3032: Skew-T Log-P

Skew-T Log-P

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Title text: The most important quantity for meteorologists is of course the product of latent pressure and temperostrophic enthalpy, though 'how nice the weather is' is a close second.

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Explanation

This is one of 52 incomplete explanations: Created by a BOT CLEANING UP AFTER DAVE - This needs an explanation. Table not filled out. Also the title text was not mentioned at all. I added a very simple start to this, but nothing about what the product actually means, please expand... Do NOT delete this tag too soon. If you can fix this issue, edit the page!

A skew-T log-P diagram is often used to plot atmospheric soundings made by sending a weather balloon up into the air. The name comes from the temperature (T) lines being skewed at a 45-degree angle, and the pressure (P) lines being logarithmic in scale.

Because the diagrams have a lot of lines on them (isobars, isotherms, adiabats, and mixing ratios, and that's before plotting the actual measurements of temperature and dew point temperature), they can be hard to understand. The comic pretends to offer an explanation of one such diagram, but most of the explanations are blatantly incorrect or humorous in nature. The diagram appears to have measurements from two separate weather balloons, with solid lines for rising balloons and dashed ones for popped balloons falling back down, whereas two lines and various styles of lines in real diagrams generally mean different measurements from the same balloon (or other sensor platform) track. See details in the table below. Many weather balloons do indeed pop, as they're designed to do this after reaching a certain height high in the atmosphere.

In the title text it is stated that "The most important quantity for meteorologists is of course the product of latent pressure and temperostrophic enthalpy, though 'how nice the weather is' is a close second". So it jokes by comparing a non-existent, complicated-sounding product (temperostrophic enthalpy is not actually a thing) with a simple sentence about how nice the weather is.

An actual Skew-T Log-P diagram, with several real annotations. The X-axis has temperature (blue diagonal lines in diagram) and the Y-axis has pressure in millibars.

The true design of a Skew-T Log-P diagram is intended to best represent the nature of the weather in any given column of atmosphere. The pressure (vertical axis, with pressure being closely but not exactly inversely related to altitude) is shown as a logarithmic scale (i.e., Log-P) because it makes altitudes nearly evenly spaced. Plotting pressure proportionately (which must also be from top to bottom, to match its general relationship with altitude) would space features out in ways that would be hard to use and interpret, whereas the logarithmic scale is far more pragmatic. The temperature scale is deliberately tilted, rather than orthogonal, which (together with the logarithmic nature of the inverted pressure scale) allows the typical way that temperatures fall with altitude(≈as pressure falls) to trend roughly vertically, give or take the notable changes that are key to understanding the forecast. Other measurement lines, differently skewed and often also curving across the temperature/pressure skewed-log 'grid', represent various other idealistic relationships (where both T and P vary, keeping another measure constant) that are useful references to meteorologists.

Upon this style of graph are plotted the actual measurements obtained by releasing a weather balloon or other sensor. As well as the variation of actual temperatures and pressures, other retrieved and calculated data is plotted, such as the dew point. The dew point, a function of the air's water content, temperature, and pressure, is where condensation begins. By observing how the actual measurements and dew point line converge and cross, the development and nature of clouds can be tracked and pinned to specific cloud layers. Further details may also be included, such as wind-direction and wind-speed indications (often to the side of the plot) to give a visual cue about possible wind shear and/or to suggest which direction of adjacent weather-station readings may hold clues as to what changes may later blow in above the current site.

Table with terms

Item in comic	Correct?	Explanation
Pressure latitude	No	Pressure altitude is the height above a standard datum plane, a theoretical level where the pressure of the atmosphere is 1013.24 millibars (29.921 inHg). It's essentially an estimate of altitude calculated from atmospheric pressure.
Enthalpic pressure
Entropic density
Latent heat of cooling
Isobars	Yes	Lines denoting equal ("iso-") air pressure ("-bar"), probably most often recognized as the indicators of how ground-level pressures change (or not) across the horizontal area depicted on a weather map. In this type of chart, which depicts data obtained from above a single point, it has the same meaning but is instead a pre-existing reference line across which the actual data is plotted, and does not itself indicate the nature of any wind.
Omnitrophic wind	No	Something "omnitrophic" would apparently be "all-eating", in some scientific sense. An omnitrophic wind would probably be a concerning phenomenon. Probably a play on something like geostrophic wind ("geo"+"strophic" being from "Earth curving"), a theoretical state of wind that results from an exact balance between the Coriolis force and the pressure gradient force.
Isomers	No	Different forms of molecules with the same formula, with the atoms or functional groups arranged differently. An example would be propanol, which has three isomers. One of the most common isomers of propanol has its OH functional group in the middle, so is called isopropyl alcohol or isopropanol. However, these are actually isotherm lines, representing equal temperature.
These lines are slightly different because Dave messed them up	No	Indicating isotherms (or, according to the comic, "isomers"), the suggestion is that slightly wrong lines were drawn by Dave and had to be corrected. The real reason for the not quite identical lines is that the measured temperature at a given pressure can be converted to or from the potential temperature that the same air would have if at a standard pressure (holding the same amount of heat energy). For practical reasons, both for composing and interpreting the eventual plot, each of the slightly differently skewed isotherms are given, usually in clearly differentiable styles of line.
Line of constant thermodynamics	No
Uncomfortably moist adiabat	Wrongly placed, unusually qualified	This labels a segment of isotherm, which is the exact 'opposite' of an adiabat. An adiabat is a line along which temperature can change for a given mass, without changing the amount of energy. This is primarily made possible by changing the density (by a change in pressure) of the gas. There are typically two types of adiabat, marked for reference on the plot, "dry adiabat" (curves across the isotherms perpendicularly, to create a largely square but slightly curved grid with them), and "moist/saturated adiabat" (the latter's heat-maintaining profile is influenced greatly by the humidity content, and produces graphing lines vastly different from the equivalent "dry" versions). Randall has declared this (erroneous) type of adiabat to be "uncomfortably" moist, so presumably not totally saturated but also not subjectively 'pleasant'.
Oops, the balloon flew through a ghost	No	Ghosts do not exist.[citation needed] However, one of the purported effects of ghosts (such as in the film *The Sixth Sense*) is a transient/local lowering of temperature around and/or inside them. The line is interpreted as showing a local low temperature encountered at this pressure(/altitude). This line, however, is probably the dew point line, indicating that in passing through this layer of the atmosphere, a drier band of air was encountered which would theoretically be cooled a lot more before the water-vapor oversaturates it and liquid water droplets form.
No birds up here :(	Yes*	This point is near the top of the diagram, with an air pressure of about 110 millibar - about 15 kilometers (50,000 feet) above sea level. This is well above the highest flight height of any known bird species. However, this information is irrelevant to the purpose of a skew-T log-P diagram.
Track of rising weather balloon	Yes, partially	Although there are other ways of recording these details, this is typically the record of a rising balloon. However, it would be a track of the balloon through the varying pressures and temperatures that it records (as the second line of this type records the measurements of dew point at each pressure value). Moreover, circumstances that would make the recorded data plot out a neat figure-eight knot (see "Seems bad", below) are very unlikely.
Track of popped balloon falling back down	Possible, partially	A standard plot track will include two strong lines, as this has, representing not two balloons but the recorded temperature track and the dew point track, both against the (altitude surrogate) progressive pressure changes at each pressure-point. A further pair of tracks as dotted lines could possibly be from a different launch (earlier, later or simultaneous from an adjacent location) as an analytical reference, but may indeed be the additional results obtained as the scientific package rapidly descends once the balloon pops.
Meteogenesis	No	The chart purports to show the path of two weather balloons crossing and labels the space between them with a new word. The root "meteo" means something high up (in this case, balloons) and "genesis" means creation. The implication is that a new balloon was created, though no third flight path is shown so it presumably did not fly separately or was not tracked. In reality, one of the tracks (almost certainly the left one) is the track of the measured dew point. Where the line of the existing conditions cross this line is where the moisture will precipitate out and form clouds, a process that might well be called "meteo+genesis", but isn't.
Seems bad	Not a common feature	The path of the balloon loops around in the shape of a figure-eight knot, which would indicate very chaotic conditions at that point, if taken as positional informtion As the actual Skew-T Log-P graph does not record positional information, this is best interpreted as having encountered a fluctuating temperature as the pressure decreases, continuing as something (possibly vertical wind shear, or some form of compression waves, encounter the instruments) creates a temporary increased in external pressure and then circumstances return it to its more typical altitude-induced pressure-drop. Though this is not impossible to naturally happen, it might even be best interpreted as the instruments being deliberately 'buzzed' by a passing aircraft or rocket, including some intermittent form of thermal backwash as the interfering craft criss-crosses the balloon's physical track in a briefly complex encounter. Whatever happened to the temperature/pressure track, it apparently did not significantly change the associated dew-point/pressure track (if the pressure did indeed temporarily rise, the related dew points repeated themselves each time the pressure values were re-encountered).
Dew point	Wrongly represented	The temperature at which water condenses out of the air, and therefore dew starts to form, given the amount of water vapor in the air. It is shown here as an actual single point, when it should be a line (typically the leftmost solid plotted line) representing the temperature at which dew should form at any given pressure.
Humidor	No	In reality, is a container that is used maintain a more controllable humidity within which to store smoking products. In the graph, points at the line that is probably representing the dew point, which is represents the actual humidity encountered.
Heavyside layer	No	Probably a misspelling of the (Kennelly-)Heaviside layer, also called the E region of the ionosphere that was co-discovered by Arthur E. Kennelly and Oliver Heaviside. In this diagram it is apparently labeling a heavily marked isotherm, or line of constant temperature - most likely the 0°C line, the freezing point of water that is of great importance to meteorologists, pilots, etc.
These lines are tilted because the wind is blowing them	No	The wind is not actually a derivable featured of this diagram, which does not have data of either direction or strength of air movement. These lines are actually dry adiabats (see above), possibly two sets due to a similar renormalized interpretation, as with the isotherms, at a given reference pressure.
Don't stand here or you might get hit by a balloon	No	This is a diagram of some measurements, not a place you could stand.

Transcript

This is one of 27 incomplete transcripts: Do NOT delete this tag too soon. If you can fix this issue, edit the page!

How to interpret a skew-T log-P diagram

[The comic shows a skew-T log-P diagram. On it are various labels, including isobars, comments, and other interpretations of the diagram.]

[Left to the diagram is an upwards-pointing arrow with the label "Pressure Latitude". Right to the diagram is a downwards-pointing arrow with the label "Entropic Density". Below the diagram is a right-pointing arrow with the label "Enthalpic Pressure".]

[The remaining various labels are inside the diagram.]

Trivia

• Even though this comic was released on New Year's Day 2025, it was not a New Year comic.
  • Only second time this has happened since New Year comics became a regular thing from 2011.

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Discussion

...did the ip address user really just have chatgpt write an explanation of this page without a proper understanding of what is happening in this page? the only reasonable content in this explanation page right now was contributed by other users and is a couple lines at the beginning. also they didn't even try to hide that the entire text was AI-generated - Vaedez (talk) 05:09, 2 January 2025 (UTC)

Better than hiding it. I'm guessing they just wanted to help but didn't know what the graph in question was. But I'm gonna go out on a limb and assume this mostly vapid explanation is as good as no explanation, and remove it for now. 108.162.216.132 05:42, 2 January 2025 (UTC)

Bringing a whole new meaning to the default "Created by a bot" unfinished message --172.70.214.157 05:43, 2 January 2025 (UTC)

skewtie --172.70.206.65 05:47, 2 January 2025 (UTC)

So, all the real nerds are still on holiday?Tommyds (talk) 09:39, 2 January 2025 (UTC)

I know enough about the subject to appreciate the comic, and generally recognise the real vs. not-so-real things in it to a decent degree, but (as of my first viewing, slightly after the first linkless "seems to be about the weather" text went in) couldn't really quickly muster a decent explanation together. From the above conversation, looks like I could have spent time on it, but it was late night/early morning and I was hoping something better would arrive while I was asleep. (From the above comments, and what little has effectively been added, I was wrong. Might poke away at it, now I've dealt with my morning tasks and had my dinner.) 141.101.99.164 13:09, 2 January 2025 (UTC)

I think I'm a nerd, but I really don't see much difference between Randall's version and the diagram referenced in Wikipedia. The humor, if there is any, is either a superb success or a dismal failure, either way being very much like weather forecasting. 172.71.23.88 14:00, 2 January 2025 (UTC)

Newbie here. I wondered whether the timing of this may be related to the recent publishing of a log-log chart by the Economist newspaper called "The Chart of Everything" (https://www.economist.com/interactive/christmas-specials/2024/12/21/the-chart-of-everything) based on the paper "All objects and some questions" (https://pubs.aip.org/aapt/ajp/article/91/10/819/2911822/All-objects-and-some-questions). If not, I hope you find it diverting. Stevejohnston (talk) 13:51, 2 January 2025 (UTC)

The former is (as expected) hidden behind a pester-/registration-/pay-wall that I'm not going to take the time and effort to navigate past/through. Maybe the second isn't, but want to recover from the experience of the first one, first. But, without breaking copyright rules, it could have been better if you had chosen a different link in the initial case. (Don't rely on us being paid-up subscribers, or maybe employing devious browser-scripts/noscripts to dodge the issue.) 141.101.98.244 16:16, 2 January 2025 (UTC)

That's disappointing. I was pretty sure the Christmas specials weren't pay-walled. Apologies for not checking first. Here is the direct link to the American Journal of Physics article image of the chart https://pubs.aip.org/view-large/figure/89607967/819_1_5.0150209.figures.online.f2.jpg hopefully you'll now see where my suggestion originated, and apologies for wasting your time. -- Stevejohnston (talk) 14:19, 3 January 2025 (please sign your comments with ~~~~)

Don't apologise. It's what online news-sites do (mostly... e.g. bbc.co.uk doesn't even have ads, as a non-commercial broadcaster, though I believe bbc.com does do some lighter-than-other-places additional bumf; and, other than them, maybe pop-up "spamvertising" sites that want their extremely biased output seen by as many gullible people as they can). Whether that's a full-blown "you're not getting in without paying" (give or take various ways round it) or just one of those annoying ones that pushes play-on-open videos on maybe-related subjects, behind a large notice pestering you to accept a couple of hundred "associate" cookies, it's something I'd be surprised not to encounter, just to try to balance costs and (by whatever means) incomes whilst maintaining their chosen profile of visitors/customers.

But, having become a 'customer' personal or institutional subscription/registration?), you might not realise that those who haven't yet become a 'member' might have to weave their way through the "five free views, and a lot of pop-ups reminding you of that" process to get their first look at any given thing. I'm sure you have encountered the same (elsewhere, at least), as a curious denizen of the web. It's how net-economics seem to mould the online news ecosystem. (I even get pestered by news.bbc.co.uk, but much less harshly and barely any effort needed to shrug off the pesters, given that I don't want to use their iPlayer/News/Weather apps but only take news articles and weather info directly from the web-pages.)

PS., the original aip.org link actually works well enough, I found out, once I'd made time to absorb any further battles with web-pages (though, as it turned out, there were none, except for a sensible cookie-control with a simple (not hidden!) "reject all" option... bliss!). Interesting, and (nicely!) info-dense. Maybe a deeper dive could hit registration-requirements, but what I saw looked like I was straight in. Will look at the direct picture link once I've submitted this (long-winded!) reply, but I think I know what it is that you mean now. edit:Yep, that's what I thought you meant. Again, interesting! 141.101.99.164 15:48, 3 January 2025 (UTC)

I feel like we should make a note about how this comic was published on New Year's Day in EST 141.101.109.192 14:44, 2 January 2025 (UTC)

In a number of ways, we already are doing. At least two of them predating your suggestion, but haven't checked the history for exact timings on all the non-automatic indicators. 141.101.98.244 16:16, 2 January 2025 (UTC)

The date is at the top... But I have rewritten the trivia about it, since it is clearly the first 2025 comic as it was released on New Years Day. In stead the trivia now mentions the odd fact that it is not a comic about New Year. A rare incident. But then again the Christmas Day comic was not about Christmas either, which was a first. For New Year a second since they began as a regular thing. But at least we did get a New Year comic this New Year just before in the last comic of 2024. --Kynde (talk) 16:33, 2 January 2025 (UTC)

The Heavyside/Heaviside layer is also the heaven-like destination in the musical (and movie) /Cats/. Nickdenny (talk) 21:15, 2 January 2025 (UTC)

xkcd wouldn't make a typo, right? "ChatGPT, make a list of the top 10 misspelt scientist names!"

..."10. Turing - Frequently misspelled as "Turing" or "Turing." Alan Turing is known for his foundational work in computer science and artificial intelligence."...Congrats, ChatGPT, you passed the Turing test with flying colors, only a human could be such a complete twit. 09:40, 7 January 2025 (UTC)
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