explain xkcd - User contributions [en]

3024: METAR

2024-12-16T00:57:30Z

ThatGuyDavid09: /* Trivia */ Added translation of the dollar sign in the metar, referring to required maintenance

{{comic
| number = 3024
| date = December 13, 2024
| title = METAR
| image = metar_2x.png
| imagesize = 640x360px
| noexpand = true
| titletext = In the aviation world, they don't use AM/PM times. Instead, all times are assumed to be AM unless they're labeled NOTAM.
}}

==Explanation==
{{incomplete|Created by a BOT WITH NO SIGNIFICANT OTHER :( (OTHER THAN AN A380). Do NOT delete this tag too soon.}}

In aviation, the {{w|METAR}} (Meteorological Aerodrome Report) is used to give pilots a brief overview of the current meteorological conditions at the airport. The METAR follows a specific structure and makes heavy usage of abbreviations, which makes it hard to read for anyone not familiar with it. The comic makes fun of that by assuming meaning of the words based on what non-aviation people might think the different elements of the METAR report may represent. The METAR in the comic is fairly alarming, describing dangerously fast winds, a possible tornado, freezing volcanic ash (in New York!), lightning, and impossibly high atmospheric pressure.

{| class="wikitable"
|-
! Code !! Real Meaning !! According to the comic
|-
| METAR
| Type: Meteorological Aerodrome Report
| The comic assumes that this is just a spelling error and it should be "meter"
|-
| KNYC
| Station ID: 4 character identifier; for an airport, this would be the {{w|ICAO airport code|ICAO code}}. In this instance the identifier represents the automated weather station at Belvedere Castle in Central Park, NYC. Airport, weather, and radio station call signs share a common heritage.
| "Station ID", which is actually correct, although people unfamiliar with METAR-reporting stations could presume that this is an AM radio broadcaster's name.
Among AM radio stations, KNYC is not a current call-sign (though {{w|WNYC}} is, and indeed serves New York City), but (among the 'western' subset of US stations) currently {{w|KNCY (AM)|KNCY}} serves the area around Omaha, Nebraska (being based in Nebraska City), and {{w|KYCN}} covers Wheatland, Wyoming.
|-
| 251600Z
| Time of observation: 25th day of the month at 4 PM UTC. Z is not part of the time, but simply global shorthand for {{w|Military time zone|"Zulu" time}}, i.e. {{w|Coordinated Universal Time|UTC}}. Normally most stations would report at a particular time every hour, in this particular case either 15:51 or 16:51 would apply, but more frequent reports are made during unusual and rapidly changing weather events (as may be the situation, in this case).
| Misreading the "Z" as a 2, resulting in a nonsensical time. Hours greater than 24 are sometimes used to indicate a time after midnight, e.g., in Japan 17~25h means from 5 P.M. to 1 A.M. the following day. A normal METAR does not use more than 24 hours, instead incrementing the day, so 25 hours further adds to the nonsensical nature of the interpretation.
|-
| 18035G45KT
| Wind direction and speed: Wind direction 180° (directly from the south, degrees based on 0°=magnetic north), speed 35 knots, gusting to 45 knots. This is quite stormy weather.
| Instead of interpreting the first 5 digits as direction and speed, it is assumed that it is one big number and the G45 stands for the time span in which this was observed with "G" standing for "good". 18,035 knots is an unrealistically high wind speed, faster than orbital velocity; the {{w|jet stream}} typically contains the highest winds on Earth, and may reach about 250 knots.
|-
| 6SM
| In weather reports related to aviation, "6SM" stands for 6 statute miles of visibility, meaning that objects can be seen clearly up to 6 miles away.<ref>https://weather.cod.edu/notes/metar.html#:~:text=6SM%2DVisibility,SM)%20up%20to%2010%20SM.</ref> This indicates clear enough weather to fly without instruments; the value has a max range of 10SM.
| The comic interprets "6SM" to humorously mean a "Size '''6 Sm'''all".
|-
| VCFCFZVA
| In the vicinity (VC): funnel cloud (FC) and freezing (FZ) volcanic ash (VA). This sounds somewhat unusual for New York City.{{Citation needed}}
| A riff on the repeated letters which give off the impression the string is constructed by randomly keypresses on the keyboard, as exemplified by a [[1689: My Friend Catherine|cat on the keyboard]]. There is a long history of this problem, as well as [http://bitboost.com/pawsense/ attempted solutions].
This string may or may not actually look like the pattern of characters that a walking cat could produce (and be reliably detected). All the letters are in a cluster at the lower left of the (QWERTY) keyboard, with some adjacently paired characters perhaps indicative of stepping on multiple keys and other neighbouring keys having been stepped over, not uncommon of an oblivious feline wandering across your desk. But the repeated cluster of "CFCF", and other implied paw-press events, seem less likely to emerge even from a rapid quadrupedal gait. A more casual stroll would likely also create single-character duplications, unless the keyboard repeat delay was set unnaturally high.
|-
| +BLUP
| Heavy (+) blowing (BL) unknown precipitation (UP)
| Riffing on the fact that it looks like an onomatopoetic word
|-
| NOSIG
| No significant change is expected to the reported conditions within the next 2 hours
| The comic assumes that the transmitter of the METAR report wants the receivers to know that they do not have a significant other, which the comic finds sad. The observer could be trying to abuse the METAR report as a dating platform.
|-
| LTG OHD
| Lightning overhead
| OHD is interpreted as "overheard" instead of "overhead", indicating that they did not observe it themselves and instead just overheard people talking about it.
|-
| A3808
| Altimeter setting: (calculated) air pressure at {{w|Mean_sea_level|mean sea level}} at the airport is 38.08. The value of 38.08 inHg is extremely high. The standard atmospheric pressure used in aviation is 29.92 inHg; the highest recorded surface pressure on Earth was 32.01 {{w|Inch_of_mercury|inches of mercury (inHg)}}<ref>https://learn.weatherstem.com/modules/learn/lessons/125/18.html</ref> ("A" for inHg, used primarily in USA, Canada and Japan; "Q" would indicate a value in hPa). This is used to adjust the altimeter in the aircraft to the local air pressure, instead of using the standard setting used in higher air spaces.
| The comic says that the observer saw an {{w|Airbus_A380|Airbus A380-800}}, a very large passenger plane. Note: The {{w|List_of_ICAO_aircraft_type_designators|ICAO aircraft type code}} for the Airbus A380-800 is A388 and not A3808.
|-
| RMK
| Beginning of the section with remarks
| Remarkable. A comment about the A380.
|-
| AO2
| The weather station is automated (A) and has a precipitation discriminator (O2), which can tell the difference between liquid and frozen precipitation.
| A reference to the {{w|fan fiction|fanfic}} site [https://archiveofourown.org/ Archive of Our Own], often abbreviated as AO3 ('''A'''rchive '''o'''f '''O'''ur '''O'''wn, or AOOO). It's nonsensical to describe this site as having a precipitation discriminator.{{Citation needed}}
|-
| SLP130=
| {{w|Sea_level_pressure|Sea-level pressure}} is 1013.0 hPa (approx. 29.91 inHg). The equal sign signifies the end of the METAR.
| "SLP" interpreted as abbreviation for sleepy, the numbers as a time, and the = sign as "around" (maybe confused with ≈)
|-
|
|
|-
| NOTAM (title text)
| Not part of a METAR report, but instead another aviation abbreviation. It stands for {{w|NOTAM|Notice to Air Missions}} (previously Notice to Airmen).
| Parsed as "not A.M.", indicating that a given time is to be interpreted as P.M. While AM and PM are indeed not used in aviation, as the comic says, they use a 24-hour clock system, not an "A.M.-by-default" 12-hour clock system.
|}

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

:Decoding a METAR report:

:[A METAR report is shown with annotations. The report is:]

:<code>METAR KNYC 251600Z 18035G45KT 6SM VCFCFZVA +BLUP NOSIG LTG OHD A3808 RMK A02 SPL130=</code>

:[The annotations are:]

:* <code>METAR</code> "METER" (Usually misspelled)
:* <code>KNYC</code> Station ID
:* <code>251600Z</code> Time (25:16:002)
:* <code>18035G45KT</code> Wind speed has been 18,035 knots for a good 45 minutes now
:* <code>6SM</code> Observer is a size 6 small
:* <code>VCFCFZVA</code> Sorry, the station cat walked on the keyboard
:* <code>+BLUP</code> Weird noise the sky made earlier
:* <code>NOSIG</code> Observer has no significant other :(
:* <code>LTG OHD</code> We overheard someone saying there was lightning
:* <code>A3808</code> Hey look, an Airbus A380-800!
:* <code>RMK</code> Remarkable!
:* <code>A02</code> Fanfic Archive equipped with a precipitation sensor
:* <code>SLP130=</code> Observer got sleepy around 1:30

==Trivia==
The actual last three KNYC METAR strings, that were generated at about the actual time of publication, were:
KNYC 131651Z AUTO VRB03KT 10SM CLR M01/M14 A3066 RMK AO2 SLP374 T10061144 $
*''New York, Central Park : 13/Dec/2024, 16:51 UTC (11:51am local time) : Fully Automated Report : Variable wind, no more than 3 knots : 10 (or more) statute miles visibility : No clouds below 12,000 ft (3,700 m) : −1 °C (about 30 °F), dew point at −14 °C (about 7 °F) : Altimeter at 30.66 inHg (1038.3 hPa) : Precipitation discriminator present : Sea-level pressure at 1,037.4 hPa (30.63 inHg) : Temperature -0.6 °C (conversion from exactly 31 °F) , dew point -14.4 °C (from exactly 6 °F) : Maintenance check required (indicated by dollar sign)''
KNYC 131751Z AUTO 10SM CLR 00/M16 A3066 RMK AO2 SLP374 T00001156 10000 21028 56006 $
*''New York, Central Park : 13/Dec/2024, 17:51 UTC (12:51pm local time) : Fully Automated Report : (no wind measured) : 10 (or more) statute miles visibility : No clouds below 12,000 ft (3,700 m) : 0 °C (about 32 °F), dew point at −16 °C (about 3 °F) : Altimeter at 30.66 inHg (1038.3 hPa) : Precipitation discriminator present : Sea-level pressure at 1,037.4 hPa (30.63inHg) : Temperature ±0.00 °C (from exactly 32 °F), dew point -11.56 °C [sic, -11.67 °C would be from exactly 11 °F] : 6 hour maximum 0.00 °C (32 °F) : 6 hour minimum -10.28 °C (from 13.5 °F) : 3 hour pressure tendency, falling by 0.6 millibars (0.018 inHg) : Maintenance check required''
KNYC 131851Z AUTO 10SM CLR 00/M16 A3066 RMK AO2 SLP377 T00001156 $
*''New York, Central Park : 13/Dec/2024, 18:51 UTC (1:51pm local time) : Fully Automated Report : (no wind measured) : 10 (or more) statute miles visibility : No clouds below 12,000 ft (3,700 m) : 0 °C (about 32 °F), dew point at −14 °C (about 7 °F) : Altimeter at 30.66 inHg (1038.3 hPa) : Precipitation discriminator present : Sea-level pressure at 1,037.7 hPa (30.64 inHg) : Temperature -0.6 °C (from exactly 31 °F), dew point -11.56 °C [sic, again probably from +11 °F] : Maintenance check required ''







==References==
{{reflist}}
[https://e6bx.com/metar-decoder/ ''Actual'' Metar Decoder]

{{comic discussion}}

[[Category:Weather]]
[[Category:Aviation]]
[[Category:Emoji]]

684: We Get It

2023-02-13T16:02:31Z

ThatGuyDavid09: /* Explanation */ Removed random reference to Avatar

{{comic
| number = 684
| date = January 4, 2010
| title = We Get It
| image = we_get_it.png
| titletext = The most brutal way I've ever seen someone handle this was 'Oh, you have a girlfriend. Are you going to get married?' 'I, uh, don't know--' 'Well, do you love her?' '...' 'Anyway, what were you saying about the movie?'
}}

==Explanation==
The comic illustrates how someone in a new relationship tends to be overly eager and giddy to let everyone know about it, while others tend to not be all that interested. The character on the left is so excited to let everyone know, that he goes off panel to get a ladder and a loudspeaker. The second character thinks it should be legal to murder (or at least wound) him for this annoyance if he acts like this for more than a week. The first character is so giddy that he doesn't notice what he has done and even tries to mention his girlfriend again.

The title text describes a very harsh way to respond to this annoyance. Since someone in the position of the first character is so excited about the new romantic commitment in their life, their friend brings up the much greater and more serious commitment of marriage. If they have only been dating for a week or less, it is probably much too early for them to seriously think about marriage. The friend then implies that if they aren't already excited to marry her, then he doesn't really love her. The end result is that the first character doesn't want to talk about his relationship anymore.

==Transcript==
:[Cueball and a friend are talking.]
:Cueball: Avatar? Yeah, I saw it last week with...

:[Cueball walks out of the panel.]

:[Cueball returns with a ladder and megaphone.]

:[Cueball stands on top of the ladder, shouting through a megaphone.]
:Cueball: '''''...MY GIRLFRIEND.'''''

:Friend: You know, if this phase of your relationship lasts more than a week, I'm legally allowed to stab you both.
:Cueball: What phase?
:Cueball: So, did I mention I'm seeing someone?

{{comic discussion}}
[[Category:Comics featuring Cueball]]
[[Category:Multiple Cueballs]]

1381: Margin

2022-10-25T18:08:33Z

ThatGuyDavid09: /* Explanation */ Modified link for non-constructive proof to link to correct wikipedia page

{{comic
| number = 1381
| date = June 13, 2014
| title = Margin
| image = margin.png
| titletext = PROTIP: You can get around the Shannon-Hartley limit by setting your font size to 0.
}}

==Explanation==

This is a reference to {{w|Fermat's Last Theorem}}, of which {{w|Pierre de Fermat}} claimed he had a proof that was too large to fit in the margin of a copy of ''{{w|Arithmetica}}''. Despite its simple formulation, the problem remained unsolved for three centuries; it was cracked only with advanced techniques developed in the 20th century, leading many to believe that Fermat didn't actually possess {{w|Fermat's Last Theorem#Fermat's conjecture|a (correct) proof}} (see [[#trivia|trivia]]).

In the comic, the person writing in the margin attempts to pull a similar trick, without actually having any proof, by claiming that he has found a proof that information is infinitely compressible, but pretending not to be able to show it due to lack of space in the margin. In this particular case, however, this approach backfires, precisely because if information was actually infinitely compressible, the writer ''would'' be able to fit the proof in the margin (due to his own proof). The writer realizes that if he had a proof he should be able to fit it into the margin, and thus he realizes that he cannot pull this trick. Or perhaps the writer really thought he had a proof, but then realized that his statement was a counterexample, and was disappointed that his idea for a proof was wrong.

What it seems he did not realize, is that it would be impossible to read the proof if the writer actually was able to compress his proof to fit in the margin. This is because you would need to know the algorithm described in the proof before you could decompress the proof text so you can read it. So he could actually have used this trick instead, writing that he had compressed it into - say a dot "'''.'''" - and then people would have to find his proof to read it. And since they cannot find such a proof - they could not check his dot. Unfortunately this would also have backfired - because there is already a {{w|Pigeonhole principle#Uses and applications|proof that this is not possible}}!

Another thing that he probably didn't realize, is that finding a proof for something being possible does not necessarily mean inventing an actual algorithm to do that particular thing. If the person claimed having found a {{w|Existence theorem|non-constructive proof}} for such an algorithm, his statement at least wouldn't contradict itself.

The title text, yet another [[:Category:Protip|protip]], makes a reference to the {{w|Shannon–Hartley theorem}}, which limits the maximum rate at which information can be transmitted. Setting the font size of text only changes its ''representation'' on the screen, and not the actual characters themselves. Trying to decrease the amount of space needed to store or transmit it like advised would be nonsensical. Another possible interpretation is that if you set the font size to 0, the text cannot be seen, and therefore, nothing is being transmitted period.

In the case of actual printed paper, decreasing the font size is valid technique for information compression (more information on the same page), as used in ie. {{w|microform}}. However, this comes at the cost of an increased spatial bandwidth (number of black/white transitions per distance). In the end, the resolution of the printer/paper/microscope chain limits the minimal font size that remains useable (above the {{w|Nyquist rate}}).

==Transcript==
:[Written on the right margin of a page:]
:I have
:discovered
:a truly
:marvelous
:proof that
:information
:is infinitely
:compressible,
:but this
:margin is too
:small to...

:...oh

:never mind :(

==Background to Fermat's Last Theorem==

*Fermat's Last Theorem states that no three positive integers ''a'', ''b'', and ''c'' can satisfy the equation ''a''''n'' + ''b''''n'' = ''c''''n'' for any integer value of ''n'' greater than two.
**In the case with n=2, a b and c are the sides of a {{w|Pythagorean theorem|right triangle}}. There are an infinite number of integer solutions for a, b and c, such as ''3''''2'' + ''4''''2'' = ''5''''2''. This was known to Euclid, but was used by land surveyors in Egypt and Mesopotamia over 1000 years before Euclid's time.
*Fermat's Last Theorem was {{w|Wiles' proof of Fermat's Last Theorem|solved}} in 1995 by {{w|Andrew Wiles}} with some assistance by {{w|Richard Taylor (mathematician)|Richard Taylor}} who helped him close a gap in his original proof from 1993.
**The proof involved some of the most complicated mathematics used today, and it has been speculated that only a handful of people in the world would be able to understand it.
**For people interested in the subject, {{w|Simon Singh}} has written a [http://simonsingh.net/books/fermats-last-theorem/the-book/ popular science book] about it, called ''{{w|Fermat's Last Theorem (book)|Fermat's Last Theorem}}''.
***[https://www.youtube.com/watch?v=qiNcEguuFSA Fermat's Last Theorem - Numberphile]
***[https://www.youtube.com/watch?v=FXbsIbRVios Fermat's Last Theorem (extra footage) - Numberphile]
*There are US Patents in this very area, analyzed by [http://gailly.net/05533051.html Jean-loup Gailly].

{{comic discussion}}
[[Category:Protip]]
[[Category:Math]]
[[Category:Comics with lowercase text]]