Skew-T school gets a bad grade

It's pretty common to see various aviation organizations such as AOPA attempt to explain the Skew-T log (p) diagram, but very few are successful. For example, in this piece from Tom Horne, he attempts to explain some basic things about the Skew-T, but does a poor job doing so. It's understandable that he did not intend for this short article to be an exhaustive explanation of this diagram, but let's look at the bad information from this article so you can sort out fact from fiction.

The first image (shown above) at the beginning of the article says in the figure caption...


"One glance can tell a lot. The close temperature-dew point [sic] spread from the surface to 600 mb/ about 14,000 feet indicates cloudiness between those altitudes. A rising air parcel is colder than temperatures aloft, so convection shouldn’t happen. The freezing level is just above 700 mb/about 10,000 feet. Looks like a quiet, if IMC, situation."


A number of points to make here. First, we don't know if this is a RAOB or from a numerical weather prediction model. Given the smoothness of the data, it's likely from a model, but he does not make that distinction in the article. Second, the tops are more likely to be be above 600 mb. Notice the temperature and dewpoint start to diverge when the static air temperature is colder than -10°C (just above 600 mb where he says the cloudiness likely ends). But they stay pretty close together above that. This is very common and likely means that the atmosphere above 600 mb is saturated with respect to ice and not liquid water. The dewpoint is a better measure of saturation when the temperature is warmer than -12°C. It's very possible that the tops here are well above this and might be as high as 400 mb (24,000 feet) strictly based on the dewpoint depression.


Third, he says that the air parcel is colder than the temperatures aloft and that convection shouldn't happen. That's because on the diagram he provided, a surface-based parcel (dotted dark blue line) is being lifted. However, notice that when a most unstable parcel is added (magenta line), that there is plenty of instability and convection could build up to the equilibrium level at 250 mb (~35,000 feet). This doesn't imply that convection will develop, but it's not as stable as he makes it seem because he is likely not aware of the concept of elevated convection.


Further down he says,


"Hodographs show the turning of wind direction with altitude and are used to predict tornados [sic]."


This is true. However, hodographs can help understanding storm type and severity. Will these be pulse-type storms or supercells? It can also tell if storms may split creating possibly right- or left-turning cells which also can be severe. And yes, they can tell you the bulk shear in the atmosphere that is responsible for thunderstorms that can produce a tornado.


Next he says,


"Also, in a northeast-southwest orientation are dashed lines, which represent the saturation mixing ratio—the amount of water vapor in the atmosphere compared to a mass of dry air. Some Skew-Ts don’t show these, but they’re used to determine where dew points and temperatures merge."


This is really confusing. The saturation mixing ratio can be used to determine where unsaturated air will saturate in a rising parcel of air. That's called the lifted (or lifting) condensation level (LCL). What he should have said, is that these lines define the change of dewpoint temperature in a rising parcel of unsaturated air. They can also be used to determine the relativity humidity at any altitude and are used to determine where icing may be the most intense when air is saturated.


He goes on to say,


"Superimposed on this clutter of diagonal lines are the traces for the observed, or environmental, temperature and dew point [sic] temperatures. All those lapse rates and ratios are standardized measures—but the temperature and dew point [sic] traces are from weather balloons with radiosonde transmitters, temperature profilers, and satellites with remote-sensing equipment."


While he is correct that the temperature and dewpoint can be plotted using data collected from sensors, the data can also come from numerical weather prediction models which he doesn't really mention except at the very end when it points out websites that can provide a Skew-T diagram from a model. Interpreting a Skew-T from a model is much different than interpreting one from a radiosonde launch (RAOB).


Next he gets into the parcel theory (he doesn't use that specific term) which is a fairly complex concept and he says,


"Air parcel analysis becomes very important in determining the chance, and severity, of thunderstorms. We all know that warm air rises—but only if the air around it is cooler. How much cooler? The gap between parcel and observed temperatures tells the tale. A wide gap means a lot of what’s called convective available potential energy (CAPE). CAPE is frequently posted with the chart. Values of 1,000 to 2,500 mean thunderstorms are likely; 2,500-4,000 indicates widespread severe storms; and above 4,000, extreme storms with large hail."


Once again, he emphasizes the observed temperature. This may also be a more useful model forecast temperature. CAPE simply describes how the atmosphere is poised in terms of convective potential and says there is sufficient moisture and instability but says nothing about whether enough outside energy contribution (or lift) is present. CAPE simply says that there is enough fuel for a fire, but doesn't tell you if there's a spark available to light that fuel.


Here's a big error on his part. He says, "If the temperature line is running straight up and down, then you’ve got an isothermal situation." That's not correct unless you are looking at a Stüve diagram. An isothermal layer is one that does not change temperature with height and it follows the isotherms which are skewed to the right on the Skew-T diagram. He follows this with, "If it’s leaning to the right, there’s an inversion." Sure, but it has to lean pretty far to the right such that the temperature line is showing an increase of temperature with height. If the temperature is straight up and down, then the temperature is decreasing with height.


He continues with explaining stability as he says,


"Inversions, for example, suppress cloud buildups because their warmth can exceed the temperature of any rising air from below, preventing it from climbing—unless it has so much energy that it can zoom up through it!"


Yes, inversions imply a stable layer in the atmosphere. They do suppress the motion of air that is being displaced vertically. So inversions can limit the vertical growth of cumuliform-type clouds. But he doesn't connect the parcel's lapse rate to this concept so he makes it very confusing to understand what is going on. He's saying as the parcel rises, it cools dry adiabatically and in the presence of an inversion the parcel will reach a temperature that is colder than the surrounding air (due to the inversion) and will not be buoyant.


Certainly an air parcel can "zoom" through an inversion, but this takes a lot of outside energy to do so. It won't magically penetrate through the inversion without some help. A rising saturated parcel of air in building deep, moist convection can penetrate through the tropopause and into the stratosphere since the updrafts in thunderstorms have substantial momentum and may overshoot the parcel's equilibrium level. The tropopause is the mother of all inversions and keeps our weather limited to the troposphere.


Lastly, he lists some websites in the article to view Skew-T diagrams, but leaves out one of the best from NOAA, namely, https://rucsoundings.noaa.gov. If you want to learn more about a Skew-T diagram, consider purchasing the most comprehensive book for pilots, The Skew-T log (p) and Me: A primer for pilots found at https://avwxtraining.com/skewt.


So Tom gets a C- as a grade for this article. Just as a reminder, when you read any weather-related article, be sure you look at the source. Experts in the field will give you the most accurate information. For those others, buyer beware.


Most pilots are weatherwise, but some are otherwise


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

68 views0 comments