3032: Skew-T Log-P

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Skew-T Log-P

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.

Explanation

Skew-T log-P diagrams are commonly used to plot atmospheric soundings collected by weather balloons or other methods. The name comes from the temperature (T) lines being skewed at a 45-degree angle and the pressure (P) lines being logarithmic in scale. Although it looks very much like a cross-sectional diagram, it shows non-positional information derived from passing (generally) vertically up through the atmosphere from the initial reference location.

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 comprehend. 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 either measurements from two separate weather measurements or the measured temperature and dewpoint from a single balloon, with solid lines for the primary balloon's two streams of data (often disambiguated by the chosen hue of the line) and dashed ones for the secondary set of data (popped balloon(s) falling back down, a separate second survey balloon rising or estimates derived from weather-radar data). See details in the table below. Many weather balloons are designed to rupture after reaching a certain height high in the atmosphere.

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 inversely related to altitude, more or less) 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 through some 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.

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.

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	No	Enthalpy is the total internal energy of a thermodynamic system plus the product of the system's pressure and volume. Essentially, it represents the energy contained in a system, and is independent of the means or sequence of operations that the system went through to achieve its current state.
Entropic density	No	Entropy is a quantity that shows many physical processes can only go in one chronological direction. An egg can easily be scrambled (increasing its entropy), but it is very difficult to "un-scramble" an egg[citation needed] (which would decrease its entropy). Ordered systems have low entropy, with differences in temperature, pressure, potential energy, or the like. Disordered systems have high entropy, without temperature, pressure, electrical, or other differentials.
Latent heat of cooling	No	Latent heat is the energy absorbed or released by a system during a constant-temperature process, like melting, freezing, boiling, or condensing. Cooling is a process of lowering a temperature, and therefore not a constant-temperature process. The humor comes considering the "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. For example, propanol with the basic formula of C3H8O, or propane with a single -OH functional group attached. There are two distinct positions that the hydroxyl group can sit, 1-propanol has it at either end, while 2-propanol has it connected to the middle. 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. The "Dave" of the description may be David Bolton, whose 1980 paper introduced a means of calculating the atmosphere's potential temperature.
Line of constant thermodynamics	No	Thermodynamics is a branch of physics concerned with work, temperature, heat, the way they relate to entropy, energy, enthalpy, and the physical properties of radiation and matter. As a field of study, Thermodynamics does not have constant lines, except perhaps as a means of describing a consistent avenue of research.
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	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 information 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, encountering the instruments) creates a temporary increase 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 to 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	In the metaphysical cosmology of the musical Cats, the Heavyside Layer is a blissful afterlife which all the cats in the musical long for. It is likely included here as a comical 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. This layer is extremely far up in relative atmospheric terms, straddling the boundary to 'space', and would be shown as a broad horizontal feature significantly above the existing "No birds up here" label, if it all. The labeled item, in this diagram, is a heavily marked isotherm, or line of constant temperature. Most likely this is the 0°C line, marking the freezing point of water that is of great importance to meteorologists, pilots, etc. With a few caveats, passing 'through' this line marks a transition between any precipitation tending to be liquid rain and of it being snow/hail-forming (or, on passing down through it, to start melting such things), and if the measurement lines start above-left of this line then any falling (or condensing) water is almost certainly going to appear as one frozen form or other. Above-ground, it suggests important implications of potential ice-formation (affecting weight and lift dynamics) upon aircraft beyond any given altitude, should the existing moisture load be high and the dew-point be inconveniently close.
These lines are tilted because the wind is blowing them	No	The wind is not actually a derivable feature 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	On the misguided basis that this is a cross-sectional diagram, this would be the imagined release-point for the balloon(s) involved. And, if you're particularly (un)lucky with the winds, where they eventually fall straight back down to once the balloon has popped. However, this is a diagram of some measurements for a location, not directly indicating a range of places you could choose to stand, and the bottom of the lines indicate the conditions at the release point (and possibly then the point of landing), regardless of where those lines appear to be rooted. To be accurate, the whole width of the the table (and at a 'height' that represents the actual recorded ground-level pressure for that location and time) is where any 'danger' may be. The person initially releasing the balloon would not normally be too fazed by being struck by a wind-buffeted balloon (if anything, they'd be more concerned at damaging it prior to release). The attached remains of a popped balloon that is no longer buoyant will generally also act as a form of parachute (together with any actual drogue chute) to make any the light (and often well padded) payload descend slowly enough to not be a falling danger. Having traveled tens of kilometers up, before gently coming down, the chances of any given balloon landing in any given awkward spot (let alone the point of release) are low. Where possible, the sensor package and the remains of the balloon may be recovered, but the largest danger may instead be the environmental effects of the fragments of burst balloon, scattered to the very winds they were originally measuring.
Temperostrophic enthalpy (Title text)	No	The largely nonsensical first word perhaps could be interpreted as "time-warping", and allude to the varying passage of time experienced by those who do or do not understand these charts, on having to examine them.

Transcript

This transcript is incomplete. Please help editing it! Thanks.

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".]

[Two solid and dashed lines extend from the top line to the bottom line of the diagram.]

[The remaining various labels are inside the diagram.]

[Labels on the left:]

[An arrow pointing to a densive dashed segment attaching to one solid line:] Latent heat of cooling

[A label lying on one horizontal guide:] — Isobars —

[An arrow pointing to the intersection of one solid and dashed line:] Omnitrophic wind

[A label lying on one left-downward guide:] Isomers

[A label lying on one right-downward guide:] Line of constant thermodynamics

[An arrow pointing to a solid dashed segment on one left-downward guide:] Uncomfortably moist adiabat

[An arrow pointing to the same solid line as ‘latent heat of cooling’:] Humidor

[An arrow pointing to a steep peak on the same solid line:] Oops, the balloon flew through a ghost

[To adjacent arrows pointing to two left-downward guide not perfectly coinciding to each other:] These lines are slightly different because Dave messed them up

[An arrow pointing to the end of solid and dashed lines on the bottom line:] Don’t stand here or you might get hit by a balloon

[Labels on the right:]

[An arrow pointing to the top line:] No birds up here :(

[An arrow pointing to one solid line generally:] Track of rising weather balloon

[An arrow pointing to one dashed line generally:] Track of popped balloon falling back down

[An arrow pointing to crossing of two solid lines, The area enclosed by which is painted black] Meteogenesis

[An arrow pointing to a knot on one solid line:] Seems bad

[An arrow pointing to a dot] Dew point

[An arrow pointing to a highlighted left-downward guide:] Heavyside layer

[Text written sideways below the line:] These lines are tilted because the wind is blowing them

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)