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 Soundngs or Skew-T plot graphs

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PostSubject: Soundngs or Skew-T plot graphs   Tue Apr 13, 2010 7:07 pm

An often ignored bit of information that can be very useful in determining likely atmospheric conditions for unstable weather and more importantly storms.

Soundings are graphics plotted by meteorologists from data collected by sondes - these are computer based devices sent up with weather balloons periodically during the day. Most offices will send up two, one at 12Z which is UTC time or 11:30pm and 00Z which is 11:30am. They regularly send up further balloons to gauge the conditions but these traces are not normally available online to the public.

If you can't obtain these skew-T traces from your own Met office site online you can obtain the sounding from the University of Wyoming site http://weather.uwyo.edu/upperair/sounding.html

From the drop down list pick your country of origin and from the other drop down list pick 'GIF Skew-T'. This selection will give your the sounding graph plotted or if you wish pick TEXT and you will get all the information you need, but the graphic is a little better to read.

Soundings gather information on wind speed, dew point and temperature through each level of the atmosphere, right up to 300hpa or until the balloon bursts which is sometimes why soundings are incomplete.

Storm chasers should familiarise themselves with soundings. It's not a requirement, but you get a bit of heads up on what the atmosphere is doing at these times if storms are 'forecaast', they will help you understand what requirements are needed for storm initiation. Things such as updraft speed, lapse rate, cloud base and cloud top levels, anvil height and whether the levels in the atmosphere will be conducive to instability.

There are about 50 or so indices (data items) on a skew-t but for chasing purposes there are only a handful that you really need to know. CAPE, LFC, LCL, CIN , LI and CAP - the other indices you can look at and work it out altogether.

Explanations & graphics courtesy/credit to Meteorologist, Jeff Haby at http://www.theweatherprediction.com/

1. What is CAPE?

CAPE (Convective Available Potential Energy) is the integration of the positive area on a Skew-T sounding. The positive area is that region where the theoretical parcel temperature is warmer than the actual temperature at each pressure level in the troposphere. The theoretical parcel temperature is the lapse rate(s) a parcel would take if raised from the lower PBL. (Planetary Boundary Layer)

2. How is CAPE determined?

The positive area on a sounding is proportional to the amount of CAPE. The higher the positive area, the higher the CAPE. The positive area is that area where the parcel sounding is to the right (warmer) than the environmental sounding. The units of CAPE are Joules per kilogram (energy per unit mass). The sounding at the bottom of this page shows a CAPE value of 2,032 Joules per kilogram.

3. Operational significance of CAPE:

CAPE
1 - 1,500 Positive
1,500 - 2,500 Large
2,500+ Extreme


High CAPE means storms will build vertically very quickly. The updraft speed depends on the CAPE environment.

Hail: As CAPE increases (especially above 2,500 J/kg) the hail potential increases. Large hail requires very large CAPE values.

Downdraft: An intense updraft often produces an intense downdraft since an intense updraft will condense out a large amount of moisture. Expect isolated regions of very heavy rain when storms form in a large or extreme CAPE environment.

Lightning: Large and extreme CAPE will produce storms with abundant lightning.

4. Pitfalls:

a. Storms will only form and the CAPE actualized if the low level capping inversion is broken.

b. CAPE magnitude can rise or fall very rapidly across time and space.




LFC Layer of Free Convection:

1. What is LFC?

The LFC (Level of Free Convection) is the lower boundary of the most significant region of CAPE in the troposphere. It is the point at which a lifted parcel of air will become equal in temperature to that of the environmental temperature. Once a parcel of air is lifted to the LFC it will rise buoyantly on its own all the way to the top of the CAPE region.

2. How is LFC determined?

Find the region of CAPE on the sounding. The pressure level at the bottom of the CAPE region that is closest to earth's surface is the LFC. The LFC on the sounding below is at 809 mb (notice at this pressure level that the parcel temperature and environmental temperature are the same).

3. Operational significance of LFC:

Tornadoes: A LFC closer to the surface is more supportive of tornadoes in a severe thunderstorm environment. This is because the region of +CAPE (region where buoyant rising occurs) is closer to the surface.



LCL

1. What is LCL?

The LCL (Lifted Condensation Level) is the pressure level a parcel of air reaches saturation by lifting the parcel from a particular pressure level. A rising parcel of air cools, thus the relative humidity increases inside a rising unsaturated parcel. Once the RH first reaches 100% in the parcel, the LCL occurs there.

2. How is LCL determined?

First note the temperature and dewpoint that is 50 mb above the surface pressure. Draw a line parallel to the dry adiabatic lapse rate starting from the temperature that is 50 mb above the surface. Draw a line parallel to the mixing ratio lines starting from the dewpoint that is 50 mb above the surface. The intersection of these two lines is the LCL. The sounding at the bottom shows a LCL of 546 mb. It is very high since the PBL is very dry.

*note: Different computer algorithms will often have different starting conditions for the parcel. Some will use the surface temperature and dewpoint. Some will use the temperature and dewpoint that is 50 mb above surface. Some will use the average temperature and dewpoint is the PBL. Parcels of air that are used to locate the LCL originate from the PBL. Mixing of air is frequent in the PBL-- thus the use of the temperature and dewpoint toward the center of the PBL (50 mb above surface) is a good starting point.

3. Operational significance of LCL:

Cloud bases: It determines how far air needs to be lifted to produce clouds.

Tornado: In a supercell thunderstorm situation, a low LCL (closer to surface) increases the likelihood of tornadogenesis since the region of CAPE will be closer to the surface.

4. Pitfalls:

a. The LCL that is commonly plotted on a sounding is only relevant for lifting that is occurring in the PBL. Use the LCL for cases of warm season lifting resulting from low level convergence (not from upper level lifting).

b. Do not use LCL in cases where parcel is rising from the PBL due to positive buoyancy alone (use Convective Condensation Level in those cases).



CIN

1. What is CINH?

CINH (Convective Inhibition in units of Joules per kilogram) is anti-CAPE (negative CAPE) in the lower troposphere. This is the region where a parcel of air if raised from the lower PBL would sink back down again. Another term for CINH is a capping layer. The capping layer must be broken before lower PBL based lifting is able to move into the +CAPE region of a sounding and develop into deep convection.

2. How is CINH determined?

CINH is the area of the sounding between the surface and to the level at which +CAPE begins. In the CINH region the parcel will be cooler than the surrounding environment-- thus this is a stable layer. The sounding at the bottom of this page shows a CINH value 37 J/kg. This is a weak cap, especially considering daytime heating has not even begun. Convection will likely start early this day.

3. Operational significance of CAPE:

CINH
0 - 50 Weak Cap
51 - 199 Moderate Cap
200+ Strong Cap


CINH will be reduced by: 1) daytime heating, 2) synoptic upward forcing, 3) low level convergence, 4) low level warm air advection (especially if accompanied by higher dewpoints). CINH is most likely to be small in the late afternoon since daytime heating plays a crucial role in reducing CINH.

4. Pitfalls:

a. Index is only relevant to lower PBL based convection. Index is usually only relevant in a barotropic environment or in the warm sector of a mid-latitude cyclone.

b. Index is only relevant when there is a cap to be broken. If no +CAPE exists above PBL, then the CINH value is meaningless.

c. Keep in mind the factors that day that will either enhance or reduce the capping inversion. See forecast soundings.



CAP

. What is CAP?

The CAP is a stable region of the lower troposphere that impedes convection in the PBL from occurring.

2. How is CAP determined?

The CAP is determined as the maximum temperature difference between a parcel of air and the surrounding actual temperature in the lower troposphere.

Formula: Environmental temperature - parcel temperature (both in region with max temperature difference)

The CAP will always be a positive number since a CAP region has a parcel temperature that is colder than the surrounding environment. CAP is given in units of Celsius difference. This temperature difference is the amount of warming (or rather the weather conditions that will cause the parcel to no longer be colder than surrounding environment) required to produce no CAP.

The CAP on the sounding at the bottom is 1.1 (weak for a morning sounding CAP).

3. Operational significance of CAP:

CAP
0 No Cap
0.1 - 1.9 Weak Cap
2.0 - 4.0 Moderate Cap
4.1+ Strong Cap


When the CAP is less than 2.0, storms are likely to develop shortly when the only parameter holding back convection is the CAP.

When the CAP is greater than 4, help will be needed over the next few hours to break it.

4. Pitfalls:

a. Pay attention to factors that can rapidly weaken a cap: synoptic uplift, daytime heating, low level convergence

b. Does not consider elevated convection; CAP is a warm season- warm sector index

c. Index is meaningless if there is zero CAPE in troposphere



LI or Lifted Index

1. What is LI?

LI (Lifted Index) is an indice used to assess low level parcel (in)stability of the troposphere.

2. How is LI determined?

First note the temperature and dewpoint that is 50 mb above the surface pressure. Draw a line parallel to the dry adiabatic lapse rate starting from the temperature that is 50 mb above the surface. Draw a line parallel to the mixing ratio lines starting from the dewpoint that is 50 mb above the surface. The intersection of these two lines is the LCL. From the LCL, parallel the wet adiabatic lapse rate with height until the 500 mb pressure level is reached. Compare this 500 mb parcel temperature to the actual (environmental) 500 mb temperature.

LI formula = Temperature of Environment at 500 mb - Parcel temperature at 500 mb

The sounding below shows an LI of -6.2. Thus, the parcel of air raised from 50 mb above the surface to the 500 mb level will be 6.2 degrees warmer (positively buoyant) as compared to the 500 mb actual (environmental) temperature.

3. Operational significance of LI:

Instability: A negative LI indicates that the PBL is unstable with respect to the middle troposphere. This is an environment in which convection can occur. The more negative the LI the more unstable the troposphere and the more buoyant the acceleration will be for rising parcels of air from the PBL.

LIFTED INDEX
Positive number Stable
0 to -4 Marginal instability
-4 to -7 Large instability
-8 or less Extreme instability


4. Pitfalls:

a. The LI only assesses instability in one level of the troposphere. Unlike LI, CAPE is better at assessing instability in the troposphere as a whole.

b. Only use the LI for warm season convection. LI is most relevant in the warm sector of a mid-latitude cyclone or in a barotropic troposphere. LI is worthless when a shallow polar air mass moves into the PBL and is usually worthless for forecasting winter precipitation.




The terminology may be a bit foreign but once you get an understanding of the basic things to look at you can easily pick exactly where these points are on a sounding. They are complex at first and you do have to make mathematical calculations on some things to work out instability, but thankfully they've done most of the work for us.

Soundings should used as a tool and guide only and do not represent 'the current real time' atmospheric conditions and represent a window for chasers to go by. They are useful in post-storm events also to compare the atmosphere and how it strengthened or not.

The moderating team has some archived soundings and other info that can be posted here with some 'basic' terminology for easier understanding, but questions should be put forward if there is anything you want to know. As with most Met related, it's better to start at the ground floor and work your way into things, because just when you thought you knew it all - you don't - that is, for non-meteorologists!

More info soon.
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PostSubject: Re: Soundngs or Skew-T plot graphs   Sat Jun 26, 2010 12:28 am

I Swear Skew-T sounding charts are my nemesis. I REALLY Cannot get my head around these buggers
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PostSubject: Re: Soundngs or Skew-T plot graphs   Sat Jun 26, 2010 12:50 am

i look at a skew-T and remember some bits then forget the rest. think im slowly getting the hang of the things tho.
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PostSubject: Re: Soundngs or Skew-T plot graphs   Sat Jun 26, 2010 1:47 am

No comment on this front.... No
Pulse1 ( Mike ) knows the score with me and Skew-Ts.
And much the same as you guys for every time i think i get somewhere i get very confused lose the plot, give up and really need to start all over again.

I think Wazza does have some sort of feel for this though.
Thats right isnt it mate ?
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PostSubject: Re: Soundngs or Skew-T plot graphs   Sat Jun 26, 2010 5:08 am

Well I can identify the bits on a Skew-T plot and get a rough idea if it looks good or not but that's as far as I have got! I learn visually and in practice so looking at a severe weather event combined with a Skew-T plot usually helps me understand it.

As for picking out every detail I'm a long way off yet.

Have a go at looking at the University of Wyoming site, grab a Skew-T plot from there and compare it to one of those Mike has listed. That usually gives a good idea of what the differences are. If I get time later I'll post one up.

Thanks Mike for posting this info up, reward point for you!

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PostSubject: Re: Soundngs or Skew-T plot graphs   Sun Jun 27, 2010 12:53 am

Lots of things have to be taken into consideration with soundings and good chasers can and seasoned chasers who use soundings can nail areas or the likelihood of potential storm types by simply viewing the Skew-T's.

Picking on instability on these things is pretty easy and I've shown Julian what to look for at least for our storms for topic content here. The US is a totally different kettle of fish as everything is over the top there!

Isotherms, isotachs, this and that - they all mean something and all those dashed and dotted and solid lines all relate to each other. Most of the time you have to work things out mathematically - but because most chasers are just that, then, we just go for what we know!!!

In reality you 'don't really' have to use them to chase. I asked a lot of questions about them when first starting out because guys were posting these weird things and discussing the soundings....a personal thing but I would not chase without seeing and using them now. It can give some pretty good hints on the atmosphere!
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PostSubject: Re: Soundngs or Skew-T plot graphs   Mon Jun 28, 2010 10:23 pm

Here is a sounding taken from yesterday in the southern heat. It shows a stable atmosphere, not good for storms.



Now compare it to the one below and you should be able to see some differences. This is a good sounding. Stolen from Mikes website (I'm sure he won't mind).



We are pretty limited with the soundings we can get for the UK, but the basics are there.

I won't try to explain each of these as I will probably get it wrong or not quite right, but I'm sure Mike will answer any questions you may have as he's deffo the expert on these things!

Here is a direct link to Mike's website that has a lot of information on Skew-T plots. http://stormscapesdarwin.com/page30.htm
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Jun 29, 2010 3:21 am

Yeah the Wyoming sounds are a bit obscure as they don't actually show CAPE, CIN or CAP - but you can work it out. That Darwin sounding was pretty insane...although there's a small inversion down low, that really would not worry me given the numbers for instability. And our soundings give two sets of temp and DP traces, the before and after! so that in itself is ideal for comparison between times.

Your sounding shows fairly dry air...even from the surface it is dry, then moist but dries out a fair bit. Cloud tops would be high. There's cooling of the parcel but nothing extraordinary or steep - so lapse rates dull. Wind shear is rubbish also...no speed shear or directional shear which would have been ideal if it changed direction, but given the winds - they would be pretty lame.

There must have been convection on this day no? The LFC is near the surface - but looks like the CAPE is huge if it runs properly....can't be surely....unless the LFC is before that...then it would be pretty dull storm wise....mmm ..nah does not work...

what weather did you have that day?
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Jun 29, 2010 3:35 am

That was yesterdays skew mate, it was hot with no cloud in the sky whatsoever. Clear blue skies and maxed out at 31C. Like being abroad mate!

CAPE was forecast at being around 1500 and LI at -3ish last time I looked, but no fronts pushing across at all.
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Jul 20, 2010 10:59 pm

an interesting read from SkyWarn Switzerland. enjoy the read, i did study

Sounding Reference
For what you need a sounding ?

A Sounding ( radiosonde ascent ) returns the state of the atmosphere at a certain point at a certain time . Soundings find particular use in storm prediction. From them can be very easily determine the stability and other relevant parameters for predicting the storm .


The Skew -T Log P representation

This most frequently used graphical representation of a radiosonde ascent is a very quick overview of the current situation. The temperature and the dew point it at a 45 degree angle to the right shown above and the pressure in a logarithmic scale for the Y -axis. Wind speed and wind direction are normally presented as a wind flags on the right edge. Thus one can see the progress of wind speed with height better, I 've attached the wind speed as an additional scale on the x -axis. The wind directions are represented as simple lines right.
Example

Another X - scale is the mixing ratio (mixing ratio) . It is used to determine the condensation level .
Example


Additional scales

In addition to directly measured values (temperature , dew point , wind) , nor CAPE CINH and shows up to a height of 700 hPa graphically. CINH is this multiplied by the factor 5 in order to obtain the graphic representation acceptable levels. (Description of CAPE and CINH see below)


Condensation level

In a Sounding you can choose between two different condensation levels:

Lifting Condensation Level ( LCL)

The LCL is used at the height of the cloud base to determine if the convection is triggered by elevation of fronts or mountains. For this you need the average mixing ratio and the average temperature of the upper air package. As a standard here , the averaged values of the lowest 100 hPa are used. The intersection between the proportions and the Trockenadiabate is the LCL.
Example

Convective Condensation Level ( CCL)

The CCL is used when the convection is triggered Thermal. This is particularly the case with heat lightning . As with the LCL is needed for even the average value of the ratio. The intersection between the proportions and the temperature curve from the measured values of the CCL. The CCL is usually higher than the LCL.
Example


Thunderstorm prediction using a Soundings

Soundings can put in the storm forecast a significant factor; with them to assess the situation accurately and make a statement , above all, what is expected for a type of storm.
As this is only a snapshot of the atmosphere at a certain point, you should never just leave alone the current sounding , but always call the models and pay attention to changes in the next few hours .

The first one looks the best , the curve of the ascending air parcel ( Parcel in ) and compares them to the temperature curve. Parcel , the curve is located far to the left of the temperature curve, the atmosphere is stable and there are not expected thunderstorms. The Lifted Index is a value often used to describe the current stability . A value of> +3.0 ° C in the sounding of 12Z is usually too strong for the formation of thunderstorms. Exception would be a cold front approaching with a strong cooling in the amount or moist air , which flows into the lower atmosphere .
At 00Z rise to pay attention more, because there already may be sufficient alone to sunlight of the coming day to destabilize the atmosphere.

A second factor is to be expected if the storm is moisture in the lower atmosphere . The Lifted index can indeed be negative , but dry air near the ground impossible storm , as one is not tethered cap, which means that rising air packages can not reach the "Level of Free Convection (LFC ) " freely from which to them would arise . Example Cap for a strong thunderstorm .

The third key parameter for storm , raising the standard , it can not determined from a sounding . For this the model maps are consulted.
One sign of improvement can be at most a clockwise rotating wind direction. In the lower atmosphere , it points to warm air advection, which would imply raising .

you want to find out what is expected for a type of thunderstorms , you look on as the first wind profile. no greater increase of the wind is visible to the height, one can assume that will provide only short-lived storm, whose intensity is determined by the instability. With high instability to expect multi-cell , single cells at low instability . Example a multicell Soundings .
If a wind speed reaches at least about 40 knots at 300 hPa, there is the possibility of thunderstorms with a longer life span (Super ) cells. The increase of the wind with height is crucial to create an area separate from the updraft Abwindbereich . For an additional clockwise rotation of the wind with altitude supercells are likely.
The question whether it could be used in conjunction with supercell tornadoes can be as close to answering that the odds are in favor of the largest, if a strong low level jet is present. The low-level jet usually occurs in advance of cold fronts , where large temperature differences develop in the deeper layers of the atmosphere . In Switzerland , the Low -Level Jet its maximum at about 700 hPa (Example). The Jura and the Alps to have a significant impact . In rare cases it happens , however, that the low-level jet has its maximum at about 850 hPa. Then possible combination of supercells and tornadoes (Example).


Sounding parameters

From the measured values a radiosonde ascent can be all sorts of different parameters calculated. They then can the structure of the atmosphere to be assessed more accurately. Further , all parameters are explained in short, to be shown on the right.

850 Wet Bulb Theta -E
Energy in the air at 850 hPa

Sfc - 700 Man Rel Hum
Average humidity between the ground and 700 hPa

Convective Temperature
Exceeding this temperature near the ground , the trigger thermal convection is expected.
Example

Lifted Index
Difference between the rising air package and the ambient temperature measured at a height of 500 hPa
Example

Formula : LI = T500 - T500Parcel
> 0 Thunderstorms unlikely
0 - -2 Thunderstorms possible - trigger needed
-3 - -5 Thunderstorms probable
-5 - -7 Strong / severe thunderstorms . Tornadoes possible
-7 - -9 Move to Alaska
< -9 Yikes


CAPE / CINH
CAPE ( Convective Available Potential Energy ) is the most important size of the storm forecast. It is a direct measure of the energy available for convection . From this one can also calculate the maximum expected updraft speed ( Max Up Vert Vel ) in a thunderstorm.
CINH ( Convective Inhibition ) is the energy that is needed to bring an air pack on the LFC. This can be done by ground-level Erwärnmung or by raising fronts.
Example

<300 Weak convection ( showers)
300-1000 Weak thunderstorms
1000-2500 Moderate thunderstorms
2500-3000 Strong thunderstorms
3000 + Very Strong thunderstorms


Normalized CAPE
This value is approximately the same importance as the Lifted Index, is simply here over the whole range of "Level Of Free Convection ( LFC ) and the "Equilibrium Level ( EL) averaged. CAPE narrow areas which have a large height difference between LFC and El have a small value, while large areas CAPE produce a small difference in height between LFC and EL great values.

Formula: Normalized CAPE = CAPE / ( Height ( EL) - Height ( LFC) ) [m / s ^ 2]

Scale: 00-10 Narrow
10-20 Wide
> 20 Very Broad

Examples: 6:02 m / s ^ 2 (Payerne 02/06/1999 12Z)
6.63 m / s ^ 2 (Payerne 05/08/2003 12Z)
15:12 m / s ^ 2 (Payerne 06/24/2002 00Z )
26.84 m / s ^ 2 (De Bilt 12Z 06/18/2002 )

In Switzerland and Europe , the value of 10 m / s ^ 2 is exceeded only in rare cases.

Max Up Vert Vel
Maximum Aufwärtsgeschwindigkeit in the expected thunderstorms. This does not apply supercells , where the updraft can reach larger values.

Formula: sqrt (2 * CAPE)

850-600 lapse rate
Temperature difference between 850 hPa and 600 hPa is used here for calculating Exp Hail Size.

Wet -Bulb Zero
see External Description ( English). The height of the WBZ is an important prediction of hail size. Is used for calculating Exp Hail Size.

Fawbush -Miller Hail Size
Mean expected hail size in developing storms .

Exp Hail Size
Experimental maximum expected hail size.
Formula

LCL
Height of cloud base in elevation caused by (fronts , mountains) convection.

CCL
Height of cloud base on thermal convection.

LFC
= LFC Level of Free Convection. From this rate is rising , the ascending air parcel without energy from the outside to the EL. The LFC is located at a height where the ascending air parcel , the temperature curve in the direction of cuts warmer. For stable compared Schichting may never . If thermal convection is the LFC = CCL.
Example

EL
EL = equilibrium level. At this altitude, an air parcel reaches the LFC before which has once again exceeded the equilibrium state, ie it increases to go no further. During thunderstorms, the anvil spreads out in this amount.
The EL has nothing to do with the tropopause !

Theta
Value for rising air parcel (not important ).

Theta -E
Value for rising air parcel (not important ).

Mixing Ratio
Value for rising air parcel (not important ).

Showalter Index
Basically the same as the Lifted index with the change that is taken here , the air parcel from 850 hPa. The difference between an ascending air parcel and the ambient temperature is again calculated at 500 hPa. A negative Showalter index exists in all probability a wet base coat , which is an important prerequisite for thunderstorms.

Formula : SI = T500 - T500Parcel
> 4 Thunderstorms unlikely
1-4 Thunderstorms possible - trigger needed
1 - -2 Increasing chance of thunderstorms
-2 - -3 High potential of heavy thunderstorms
-3 - -5 Getting scary
-5 - -10 Extremely unstable
<-10 Head for the storm shelter


K Index
The K Index ( KI ) is composed of a large temperature drop 850-500 hPa and the Grundschichtfeuchte . Dry layers in applied levels (850 hPa and 700 hPa ) can give a false impression.

Formula : KI = ( T850 - T500 ) + TD850 - ( T700 - TD700 )
0-15 No thunderstorms
18-19 Thunderstorms unlikely
20-25 Isolated thunderstorms
26-30 Widely scattered thunderstorms
30-35 Numerous thunderstorms
36-39 Thunderstorms very likely
40 + 100 % chance of thunder storms


Modified Thompson Index
Thompson is the modified index of "Modified K Index " and " Lifted Index. The Modified K comes from the index and referred to the calculation of temperature and dew point for the 850 hPa level to the K index, which does not conform with the measured values here at this altitude, but with the averaged values between the ground and 850 hPa

Formula: TImod = KImod - LI [° C] where KImod = ( TmeanSfc. .850 - T500 ) + TDmeanSfc .. 850 - ( T700 - TD700 )
< = 28 No thunderstorms expected
28-36 Isolated thunderstorms
> 36 Widespread thunderstorms


Total Totals Index
The Total Totals Index ( TTI ) speaks during a wet base layer at the minimum to 850 hPa and below 500 hPa temperature. Both of these are important values for thunderstorm . In a dry interlayer at 850 hPa is the same problem as the K index.

Formula : TT = T850 + TD850 - 2 * T500 ()
<43 Thunderstorms unlikely
43-44 Isolated thunderstorms
45-46 Scattered thunderstorms
47-48 Scattered thunderstorms / isolated severe
49-50 Scattered t-storms/few severe / isolated tornadoes
51-52 Scattered - numerous t-storms/few-scattered severe / isolated tornadoes
53-55 Numerous thunderstorms / scattered tornadoes
56 + You do not want to know


SWEAT index
In the United States successfully angewanter index for the prediction of severe thunderstorms and tornadoes.

Formula: SWEAT = 12 * ( TD850 ) + 20 * ( TTI - 49 ) + 2 * ( WS850 ) + ( WS500 ) + 125 * (sin ( WD500 - WD850 ) + 0.2)
(WAS = wind speed , WD = Winddirection )
< 272 Thunderstorms unlikely
273-299 Slight risk - general thunderstorms
300-400 Moderate risk - approaching severe limits
401-600 Strong risk - few severe tornadoes t-storms/isolated
601-800 High risk of severe tornadoes t-storms/scattered
801 + High wind damage , but not favorable for severe weather


Craven Significant Severe Weather Index
The Craven SigSvr Index is a combined index between CAPE and wind shear between the floor and 6 km altitude.

Formula: Craven SigSvr CAPE = * 0 .6 km Windshear [m ^ 3 / s ^ 3]

Scale: 0-5 Not expected to be severe thunderstorms
50-10 Severe thunderstorms possible
> 10 Severe thunderstorms expected

Examples: 12:24 m ^ 3 / s ^ 3 (Payerne 05/08/2003 12Z)
14:14 m ^ 3 / s ^ 3 (Payerne 02/06/1999 12Z)
33.89 m ^ 3 / s ^ 3 (Payerne 06/24/2002 00Z )
60.49 meters ^ 3 / s ^ 3 (De Bilt 12Z 06/18/2002 )


Supercell Composite Parameter
The SCP is a combined index between the maximum CAPE , 3 km Storm Relative Helicity and Bulk Richardson Windshear . It was developed by the Storm Prediction Center in the U.S.. The larger the value, the higher the probability of supercells and tornadoes.

Formula : SCP = (Most Unstable CAPE / 1000 ) * ( 3km SRH / 100 ) * ( BRN Shear / 40 ) [1 ]

Scale: <= 1 Not expected to supercell
1-4 Supercells are possible
> 4 Super cells are probably

Examples: 1.76 (Payerne 05/08/2003 12Z)
3:19 (De Bilt 12Z 06/18/2002 )
6:12 (Payerne 06/24/2002 00Z )
19:44 (Payerne 02/06/1999 12Z)


Significant Tornado Parameter
The STP is like the SCP from Supercell Composite Parameter of several parts. This CAPE, Lifted Condensation Level height are the (LCL ) above ground (AGL ) , 1km Storm Relative Helicity and 6km Windshear .

Formula : STP = ( CAPE / 1000) * (( 2000 - LCL height AGL ) / 1500 ) * ( 1km SRH / 100) * (0 .6 km Windshear / 20 ) [1 ]

Scale: <= 0.5 Not expected to signifiers (> = F2) tornadoes
> 0.5 Significant tornadoes are also possible

In Switzerland and Europe , the value of 0.5 is achieved only in rare cases, because either the LCL because of low humidity at ground level is too high , or the 1km SRH is too small because of the lack of low-level jets.

Examples: 12:01 (Payerne 05/08/2003 12Z)
12:10 (Payerne 06/24/2002 00Z )
00:25 (Payerne 02/06/1999 12Z)
00:59 (De Bilt 12Z 06/18/2002 )


Experimental Supercell Index for Switzerland ( SIS)
The SIS consists of a combination of maximum wind speed between 850 hPa and 500 hPa and the 100mb Mean Layer CAPE CCL of the lower and middle altitudes (surface - 400 hPa).

Formula : SIS = ( Max Wind Speed 850_500 / 30 * 2 ) * ( CAPE CCL Sfc_400 / 500 ) [J / kg * m / s]

Scale: <1.4 It would not be expected supercell (super Cells not likely).
1.4 - 1.99 Supercells are possible ( Super Cells possible).
> = 2.0 Supercells are expected ( Super Cells expected) .

Graphics:
- Maximum wind speeds 850-500 hPa
- 100mb Mean Layer CAPE CCL (Surface - 400 hPa)
- Experimental Supercell Index for Switzerland ( CH)


KO Index
Relatively new storm Index of the German Weather Service. He describes the potential instability between the lower and upper atmosphere .

Formula : KO - Index = 0.5 * ( + ThetaE700 ThetaE500 ) - 0.5 * ( + ThetaE1000 ThetaE850 )
> 6 Thunderstorms unlikely
2-6 Thunderstorms possible
<2 Severe thunderstorms possible


Index Deep Convective
Index for the prediction of deep convection ( thunderstorms ).

Formula : DCI = T850 + Td850 - SLI
SLI = Surface Lifted Index <10 Thunderstorms unlikely
10-20 Thunderstorms possible
20-30 Strong thunderstorms possible
> 30 Severe thunderstorms possible


CS Index
Thunderstorms and wind shear index CAPE ( see also SWEAT Index ), which was successfully tested in Switzerland in the storm prediction.

SWISS00 / SWISS12 Index
By Heidi Huntrieser specifically for Switzerland ( north of the Alps ) developed thunderstorm indices for the 00Z and 12Z radiosonde ascents.

Storn Direction / Storm Motion
General tensile direction and speed of the train of thunderstorms. Supercell can in some cases significantly deviate !
Example

Shear Wind Dir Sfc - 3000 / Wind Shear Sfc - 3000
Wind Direction and Wind Speed change between the ground and 3000m above ground.

Shear Wind Dir 3000 - 6000 / Wind Shear 3000 - 6000
Wind direction and wind speed change between 3000m and 6000m on the floor above ground.

Shear Wind Dir Sfc - 6000 / Wind Shear Sfc - 6000
Wind direction and wind speed change between the ground and 6000m above ground.

Bulk Shear
Is used for bulk Richardson Number ( important).

Ri Shear
Is used for bulk Richardson Number ( important).

Bulk Richardson Number
From the Bulk Richardson Number ( BRN) can be concluded that what is expected for a Gewittertyp ( see also Energy Helicity Index).

Formula: BRN = ( CAPE / 0- 6km Shear )
<10 Thunderstorms unlikely - single cell type
11-49 Moderate chance - supercell in nature
50 + Strong chance - multi cell type


Storm Relative Helicity
The Storm Relative Helicity (SRH ) is a measure of the change of direction and speed of the wind in the lower 3000m.
Example

<80 unlikely Super Cells
80-150 Supercell Storms
150-299 F0 , F1 Tornadoes
300-499 F2 , F3 Tornadoes
> 450 F4 , F5 Tornadoes


Effective Storm Relative Helicity
The Effective SRH should describe more precisely for a thunderstorm available Helicity than the conventional SRH . The Effective SRH is not necessarily between the ground and 3 km altitude calculated , but between the X point where the ascending air parcel , for the first time at least 50 J / kg and reaches X + 3km. Especially with the Soundings in the night where a soil inversion , the value is different from the normal SRH . During the day, both values are generally identical , except the post-frontal area where it is often elevated CAPE .

Energy Helicity Index
The Energy Helicity Index ( EHI ) is needed in the U.S. as one of the best indices to predict tornadoes. It is calculated from CAPE and Storm Relative Helicity , so the two most important parameters for the determination of the expected storm type.

Formula : EHI = ( CAPE * SRH ) / 160,000 < 0.8 Tornadoes unlikely
0.8-1 F0 , F1 Tornadoes
1-4 F2 , F3 Tornadoes
> 4 F4 , F5 Tornadoes


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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Jul 20, 2010 11:02 pm

jp wrote:

Formula : LI = T500 - T500Parcel
> 0 Thunderstorms unlikely
0 - -2 Thunderstorms possible - trigger needed
-3 - -5 Thunderstorms probable
-5 - -7 Strong / severe thunderstorms . Tornadoes possible
-7 - -9 Move to Alaska
< -9 Yikes


hahahahaha
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Jul 20, 2010 11:05 pm

made me laugh aswell. got to love the swiss attitude lol!
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Sep 21, 2010 4:06 am

some good info there, but a lot of that stuff is not 'relevant' when chasing...you only need a few things to determine if there will be storms and what type and how much lightning is probable.

Since I'm doing some lightning research I'll give some tips where I can and give some added info re soundings, so if you want to know something about the plots then post the skew-t up and we can discect it and discuss what's going on.

Check this sucker out from Dec 10 2009 for Darwin and see if you can work it out...carton of beer for the correct answer!


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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Sep 21, 2010 7:06 am

I so need a couple of weeks to myself to get my head round all the charts etc. to learn more! Perhaps once my kids have got into their routines a bit more with college times and more importantly are on the road again themselves (which will stop me being taxi!).....however I know of a house husband who must be able to spend a little more time now on these things! lol!
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Sep 21, 2010 8:56 am

Wooohooo!

Ok here goes.

Nice unstable airmass, (CAPE line being to the right of the temp line). Low level directional shear present, speed shear towards the low-mids and uppers, with directional shear also present up high too. Clouds topping out real high, nearly off the chart. Lapse rates are ok too. Small amount of CAP at 0.32. Quite a moist atmosphere throughout with some dryness towards the mids. Cape and LI good.... I would get excited over this if it were from the UK!

Any good?...
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Sep 21, 2010 3:09 pm

The directional shear is weak at the surface but speed shear is quite good for updraft speed. The sounding shows steering from the SE which is pretty normal for our storms and they did in fact track NW. The directional shear aloft is typical of most thunderstorms unless the anvils backshear, meaning the anvil wash goes against the prevailing winds. So from this sounding you will know that upper winds will blow the anvil to the SW.

The CAPE is absolutely perfect. It is extremely high and very deep through the levels, especially from 500hpa down. Lapse rates are really good also, the temp line cools all the way to the mids and continues to slant to the LHS, a sign that the parcels will ice up and cause good lightning. Where it warms in the mids it still starts to cool again as it goes higher, an even better sign.

The sounding also shows A-typical storm perameters. Moist at the surface, dry, moist and then dry again. Moist storms will not prodice an abundance of lightning, which is something I mentioned to Julian. The location of the UK is just not conducive to regular lightning active storms, and a lack of heating through land mass and temps really hinders this. What icing that does occur in your storms is quickly soaked and falls way too early and without the aid of lift there is little chance of a continual electrification process. That's not to say that you can get good lightning displays, it all depends on the atmosphere at hand.

You'll never see soundings like this in the UK simply because it's a tropical environment sounding. If this was say on the eastern side of Oz there would be massive supercells..if it was in the US with a jet stream it would be a tornado outbreak. Unfortunately Darwin is not affected by the jetstream...we're a little too high up as the jet usually tracks through the centre of Oz. When we do have depressions from lows inland or close to us we have what's called 'spikes'. These are the outer bands that swirl out (like with hurricanes and the like), you get a pattern of a strong spike, a weak spike and so on and so on. So by even viewing lows one can determine which day will be better than another.

The aloft EL is so high it runs off the chart at 100hpa - storms that bust at 300hpa are usually 50k high, so these storms were well above that on this day.

Even the Dp's are massive but heating is not necessarily that high. It was an insane day for storms, but by nightfall it spectacularly went nuts around 9pm for over two hours. This sounding was representative of a storm system that produced over 22,000 recorded lighting discharges of all types in a three hour period.

The CAP is negligable considering the CIN values of 18...that's pretty much the average for Darwin once things get cooking. Good cooling at the 700hpa region from the parcel is important also because this region starts the charging process. Even though the sounding was 'drier' at 700, the temp trace is still cooling past that level.

Anyway's you were on the money with the interpretation Wazza, if you can pick that much out from my sounding then the rest is just putting it all together to form a picture of what may occur. Julian will buy your carton of beer cos if I send it it will get sent back as an illegal substance sent abroad!!! (ie cold beer!!!)
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PostSubject: Re: Soundngs or Skew-T plot graphs   Tue Sep 21, 2010 7:43 pm

Haha nice one dude! Julian will probably get some of the cider he brought with him when he visited a few weeks ago - man that stuff you could fuel your car with!

On the storm front so to speak, as you well know by now we need those imported storms for anything decent. For some reason this year we have been totally hampered by a westerly airflow, keeping anything decent at bay. I mean there were real big storms in France this year that did track north towards us but then most diverted NE away from us being blown away by the dominant westerly flow.

I think as I say we have a better chance if some Atlantic depressions this year now.
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PostSubject: Re: Soundngs or Skew-T plot graphs   Wed Sep 22, 2010 2:29 am

Agreed. The weather patterns over the UK are abismal! They just either die out or push everything SE! What you need are elevated storms - the kind that don't require a lot of shear or deep CAPE...just a decent mesoscale forcing to break through that poor excuse of an LFC!
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PostSubject: Re: Soundngs or Skew-T plot graphs   Wed Sep 22, 2010 7:57 pm

Indeed - you mentioning the CAPE there, remember that good(ish) squall line we had back in April I think it was?... That had zero CAPE! But produced lightning and very squally winds. Although quite short lived, it kicked the nuts out of our area for around 20 mins.
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