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Revision as of 16:59, 20 November 2012

Welcome to the explain xkcd wiki! We already have 4 comic explanations!

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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.


Ambox notice.png This explanation may be incomplete or incorrect: Created by an OVERLY ANALYTICAL METEOROLOGIST. More on Cueballs 2nd panel. More on Blondie's linguistics. Explanation for dummy pronoun. Title text: What is "an off-by-one error"?. Wiki links. 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 title text) software development.

The first meteorologist, Cueball, has a background in pure math. His forecast states that each of the next five hours has a 20% chance of rain. It could be that rain was certain, but it would only last about an hour and will come within the next five hours. That would give 20% for rain in each hour but certainty of rain within those hours. This corresponds to his last question in the first panel.

But before that he focuses on probability 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.

In the second panel he continues to discuss what scattered showers means. [Need expansion here]

Finally in that panel Cueball begins to explain that he has asked the management about these things, but that they have stopped replying to his e-mails. At this point he spots the security guy coming over, and the screen goes black in to a technical difficulty screen that excuses this behavior to the viewers.

Questioning these things on air is likely confusing to the watchers, although they are all valid questions. But this may lose viewers and the news network is afraid of this. The technical difficulty panel further cements this, apologizing for hiring a person with a pure math background. Often seen as one that do not understand how to talk to regular people.

When they get back on air gain a new meteorologist, Blondie, steps in. The management enquirers (on air) to make sure she is not also a mathematician. She states no, but tells that she has a linguistics degree, which the management thinks is fine, and thus believes they have prevented the problem with Cueball. 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. After one panel of this the management calls for security again.

In the title text, the news station has made the same error again, by this time hiring a software developer as the third meteorologist. This last person is 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. But again the question is valid... On such an hour forecast does 12pm refer to the hour from 12 to 1pm, from 11:30 to 12:30 or is it actually only to the weather precisely at 12:00 that is referred to?

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.


[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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