explain xkcd - User contributions [en]

3032: Skew-T Log-P

2025-01-02T21:21:05Z

Lorehouse: /* Table with terms */

{{comic
| number = 3032
| date = January 1, 2025
| title = Skew-T Log-P
| image = skew_t_log_p_2x.png
| imagesize = 569x626px
| noexpand = true
| titletext = 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==
{{incomplete|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.}}
A {{w|skew-T log-P diagram}} (the name comes from the temperature (T) lines being skewed at a 45-degree angle, and the pressure (P) lines being logarithmic in scale) are mostly used to plot {{w|atmospheric sounding}}s, which are usually made by sending a weather balloon up into the air.

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 (which may appear to have measurements from two separate weather balloons, one with two solid black lines for its measurements and the other with two dashed black lines, whereas the two lines and various styles of line generally mean different measurements from the same balloon-track or other means of obtaining atmospheric readings), but most of the explanations are blatantly incorrect or humorous in nature. See details in the [[#Table with terms|table]] below.

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 complicated product with a simple sentence about how nice the weather is.

[[File:Annotated_Skew_T_diagram.png|thumb|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 {{w|dew point}}. The dew point, a function of the air's {{w|absolute humidity|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 {{w|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==
{| class="wikitable"
!Item in comic
!Correct?
!Explanation
|-
| Pressure latitude || No || {{w|Pressure altitude|Pressure ''altitude''}} is the height above a standard datum plane, a theoretical level where the weight of the atmosphere is 1013.24 millibars (29.921 inHg). It's essentially a way of estimating altitude from atmospheric pressure.
|-
| Enthalpic pressure || ||
|-
| Entropic density || ||
|-
| Latent heat of cooling || ||
|-
| Isobars || Yes || Lines denoting equal ("{{wiktionary|iso-}}") air pressure ("{{wiktionary|bar-|-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 || ||
|-
| 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 iso'''therm''' 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 || ||
|-
| 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.{{cn}} 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 shows such a local low temperature at this point, probably for some typical meteorological reason that somebody should edit into this sentence here.
|-
| 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 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 || ||
|-
| Track of popped balloon falling back down || ||
|-
| Meteogenesis || No || The chart shows 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.
|-
| Seems bad || || The path of the balloon loops around in the shape of a {{w|figure eight knot}}, which would indicate very chaotic conditions at that point.
|-
| 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'' singular point, when it should be a line (typically the leftmost solid plotted line) representative of the temperature at which dew should form at any given pressure.
|-
| Humidor || No || In reality, is a {{w|Humidor|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 nature of the ''actual'' humidity encountered.
|-
| Heavyside layer || No || Probably a misspelling of "Heaviside," the surname of the co-discoverer of what we now call the E region of the ionosphere. Co-discovered by {{w|Arthur E. Kennelly}} and {{w|Oliver Heaviside}}.
This reference also occurs in the musical {{w|Cats (musical)}} and {{w|T. S. Eliot}}'s {{w|Old Possum's Book of Practical Cats}}, where the Heaviside layer is the fictional "heaven" to which the cats aspire.

In this diagram it is apparently labeling a heavily marked isotherm, or line of constant temperature. Most likely it indicates the 0°C line, this being importantly indicative of the freezing point of water.
|-
| 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 || ||
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

: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 [[:Category:New Year|New Year comic]].
**Only second time this has happened since New Year comics became a regular thing from 2011.

{{comic discussion}}

[[Category:Charts]]
[[Category:Weather]]

3032: Skew-T Log-P

2025-01-02T21:20:20Z

Lorehouse: /* Table with terms */

{{comic
| number = 3032
| date = January 1, 2025
| title = Skew-T Log-P
| image = skew_t_log_p_2x.png
| imagesize = 569x626px
| noexpand = true
| titletext = 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==
{{incomplete|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.}}
A {{w|skew-T log-P diagram}} (the name comes from the temperature (T) lines being skewed at a 45-degree angle, and the pressure (P) lines being logarithmic in scale) are mostly used to plot {{w|atmospheric sounding}}s, which are usually made by sending a weather balloon up into the air.

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 (which may appear to have measurements from two separate weather balloons, one with two solid black lines for its measurements and the other with two dashed black lines, whereas the two lines and various styles of line generally mean different measurements from the same balloon-track or other means of obtaining atmospheric readings), but most of the explanations are blatantly incorrect or humorous in nature. See details in the [[#Table with terms|table]] below.

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 complicated product with a simple sentence about how nice the weather is.

[[File:Annotated_Skew_T_diagram.png|thumb|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 {{w|dew point}}. The dew point, a function of the air's {{w|absolute humidity|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 {{w|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==
{| class="wikitable"
!Item in comic
!Correct?
!Explanation
|-
| Pressure latitude || No || {{w|Pressure altitude|Pressure ''altitude''}} is the height above a standard datum plane, a theoretical level where the weight of the atmosphere is 1013.24 millibars (29.921 inHg). It's essentially a way of estimating altitude from atmospheric pressure.
|-
| Enthalpic pressure || ||
|-
| Entropic density || ||
|-
| Latent heat of cooling || ||
|-
| Isobars || Yes || Lines denoting equal ("{{wiktionary|iso-}}") air pressure ("{{wiktionary|bar-|-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 || ||
|-
| 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 iso'''therm''' 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 || ||
|-
| 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.{{cn}} 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 shows such a local low temperature at this point, probably for some typical meteorological reason that somebody should edit into this sentence here.
|-
| 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 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 || ||
|-
| Track of popped balloon falling back down || ||
|-
| Meteogenesis || No || The chart shows 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.
|-
| Seems bad || || The path of the balloon loops around in the shape of a {{w|figure eight knot}}, which would indicate very chaotic conditions at that point.
|-
| 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'' singular point, when it should be a line (typically the leftmost solid plotted line) representative of the temperature at which dew should form at any given pressure.
|-
| Humidor || No || In reality, is a {{w|Humidor|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 nature of the ''actual'' humidity encountered.
|-
| Heavyside layer || No || Probably a misspelling of "Heaviside," the surname of the co-discoverer of what we now call the E region of the ionosphere. Co-discovered by {{w|Arthur E. Kennelly}} and {{w|Oliver Heaviside}}.
This reference also occurs in the musical {{w|Cats (musical)}} and {{w|T. S. Eliot}}'s {{w|Old Possum's Book of Practical Cats}}, where the Heaviside layer is the fictional "heaven" to which the cats aspire.
In this diagram it is apparently labeling a heavily marked isotherm, or line of constant temperature. Most likely it indicates the 0°C line, this being importantly indicative of the freezing point of water.
|-
| 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 || ||
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

: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 [[:Category:New Year|New Year comic]].
**Only second time this has happened since New Year comics became a regular thing from 2011.

{{comic discussion}}

[[Category:Charts]]
[[Category:Weather]]