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Revision as of 01:59, 30 October 2013

Welcome to the explain xkcd wiki!
We have an explanation for all 1 xkcd comics, and only 0 (0%) are incomplete. Help us finish them!

Latest comic

Go to this comic explanation

Meteorologist
Hi, I'm your new meteorologist and a former software developer. Hey, when we say 12pm, does that mean the hour from 12pm to 1pm, or the hour centered on 12pm? Or is it a snapshot at 12:00 exactly? Because our 24-hour forecast has midnight at both ends, and I'm worried we have an off-by-one error.
Title text: Hi, I'm your new meteorologist and a former software developer. Hey, when we say 12pm, does that mean the hour from 12pm to 1pm, or the hour centered on 12pm? Or is it a snapshot at 12:00 exactly? Because our 24-hour forecast has midnight at both ends, and I'm worried we have an off-by-one error.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Created by an OVERLY ANALYTICAL METEOROLOGIST - Please change this comment when editing this page. Do NOT delete this tag too soon.

Although we’re constantly exposed to them, many (most?) people don’t understand the details of how to properly interpret weather forecasts. This comic takes this to the ridiculous extreme of the weather reporters themselves not understanding, and asking questions about it while on-air. It shows questions asked by three different people with different backgrounds: mathematics, linguistics, and (in the alt-text) software development.

The meteorologist with a background in math (Cueball) had a background in probability. This is shown when he asks if each hour is independent or correlated. If each hour were independent, there would have been a 67.232% chance to rain at least once. However, if the hours had been correlated, the chance would be less, since if it didn't rain in the first hour, it would decrease the chance of rain in the next hours. However, it would make it more likely of raining in all 5 hours, as it would be a .032% chance if it wasn't correlated. But if it was correlated, rain in the first hour would make it more likely to rain in the subsequent hours.

Questioning this on air, however, is wholly useless, and more than likely confuses the watcher more than helps them. The technical difficulty panel further cements this, stating that it was an error, most likely due to not knowing the background of said person before allowing them to be on television: a poor decision. [citation needed]

A new meteorologist, Blondie, steps in, stating that she has a linguistics degree. A person off-panel confirms with her that she is not a mathematician, wanting to prevent the error made earlier. However, this proves to be in vain, as Blondie goes into a tangent once more but from a linguistics standpoint, rather than a mathematical one, detailing the true meaning of the word "it" as referring to the weather.

In the alt-text, the news station has appeared to make a similar error, but this time with a software developer, stating concerns about the feasibility of the time system used to correlate to the weather patterns. This, however, is of no concern; the information being conveyed is to people, who would probably be able to interpret it easily.

Answering the Comic's Questions

Management would certainly answer the mathematician's questions! The questions themselves have been asked of meteorologists before, and NOAA has published relevant answers for probability of precipitation, as well as timing and the meanings of particular forecast words. The naming is also addressed here.

Regarding probability of precipitation, NOAA forecasts give the probability that it will rain at all at any given point in an area. To rephrase it, it is the probability of rain occurring at all within a forecast area, multiplied by the percentage of area affected by the rain. The "forecast area" is a clearly defined area of land and can be seen in the map of any official National Weather Service forecast. Here is an example.

Regarding the timing of the forecast, an hourly forecast gives the probability for each particular hour, stretching from the time listed to right before the next hour listed. So, the forecast for 12 PM describes the time period from 12 PM to 1 PM. The forecasts for individual hours can be correlated; for this reason, the NOAA generates forecasts that stretch over longer time periods, giving a useful estimate for that time range. Thus, the chance of rain for "Today" specifically means: what is the chance of it raining at any given location during any time between 6 AM and 6 PM?

Regarding phrases like "scattered showers", this specifically means a 25-54% probability of precipitation from convective cloud sources. Other phrases, and when they are used, are detailed in the chart at the end of this PDF.

So, to conclude:

  • "How likely is it to rain this afternoon?" We don't know, you need to show the 12 PM - 6 PM forecast, not the hourly.
  • "Is each hour independant? Correlated?" Hourly values are given for that hour only. They can be correlated, hence why they can't be used to calculate the answer to "How likely is it to rain this afternoon?"
  • "Is rain guaranteed and we're just unsure of the timing?" It's not guaranteed for any individual spot or time. It's still probabilistic.
  • "It says 'scattered showers.' Is this the chance of rain somewhere in your area?" Yes, it is, and it means the the rain will come from convective cloud sources with a probability of precipitation somewhere between 25 and 54%.
  • "How big is your area?" It's detailed in the forecast the mathematician would be reading from.
  • "What if you have two locations you're worried about?" You would check two separate forecasts, one for each area.
  • "Hey, when we say 12pm, does that mean the hour from 12pm to 1pm, or the hour centered on 12pm? Or is it a snapshot at 12:00 exactly?" It means the hour from 12pm to 1pm.

Transcript

[Cueball is presenting a weather forecast while seated with his arms resting on a table. The graphic to the left of Cueball shows five hours from 12pm to 4pm, each with a rainy cloud icon and the figure 20%. The "News 4 Weather" logo is shown on the bottom left.]
Cueball: Our forecast says there's a 20% chance of rain for each of the next five hours.
Cueball: How likely is it to rain this afternoon? It's a simple question, but I don't know the answer. Is each hour independent? Correlated? Or is rain guaranteed and we're just unsure of the timing?
Cueball: It says "scattered showers." Is this the chance of rain somewhere in your area? How big is your area? What if you have two locations you're worried about?
Cueball: I've asked management, but they've stopped answering my emails, so—hang on, the security guy is coming over.
[A black screen is shown with white text:]
Technical Difficulties
We apologize for hiring a meteorologist with a pure math background.
We'll be back on the air shortly.
News 4
[Cueball is replaced with Blondie.]
Blondie: Sorry about that. Hi, I'm your new meteorologist.
Off-panel voice: And you're not a mathematician, right?
Blondie: No. I do have a linguistics degree.
Off-panel voice: That's fine.
Blondie: It might rain this afternoon.
Blondie: But what is "it" here? Is it a true dummy pronoun, as in the phrase "It's too bad?" Or is the weather an entity?
Blondie: Also, what if I say, "It's hot out, and getting bigger?"
Off-panel voice: Security!


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You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • List of all comics contains a table of most recent xkcd comics and links to the rest, and the corresponding explanations. There are incomplete explanations listed here. Feel free to help out by expanding them!
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