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

[Bob Schafer]

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

 

[Jeff Snyder]

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.

 

[Bob Schafer]

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

 

[Jeff Snyder]

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.

 

[mikegeukes]

Meteorologist Jeff haby has some information on MaxT

FORECASTING SURFACE HIGH USING 850-mb TEMP
http://www.theweatherprediction.com/habyhints/278/

PREDICTING HIGHS, LOWS AND TEMPERATURE TRENDS
http://www.theweatherprediction.com/tempfx/

SKEW-T: A LOOK AT MAXT
http://www.theweatherprediction.com/habyhints/296/

Source; Haby Hints

Mike

 

[Bob Schafer]

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