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

[Rockwell Schrock]

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?

 

[Michael P. Morris]

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.

 

[Bob Schafer]

Here's a really great place to take a tutorial on interpreting Skew-T's/Soundings:

http://www.met.tamu.edu/class/ATMO151/tut/.../soundmain.html

Bob

P.S. I like Unisys's soundings better.......

http://weather.unisys.com/upper_air/skew/index.html

.....but everyone has their own tastes.

 

[mikegeukes]

Example of a Loaded Gun or sometime called Goal Post Sounding:

[Broken External Image]:http://www.srh.noaa.gov/oun/severewx/figure6.gif

Source: http://www.srh.noaa.gov/oun/severewx/figure6.php

Mike

 

[Rockwell Schrock]

So, basically, the storms spring up once an air parcel becomes warmer than the temperature inversion (the scenario the dotted lines represent in that above diagram)? Is that correct?

 

[Zach Bailey]

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).

 

[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)?

 

[Zach Bailey]

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

 

[Rockwell Schrock]

Ah, I see it now. That makes a lot of sense.

Anyway, thanks to everyone for your information. I like those Unisys plots much better than the other ones, and now I can actually interpret them! w00t

Thanks all.

 

[Zach Bailey]

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

 

[Glen Romine]

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

 

[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?

 

[Robert Dewey]

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.

 

[Glen Romine]

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

 

[Rockwell Schrock]

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?