

added Jan 11 2011
I am deleting the two attachments below as they are not working.
As the diagram was to show how to use the skew-t I will do another one explaining this, but it may be some time before I am able to do this.

diagram 9 below

diagram_9.JPG

below is diagram 10 with the explanation before it.

This was a check to see if the GFS overall forecast would give convective cloud.

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: diagram 10


diagram_10.JPG

This just about concludes the explanation of Skew-T diagrams and how they can be used to predict whether, or not, convective cloud is expected. Further tutorials will be made as and when situations arise, for more insight into thunderstorm prediction,frontal cloud, how to use the winds alongside the skew-T, maximum and minimum temperatures, will there be fog overnight, will it snow or rain.

thank you to everyone who has read/used them. Please continue to give me any feed back.

John

Edited by johnholmes, 11 January 2011 - 13:42 .


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#2 johnholmes

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Posted 25 April 2005 - 19:09

hi everyone
I'm not getting much response to this. I really would appreciate some feedback, good or bad, about it PLEASE!

regards

John

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please take care with sunbathing-it can give problems years down the line with skin cancer as I discovered


#3 kold weather

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Posted 25 April 2005 - 19:25

Hi John,thats a great guide there mate,they are probably the most useful tool that exsits for forecasting at least convective weather and that guide would certainly help people who don't understand things like that.
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#4 Roo

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Posted 25 April 2005 - 19:30

yep...I'm going to sit down tomorrow (when I'm not suffering from evening sickness!) and have a proper look at it....

thanks so much John...I just know it will be useful to ejits like me! :)
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#5 BrickFielder

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Posted 26 April 2005 - 11:30

The explanations are fine keep it nice and simple.There are some good but in some case complicated explanations on habyhints.

Habyhints

covering all these subjects.

291. SKEW-T: A LOOK AT TP (TROPOPAUSE)
292. SKEW-T: A LOOK AT FRZ (FREEZING LEVEL)
293. SKEW-T: A LOOK AT WB0 (WET BULB ZERO LEVEL)
294. SKEW-T: A LOOK AT PW (PRECIPITABLE WATER)
295. SKEW-T: A LOOK AT RH (RELATIVE HUMIDITY)
296. SKEW-T: A LOOK AT MAXT (MAXIMUM TEMPERATURE)
297. SKEW-T: A LOOK AT TH (THICKNESS)
298. SKEW-T: A LOOK AT L57 (700 TO 500 MB LAPSE RATE)
299. SKEW-T: A LOOK AT LCL (LIFTED CONDENSATION LEVEL)
300. SKEW-T: A LOOK AT LI (LIFTED INDEX)
301. SKEW-T: A LOOK AT SI (SHOWALTER INDEX)
302. SKEW-T: A LOOK AT TT (TOTAL TOTALS)
303. SKEW-T: A LOOK AT KI (K INDEX)
304. SKEW-T: A LOOK AT SW (SWEAT INDEX)
305. SKEW-T: A LOOK AT CAPE (PURE INSTABILITY)
306. SKEW-T: A LOOK AT CINH (CONVECTIVE INHIBITION)
307. SKEW-T: A LOOK AT SBLCL (SURFACE BASED LCL)
308. SKEW-T: A LOOK AT CAP (CAPPING LAYER)
309. SKEW-T: A LOOK AT LFC (LEVEL OF FREE CONVECTION)
310. SKEW-T: A LOOK AT EL (EQUILIBRIUM LEVEL)
311. SKEW-T: A LOOK AT MPL (MAXIMUM PARCEL LEVEL)
312. SKEW-T: A LOOK AT STM (STORM MOTION)
313. SKEW-T: A LOOK AT HEL (HELICITY)
314. SKEW-T: A LOOK AT EHI (ENERGY HELICITY INDEX)
315. SKEW-T: A LOOK AT BRN (BULK RICHARDSON NUMBER)
316. SKEW-T: A LOOK AT SRDS (STORM RELATIVE DIRECTIONAL SHEAR)
317. SKEW-T: A LOOK AT Tdd (DEWPOINT DEPRESSION)
318. SKEW-T: A LOOK AT MW (MAXIMUM WIND)
319. SKEW-T: A LOOK AT STORM UVV
320. SKEW-T: A LOOK AT CCL (CONVECTIVE CONDENSATION LEVEL)
321. SKEW-T: A LOOK AT TEI (THETA-E INDEX)
322. SKEW-T: A LOOK AT 0-3 KM SHEAR
323. SKEW-T: A LOOK AT SURFACE RH
324. SKEW-T: A LOOK AT THETA-E
325. SKEW-T: A LOOK AT EI (ENERGY INDEX)

The problem is there is really too much information and it is not clearly demonstrated or explained and which are the important bits. The ones I tend to look at are CAPPING LAYER(INVERSIONS), Tdd,CAPE,TROPOPAUSE,LI,LCL, 0-3KM SHEER and HELICITY but I am not sure whether this is what I am supposed to look at. So carry on with the excellent diagrams which make it a lot clearer.

#6 Verglas

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Posted 26 April 2005 - 12:27

I did have a look at :

http://www.ems.psu.e...s/skew_plot.htm

to see what it said to try and interpret the diagram but ran out of time and saved as a favourite to go back to. The site seems to have other useful info too.
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#7 johnholmes

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Posted 26 April 2005 - 13:51

thanks for the input to other sites.
At the end of my explanation of skew-T diagrams I have a list of sites for further explanation for those who want it. Equally the object of the Net Wx guides has always been, and I hope always will be, to explain in as non technical terms as possible to anyone with no, or very little knowledge, of the subject, such that they may gain a growing knowledge of the fascinating and highly complex science of meteorology. The difficult bit is making simple explanations without appearing to talk down to anyone. In this I hope I am succeeding.

regards

John

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#8 Darren Bown

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Posted 27 April 2005 - 11:03

Hey John, this is extremely useful. It is a difficult diagram to understand, but with this, i can understand it!

Just one question

On the diagram with the ELR on it, between 1000mb and 960mb or so, does the ELR follow the temperature lines (at 45 degrees) if this is so, then does the ELR stay at about 18C for the first 800mb, or is it actually warming up (as if you are reading the temperature vertically)?

Hope that made sense.

Daz

Edited by Dazmaster75, 27 April 2005 - 11:04 .

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#9 Anti-Mild

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Posted 27 April 2005 - 12:22

Good explanation John, one mistake though. In diagram 3 you say that when the DALR line crosses the 900mb line the temperature is 11c, it is actually 17c, a drop of 3c not 9c.

Also could you post a chart with the potential for t-storms?

Thanks, AM
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#10 BrickFielder

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Posted 27 April 2005 - 13:31

So clouds cannot develop beyond about 980mb. I think from the comments you need to have a few more diagrams with clear pointers to reinforce your points after each step in your discussion. Perhaps a little legend so that we don't forget which line is the wet bulb temperature and the ELR and DALR might help as well.



#11 johnholmes

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Posted 27 April 2005 - 19:15

hi
only just noticed the last 3 posts, will try and answer them tomorrow as bit busy with own web page and putting up the T+168 last check.

regards

John

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#12 Paul B

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Posted 27 April 2005 - 19:22

Thanks John for this...I will make it a sticky. :p
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#13 Adi F

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Posted 27 April 2005 - 19:58

Well done John very informative and I am eagerly anticipating the next in the series. This has been a very beneficial refresher to the small amount of training that I had in the subject 16 years ago.

I would agree with Brickfielder, some sort of pointer would be very useful.



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#14 Verglas

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Posted 28 April 2005 - 00:30

please can you explain what's happening on the cumbria chart for 28th 12.00hrs where the left trace exits stage left, is this an inversion (which may affect radio propogation) ?
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#15 johnholmes

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Posted 28 April 2005 - 07:09

please can you explain what's happening on the cumbria chart for 28th 12.00hrs where the left trace exits stage left, is this an inversion (which may affect radio propogation) ?

<{POST_SNAPBACK}>


short answer - No; you are looking at the 'dewpoint' line which, as you put it, 'exits stage left', not the temperature line. Can I ask anyone not conversant with the skew-T diagram PLEASE be patient while I slowly go through an explanation of all the different lines etc on them. By the end of the tutorial I hope we can all understand the main basics behind the diagram.
To repeat, the line you refer to is the dewpoint line, this simply shows how dry the atmosphere is at that level.

regards

John

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#16 johnholmes

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Posted 28 April 2005 - 07:19

Hey John, this is extremely useful. It is a difficult diagram to understand, but with this, i can understand it!

Just one question

On the diagram with the ELR on it, between 1000mb and 960mb or so, does the ELR follow the temperature lines (at 45 degrees) if this is so, then does the ELR stay at about 18C for the first 800mb, or is it actually warming up (as if you are reading the temperature vertically)?

Hope that made sense.

Daz

<{POST_SNAPBACK}>

hi Daz
what you say is correct; in this instance the ELR is following parallel to the temperature line(BLUE) up to about 960mb.
hope that clears it up for you.

regards

John

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#17 johnholmes

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Posted 28 April 2005 - 07:21

Good explanation John, one mistake though. In diagram 3 you say that when the DALR line crosses the 900mb line the temperature is 11c, it is actually 17c, a drop of 3c not 9c.

Also could you post a chart with the potential for t-storms?

Thanks, AM

<{POST_SNAPBACK}>

hi AM
You have made the same mistake that a lot make; a parcel of air rises AND COOLS at the DALR(Dry Adiabatic Lapse Rate); this is the RED line and slope to the left. Follow it up that to 900mb and it is at the temp I quoted. So, no its not a mistake. If you are still unsure PM me and I'll have another go at explaining it.

regards
JOhn
edit again= there is a mistake in one sentence in the explanation of diagram 3; the DALR falls at THREE degrees C per 1,000ft NOT 1C as it says. The diagram has been amended also to take the comment by Brickfielder on board. I'm really sorry if this muddies the water more, so do feel free to PM me)

Edited by johnholmes, 28 April 2005 - 08:55 .


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#18 johnholmes

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Posted 28 April 2005 - 07:23

So clouds cannot develop beyond about 980mb. I think from the comments you need to have a few more diagrams with clear pointers to reinforce your points after each step in your discussion. Perhaps a little legend so that we don't forget which line is the wet bulb temperature and the ELR and DALR might help as well.


<{POST_SNAPBACK}>


hi B
Thank you for the constructive comment. I'll look into it and see if my VERY limited soft ware allows me to do what you suggest.

regards

John
ps
done so in dig 3, and will try and do similar in future, again tks for the suggestion
John

Edited by johnholmes, 28 April 2005 - 10:04 .


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#19 swebby

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Posted 28 April 2005 - 10:00

Yep very informative John, thanks. I had never heard of a skew T diagram until i stumbled upon this forum topic. Keep these tutorials going as it looks like there is a lot of information contained in these that will require a good deal of explaining. I also think brickfielders suggestion of attaching legends & pointers would help a lot.

S

#20 johnholmes

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Posted 28 April 2005 - 10:02

hi Swebby
already taken on board if you look at diagram 3. glad you are finding informative. If not already welcomed to Net Wx, then a big welcome, enjoy it as I'm sure you will

regards
John

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please take care with sunbathing-it can give problems years down the line with skin cancer as I discovered