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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