What is a Skew-T/Log-P plot, and how do I read it?

What is a Skew-T/Log-P plot, and how do I read it? (such as those found here) I realize that there are many ways to plot such data, but each plot contains the same basic information (right?).

Also, what trends does a chaser look for in these plots to determine an area's ability to spawn chaser-friendly storms?
 
http://squall.sfsu.edu/courses/metr200/han...outs/skewt.html

They use different colors though. The link you provided has temperature as solid red, dewpoint as solid green. The dry adiabats (explained in the article) are the red lines sloping to the upper left. The moist adiabats are the green dashed lines. The isotherms here are blue and the isohumes are kind of yellow.

As far as good chasing stuff, you want to have a moist layer near the surface, topped by an inversion, with a colder dryer layer above it.

At least in simple terms.
 
Jul 8, 2004
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I'm surprised no one has mentioned the wind barbs on the right hand side of most skew-t visualizations. These can provide valuable information for understanding the wind speed and direction as the height increases. For good chasing, you want winds that change direction a lot with height, and also the speed difference is important. This wind veering with height is called shear and is an important concept in the formation of mesocyclones which are the "core" of the strongest storms. No shear, no meso, no tornado (in general).
 
Jul 8, 2004
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Originally posted by Rockwell Schrock
What is the \"direction\" of the barbs in relation to? Is the top of the diagram North, or is it just the top of the atmosphere (so sideways barbs could be blowing towards any compass direction)?
Ah, sorry, the direction of the barbs is traditional compass direction, and points towards where the wind is coming from (exactly the same as reading wind barbs from visual metars). So a barb pointing straight up represents winds coming from the north (or 000 deg), a barb pointing right represents winds out of the east (090 deg), downward barb is southern winds (180 deg) and a leftward barb represents winds out of the east (270 deg).

To find the windspeed, use the shape and length of the barb. A full sized line represents 10 knots, a half-sized line represents 5 knots. A triangle or flag represents 50 knots. Add all the individual symbols to find the total wind speed.

more here: http://ww2010.atmos.uiuc.edu/(Gh)/guides/m...sfcobs/wnd.rxml
 
Jul 8, 2004
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I figured I'd post a relatively famous (perhaps infamous) skew-t plot.

This is the skew-t from 3 May, 1999 which most people will recall as the day of the Moore/OKC, OK F5 tornado.

You may notice this sounding is from 18z. Typically soundings only occur twice a day, at 0z and 12z. However when forecasters know the danger of severe storms is urgent, they will release a special intermediate 18z sounding to get a good feel for the environment in the early afternoon. As we can see, for the most part the cap has eroded and the boundary layer air is moist with cool, dry air above 850mb, as well as a good veering wind profile (winds shift from south to west with height providing good shear). Not to mention the extreme amount of CAPE evident in the sounding, this synoptic setup went on to produce many destructive storms throughout the afternoon...

[Broken External Image]:http://www.personal.psu.edu/users/m/a/mas582/482/cape/TornadoJPEG.JPG
 
Apr 22, 2004
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CMI
www.atmos.uiuc.edu
Since these threads are suppposed to be educational, I'm going to pick on Zach's comments a bit (sorry!). First, the sounding image is modified from one found here....

http://www.spc.noaa.gov/staff/edwards/3may...99/modsndgs.htm

....which was a study by Roger Edwards, Rich Thompson and Jim Ladue. The directory link for the full study:

http://www.spc.noaa.gov/staff/edwards/3may...99/init3may.htm

The sounding, and CAPE value posted by Zach, is for a surface based parcel - which is really not appropriate. The 100 mb MLCAPE (mass mixing of the lowest 100 mb temperature and moisture profiles to determine the temp and moisture of the parcel to lift for the CAPE calculation) is more modest, around 3600 J/Kg, and even deeper mixing is probably needed to capture a realistic parcel from this sounding. See the second sounding on the first link above for the mixed parcel thermodynamic values.

Also, you can't really capture a synoptic setup from a single sounding. A nice synoptic setup composite was made for the May 3 case, and is available here:

http://www.srh.noaa.gov/oun/storms/1999050...mapfeatures.gif

While you may be able to infer some synoptic scale features from a single sounding - you would find forecasting from a single sounding alone would be rather fruitless.

Glen
 
Okay I'm a little confused now. In the first plot posted, the blue dashed line (to the right of the CAPE area) represents what? I understand the path that it follows (the dry and moist adiabats), but where does the starting value (at 1000 mb) come from?
 
Originally posted by Rockwell Schrock
Okay I'm a little confused now. In the first plot posted, the blue dashed line (to the right of the CAPE area) represents what? I understand the path that it follows (the dry and moist adiabats), but where does the starting value (at 1000 mb) come from?
I believe that's the parcel temperature, if I am reading your question right. CAPE can be calculated from any level, from the surface on up. The starting value (in the case of 1000mb), would mean the parcel was calculated from 1000mb on up.
 
Apr 22, 2004
998
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CMI
www.atmos.uiuc.edu
Originally posted by Rockwell Schrock
Okay I'm a little confused now. In the first plot posted, the blue dashed line (to the right of the CAPE area) represents what? I understand the path that it follows (the dry and moist adiabats), but where does the starting value (at 1000 mb) come from?
This is what you might hope for in an early morning sounding - in which case the starting point of the blue line is the expected afternoon high temperature, and the assumption is that the rest of the thermodynaic profile remains constant over the course of the day (often a pretty bad assumption). The green line is the constant mixing ratio line for the surface parcel - which would have to experience no mixing in this case for a lifting of nearly 2 km.

Glen
 
In that plot, the blue line is about 10C warmer than the actual air temperature at 1000 mb. If the blue line represents an air parcel's temperature, then shouldn't it match up with the temperature plot at that point? Or does that line always correspond to a forecasted high/low?
 
Dec 8, 2003
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One thing I don't understand about Unisys's soundings is the discrepancy that always seems to exist between MAXT and the yellow line's starting point.

For example, this morning's KFWD sounding:

http://weather.unisys.com/upper_air/skew/s.../skew_KFWD.html

It says MAXT 21.0, yet the yellow line is plotted from about 12. Because of this, I always just try to visualize the sounding as it would look with the yellow line shifted over to MAXT.

Can someone explain this?

Thanks.

Bob
 
Dec 9, 2003
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Oklahoma
Bob,
The yellow line and the MAXT are two independent elements (i.e. the yellow line has nothing to do with the MAXT). That yellow line is a parcel trajectory line/curve based off a mean mixed-layer parcel. In other words, mix the lowest 100mb (in the basic sense, 'mix' meaning taking the mean temperature and mixing ratio) and force the parcel to rise... The MAXT is usually a very ballpark estimate which is obtained by bringing the 850mb temperature (parcel) down dry adiabatically (~9.8C/km) to the surface. This of course assumes that, during the day, the lower levels will mix (PBL will deepen) and you'll end up with a potential temperature at the surface equal to the potential temperature at 850mb, which make for a decent estimate on days with strong insolation. Of course, this also assumes that there is no thermal advection at 850mb.
 
Dec 8, 2003
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Southeast CO
www.youtube.com
Thanks, Jeff, but I'm still confused.

That's interesting how MAXT is calculated. Is there some corresponding relationship to that and the way the NWS forecasts your local daily high temp?

I interpret your explanation to say that MAXT is essentially still a ballpark of max 10m temp, whereas the yellow line, or parcel trajectory, is based on a mean mixed layer of the lowest 100mb accounting for moisture.

The PBL is assumed to be mixed to 850mb, so with the trajectory based on the lowest 100mb, then how does its "sfc starting point" differ from the MAXT when bottom line (no pun intended) we're still looking at parcel temp, moisture or not?

Looked at from a different way....the MAXT is approximate max sfc or 10m temp for the day, based on data for 150mb deep. The trajectory is based on data from "only" 100mb deep. The trajectory initiates at the surface, or the same place as the MAXT, and is the same parameter, i.e. temperature. Moisture obviously plays a huge role in the parcel's upward trajectory, but no role at all in the lowest 100 or 150mb where we are still below LCL (today).

I've considered the possibility that MAXT is for afternoon and trajectory is just based upon obs, but this is obviously not true, or the actual obs would coincide with the trajectory at the sfc, or very nearly, and they don't. The sfc temp was 4.8C. Besides, you would have obviously made that point if it were true.

Thanks again!

Bob
 
Dec 9, 2003
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Oklahoma
Interesting question... I think the difference lies in the fact that that the MAXT is taken from a single level (850mb I believe), while the parcel trajectory uses the lowest 100mb. Here's where the difference can be significant... Imagine a sounding with a strong cap aloft (quite common on the 12z soundings courtesy of radiational cooling at the surface), taken from a site where the surface station pressure is 980mb. The MAXT will be obtained by bringing the relatively-warm 850mb temperature to the surface (980mb). Meanwhile, the parcel trajectory curve will be that of a mixed layer between 980mb (relatively-cold) and 880mb (not as warm as 850mb). The mean potential T of this 100mb thick mixed layer will be quite a bit cooler the T resulting from bringing the 850mb T to the surface. Now, if the atmosphere from 850mb to the surface was already well-mixed (constant mixing ratio and a temperature profile following the dry adiabatic lapse rate), then you would be correct in saying that the MAXT and the parcel trajectory curve at the surface would be the same. In most cases, particularly non-0Z times, the environment below 850mb isn't well mixed, in which case the MAXT will be warmer than the surface temperature of the mixed parcel. Granted, there could be situations were this isn't the case (say, a very strong inversion near 900, and cooler at 850mb, in which case the mixed layer potential T COULD be warmer than the 850mb potential T), though not too often.
 
Dec 8, 2003
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Southeast CO
www.youtube.com
Excellent. I think I get it now! <light bulb>

Here's what my misconception was:

I always thought the MAXT and trajectory were based on forecasts for the afternoon. I understood that MAXT is not as soon as this discussion started, but still believed the trajectory was (a forecast), even though the trajectory would be the sole parameter on a sounding NOT strictly derived from obs.

The reason the trajectory's starting point (at the sfc) doesn't jibe with the scf T ob is NOT because the trajectory is a forecast (again, my misconception), but because it comes from mixing the lowest 100mb and dragging that whole parcel down to its potential T (which will usually be warmer than sfc T ob), right?

Woohoo! (If that's right, LOL!)

Bob