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Sam Jowett	Posted 27/5/2003 2:53 PM (#25780) Subject: How to interpret Skew-T diagrams
Executive Location: Whitwick, NW Leics, UK, 145m amsl	How to: Interpret a skew-t diagram View an example skew T diagram Basically, a skew-t diagram shows the temperature of a vertical profile of the atmosphere. Because a rising thermal acts as an enclosed parcel of air, it cools at a different temperature to the surrounding air. Depending on how the temperature of the surrounding air falls with height, the thermal will either be able to continue to rise, or not. If it can, at some point it will condense and form cloud, if it continues to rise, a shower and rising further a cumulonimbus. By looking at the skew-t diagram we can determine how likely this is to happen. If the temperature profile falls quickly with height (leans to the left), the chances are that a rising thermal will continue to rise, if the temperature falls less quickly or even rises with height the thermal will not rise as much and cloud may not form. For more detail, see below: To interpret a skew-t diagram, first you must understand the motion of the air. Air has various properties, the ones we are interested in are moisture content (measured by the dew point) and temperature. When the ground is heated by solar insolation parcels of air are created, or thermals. As these rise they contain the properties of the air at the surface, so a particular temperature and a quantity of water vapour relative to the overall volume. As the parcel rises, the air pressure reduces (because there is less atmosphere above) and so the parcel expands. This process uses energy from the air within the parcel, causing the temperature to drop. As the temperature drops the dew point remains constant, because the total volume of water in the parcel is (basically) unchanged. If the parcel continues to rise, sooner or later the temperature is the same as the dew point, relative humidity has reached 100% and condensation occurs. This point is termed the convective condensation level (CCL). The ascent of the parcel to this point is approximately at a rate of 9.8ºC per kilometre and is termed the dry adiabatic lapse rate (DALR) because the main temperature change is caused by the expansion of the parcel. Above this point, if the parcel continues to rise, condensation continues to occur... a process that actually releases energy in the form of heat, offsetting the fall in temperature due to expansion. This means that the temperature fall with height in saturated air is lower than in unsaturated air. As the amount of water that the air can hold varies with temperature, at high temperatures when it can hold more water, more condensation can occur and therefore release more heat above the CCL. This means that at high temperatures, the temperature fall with height in saturated air or saturated adiabatic lapse rate (SALR) can be as little as 4ºC per kilometre, but this increases to nearer 9ºC per kilometre at -40ºC. Now, how this relates to the skew-t diagram, the afformentioned parcel of air will only rise if it is warmer than it's surroundings. We can determine whether or not this will be the case by releasing a balloon carrying instruments to measure the atmosphere with, a radiosonde. This process is commonly called a sounding and gives us the actual temperature with height or environmental lapse rate (ELR). A parcel of air must have a higher temperature than the corresponding point on the ELR to continue rising in normal convective conditions. If a parcel can continue rising beyond the CCL then we will get cloud... if it can rise 3km above the CCL we are at risk of showers... if it can rise to the tropopause (the top of the lower layer of the atmosphere, marked by a sharp and continuous inversion above about 300mb) we will get cumulonimbus and heavy showers and thunderstorms. The way we tell whether it will get cloudy, or if we will get thunderstorms is by using the skew-t diagram. We know the surface temperature, pressure, dew point and ELR. By following the absolute humidity axis up to the ELR from the surface pressure, we can find the CCL. By then tracing back down the DALR axis, we can see the temperature that must be reached before a parcel will be warm enough to rise to the CCL. This temperature is the potential temperature or trigger temperature which must be reached before any convective cloud will form. By then following the SALR axis (the curved one), you can see how the temperature of the parcel will continue to cool with height. If at any point it crosses the ELR, the parcel's temperature is the same as its surroundings and therefore stops rising and the cloud stops growing. If the ELR drops very quickly with height and a parcel would remain at a higher temperature as it rises, the air is termed unstable, conversley if the ELR falls slowly with height a parcel would generally cool more quickly and hence the air is termed stable, if the ELR rises with height then there is an inversion, which generally acts as a very stable cap to cloud growth. the greater the separation between the ELR and DALR/SALR the more unstable the air is (if the ELR is on the left) or the more stable it is (if the ELR is on the right). The process of cloud formation is also confused by lifting mechanisms other than convection, such as forced lifting along an otherwise stable frontal boundary, or up the side of a hill. As the mechanisms are slightly different, cooling of the air occurs at a different rate, giving a different point for cloud formation, the lifting condensation level (LCL). This mechanism is responsible for stratiform clouds and convection for cumuliform clouds. Skew-t diagrams also commonly show the environmental dew point lapse rate, generally to the left of the ELR except in supersaturation conditions, and the CAPE (Convective Available Potential Energy) index, which can also be used to indicate the extent of cloud and the strength of the convection. In the skew-t diagram below the red line is the ELR, the blue line is the dew point and the green line is the wind speed.
Andrés Davis	Posted 27/5/2003 3:00 PM (#25781) Subject: Re: How to interpret Skew-T diagrams
Manager Location: Western Brecon Beacons, Wales, UK. 1,100' ASL	Thanks Sam, Thats a BIG help. I have always looked at these when ts are forcast yet not got round to asking how to understand them.
Sam Jowett	Posted 27/5/2003 3:07 PM (#25782 - in reply to #25780) Subject: RE: How to interpret Skew-T diagrams
Executive Location: Whitwick, NW Leics, UK, 145m amsl	Apologies if this is repeating the above, just soe further stuff I wrote on the topic at some point. Data is collected by way of a little box of tricks called a radiosonde.  This is suspended below a balloon and released from a particular point on the ground.  As it rises through the atmosphere, the temperature, dew point, wind direction, wind speed, pressure, altitude etc are radio'd back to the base station at the surface.  A lot of this data is then plotted on the tephigram to give you an understanding of the weather at altitude. The temperature profile is probably the most useful information, though it is worthless without all the supporting data.  There are many axis' on a tephigram, partially what makes it appear so confusing, but you soon get used to them.  The temperature axis in the chart above is in pale purple and slopes from bottom left to top right.  The actual temperature profile (also termed ELR for Environmental Lapse Rate) is plotted in pale blue.  It is rather confused with some of the other detail on the chart above, but if you follow the ELR up the tephigram, you'll see that the temperature crosses many of the temperature axis gridlines, in this case indicating a temperature fall to nearly -40ºC at 300mb/9147m. The rise or fall in temperature with height is what let's us know whether the air is unstable or stable.  If the temperature falls quickly with height the air is unstable, if it falls slowly or rises with height it is stable.  In unstable air you get convection and cumuliform clouds, in stable air you don't get cumuliform clouds. How convection arises and how we can tell whether or not it will using the tephigram: As sunshine heats the surface (assuming there is any!), some areas are warmed preferentially over others.  For example, the tarmac of a large car park will be heated more than a grass surface.  Somewhat simplistically, this can create a bubble of warm air of a particular volume and containing a particular number of air molecules.  For the purposes of this explanation, let's assume that the quantity of atmosphere inside this bubble remains completely constant as it rises... this isn't strictly true, but is surprisingly accurate. As this bubble of air is warmer than its surroundings, it is less dense and therefore rises.  As the bubble gets higher, there is less atmosphere above it and therefore the air pressure drops.  This causes the bubble to get bigger, but remember it still has the same quantity of atmosphere inside it, it's just more spread out.  The process of expanding the bubble requires energy, a process called adiabatic expansion.  This energy that is used is effectivley the heat within the bubble.  This results in a temperature fall within the bubble.  Now remember our ELR, this is only assumed to be the environmental temperature incidentally, it is possible the radiosonde will pass through one of these bubbles and give misleading readings.  The temperature of our bubble relative to the ELR is what determines whether it will continue rising.  If it is warmer than the ELR, it is less dense than the surrounding air and will rise.  If the bubble has cooled more than the ELR as it rises, it will gradually stop rising until the air inside and outside the bubble are of the same temperature and density. When the ELR shows a marked cooling as you increase altitude, the bubble will continue to rise... this is unstable air allowing convection.  If the ELR shows any warming as you get higher (an inversion), or the bubble cools more quickly than the surrounding air, the bubbles vertical progress will be stopped.  This is stable air. The greater the difference between the temperature in the bubble and the ELR, the more self contained the bubble will tend to be.  Also the greater its bouyancy will be and the faster it will rise.  This gives rise to momentum which can overide a small amount of stability. Potential temperature (trigger temperature) is the temperature calculated from the readings from the sounding, that indicates how warm it must be to kick off convection.  I've a sneaking suspicion others could explain this more clearly, but I'll give it a go... your parcel of air at the surface can be assumed to have the same water content (i.e. dew point) as is indicated by the sounding, though diurnal variation and frontal passages can mean this can change quite significantly in the space of minutes.  You need to be aware of these potential changes, especially if trying to predict the days convective potential from the overnight soundings.  Apart from the fact there may well be an overnight radiation inversion (temperature increasing with height in the first few hundred metres), the dew point within an air mass tends to be lower overnight than by day. Anyway, on your tephigram you firstly place a dot that corresponds with the dew point temperature or mixing ratio (absolute humidity), using the pale purple axis leaning to the right and surface pressure (horizontal axis).  As the content of water vapour with in the unsaturated parcel of air can be assumed to remain constant, trace up the temperature axis from this point until you meet the ELR.  At this point, temperature = dew point and the air is saturated.  This is the condensation level also known as Normads point I think.  It is marked on the tephigram by the LCL (Lifted Condensation Level) which is slightly different to the CCL (Convective Condensation Level) due to the processes that produce saturation of the air.  They are generally at similar heights though... perhaps someone else can expand on the differences... I can't remember!   From this condensation level, trace back down the DALR (Dry Adiabatic Lapse Rate) to the surface pressure level again.  The temperature at this point is the potential temperature or trigger temperature.  I find it is better to think in terms of the rising parcel though... in the middle of the afternoon it will have a particular temperature and dew point.  As it rises, the dew point of the parcel will stay constant, hence following that axis, but the temperature in the parcel will fall at the rate for dry air, hence following that axis.  The point at which these temperatures coincide gives you condensation. This of course may not be on the ELR... this point needs to be to the right of the ELR for the parcel to continue rising and form cloud.  The further to the right of the ELR this condensation point is, the greater the bouyancy of the air and the faster the cloud will start to grow.  Above the condensation point, you need to follow the path of the SALR (Saturated Adiabatic Lapse Rate - curvy yellow axis).  This is the rate of temperature fall within the parcel when the air is saturated.  As long as this line stays to the right of the ELR, the cloud will continue to grow and again, the further to the right it is, the faster it will grow.  If at any point the SALR above the condensation point intersects the ELR, the parcel's air and environmental air are the same, the air no longer has a tendency to rise and the cloud tops out. The caps that you may have heard mentioned generally to refer to an inversion.  For a cloud to continue upward growth, the SALR must remain to the right of this relativley warm air.  If the SALR is to the right of the ELR in the main, all apart from the inversion, it is considered conditional instability.  As soon as the trigger temperature and profile move to the right of this inversion rapid convection is often acheived, because the rapid cooling of the ELR above the inversion means there is a large temperature difference between the parcel and ELR.  It is likely that there would be a lot of CAPE (Convective Available Potential Energy) in these circumstances... though it is only realised if the surface temperature and dew point are high enough to allow the cap to be broken.  LI (Lifted Index) is similar in that it measures the difference between the parcel temperature and the environmental temperature at 500hpa.  When it is negative the temperature of the parcel is higher and convection is possible at 500hpa.  This often coincides with strong convection, but if it is above a strong inversion, again it may not be realised.
Sam Jowett	Posted 27/5/2003 3:11 PM (#25783) Subject: Re: How to interpret Skew-T diagrams
Executive Location: Whitwick, NW Leics, UK, 145m amsl	If you've got any questions on that lot, do ask. It's a while since I wrote that lot and I'm not sure how confident I am that it's right... ought to be a reasonable starter though! Apologies for the size of the image btw... when I reduce it the clarity is lost.
Fujita5	Posted 27/5/2003 4:13 PM (#25793 - in reply to #25780) Subject: RE: How to interpret Skew-T diagrams
Lightning Posts: 581 Location: Clacton-on-Sea, Essex	Sam, thankyou so much for posting that.  Skew-T's are something I've wanted to understand for a very long time and I'm sure everyone will agree your explanation makes it a LOT easier to digest!! One thing I wanted to ask... while watching Silver Lining Tours' 2001 season video, a sounding was shown with 2 shaded areas marked CAPE and CIN (cap).  Can a skew-T plot easily be used to visualise CAPE vs CIN in this way, if so is there a rule of thumb to predict whether the cap will be overcome i.e. the size of the 2 enclosed areas?
Sam Jowett	Posted 27/5/2003 4:31 PM (#25797) Subject: Re: How to interpret Skew-T diagrams
Executive Location: Whitwick, NW Leics, UK, 145m amsl	No problem, CAPE is basically the area between the SALR and ELR where the atmosphere is unstable. The larger the area, the greater the bouyant energy available to a rising parcel. CIN is the opposite, it's the area between SALR and ELR where the air is stable and is only generally plotted when there is instability above, I think. This time the area is proportional to the amount of negative bouyancy, and indicates the energy required to push through the cap. The easiest way to see if the cap will be overcome is to plot your potential temperature as described above and see if you think it's feasible. You also need to be aware of the dew point though... the higher that is (similar to temperature), the more likely the cap will be broken. For some good descriptions of many of the convective indices try going to: http://www.crh.noaa.gov/lmk/soo/docu/indices.htm and http://www.wdtb.noaa.gov/resources/IC/svrparams/svrparams.htm
Fujita5	Posted 27/5/2003 8:11 PM (#25811 - in reply to #25780) Subject: RE: How to interpret Skew-T diagrams
Lightning Posts: 581 Location: Clacton-on-Sea, Essex	Thanks again.  Will look forward to seeing the soundings over this coming weekend.....
Peter H	Posted 27/5/2003 9:14 PM (#25824 - in reply to #25780) Subject: RE: How to interpret Skew-T diagrams
Flash Flood ~ Frouk Location: Dartmoor, Devon	This is quite interesting http://www.itadvice.co.uk/weatherjack/tut-soundings/Tut-soundings-P1.html. I keep reading it in the hope it will all finally stick.
Miss Dot Com	Posted 27/5/2003 9:33 PM (#25826 - in reply to #25780) Subject: RE: How to interpret Skew-T diagrams
Moderator ~ Frouk Posts: 3904 Location: Witham, Essex.	I know I'm a moaner, but is there any way to make the sounding image a bit smaller? I keep having to scroll back and forth to read everything, it would make it much easier.
Sam Jowett	Posted 27/5/2003 10:10 PM (#25836) Subject: Re: How to interpret Skew-T diagrams
Executive Location: Whitwick, NW Leics, UK, 145m amsl	OK, so I couldn't compress the image without losing clarity, but where there's a will there's a way! The things I do for you dots!
Miss Dot Com	Posted 27/5/2003 10:44 PM (#25842 - in reply to #25780) Subject: RE: How to interpret Skew-T diagrams
Moderator ~ Frouk Posts: 3904 Location: Witham, Essex.	Yeay, thanking you. Just for that I promise to really, really try hard to get the hang of it this time. That is just as soon as I remember not to ke

[John W]

I've been wondering for a while why a cumulus (generally congestus) has such a straight-line base over several Kms. Quite often you will see this in sunset photos over the sea. I came to the conclusion that ,at that altitude..ie. the altitude of the base..the temperature of the 'air parcels' reached the temp. of the surrounding air and condensation started at this point. Only this evening was I chatting about it on another site then I came on here just now and lo and behold here is the answer in great detail .I was going to ask but thought it was rather obvious..so despite a plea on the forum a couple on days ago to jump in with ANY question I succumbed to the 'dumb question syndrome.' :s

Also, another coincidence..FUJITA5..you say you were looking at SLT tour 2001. I went on tour 4 that year.June 2nd-12th . We did 9 states and 600 miles a day (averaged)

[Pogoda]

In some cases are the LCL and CCL not actually calculated from the actual temperature and dewpoint values, but rather from an average of the bottom 50 or 100hPa, as in the following sounding http://62.202.7.134/...0070107_00z.gif , blown up below. The purple lines show the LCL, the white show the CCL. (The sounding is taken from the Severeweather.ch website http://62.202.7.134/...ding_world.aspx , well worth checking out!).

My question is, when calculating LCL and CCL, how do you know when to take an average, and when to take the actual values? I can see the logic in taking an average if the surface values are obviously not representative of a typical rising parcel, but where's the cut-off point, cos you can get very different LCLs and CCLs using both methods!!

[Mickey B]

Thanks for taking the time to post all this info, im going to reading it over a few times until I can stand a chance of using the netweather extra charts !

[TrevP]

Also a good introduction here

http://www.ibmhursleyclub.org.uk/ss/gliding/Soundings/how_to_read_soundings.htm 

[Mickey B]

Thanks trev,

Im at work doing a massive print job and am about to take the plunge into that info, thats if my brain can take it !

[Dave Hancox]

Could this thread be revamped, lot of good information but dead links bit messy. Possibly a some skew -t examples for different weather events or showing heights of cloud formation etc. Maybe a guide for dummies as I keep trying to understand it and give up.

[chrisips]

I've been reading through this site recently and i think i'm slowly getting the hang of the skew-T section, i think you probably need to go through the basics section first rather than just head straight to Skew-Ts though.

http://www.downunder...uide/index.html