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A 100% chance means it is certain, so it is not really 'chance' in that case. Someone I know said that rain is always uncertain, so why not say a 99% chance then?

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This is an excellent illustration of the difference between "subjective probability" ("credence") and "objective" or "frequentist probability" ("chance"). The Stanford Encyclopedia of Philosophy explains this well.

Specifically: the forecast does not mean that the forecaster believes with 100% certainty that there will be rain. Instead, it means that, when they simulate different weather patterns using some model or other, then subject to that model's constraints they will get "rain" 100% of the time.

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    – NVZ ♦
    Commented Jan 12, 2023 at 17:41
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A 100% chance means it is certain, so it is not really 'chance' in that case.

Well, as a matter of fact, it is. The "chance" should be considered as a unit of likelihood, which ranges from "no chance" (0%) to certainty (100%)

However, the whole system of weather forecasting is beset by uncertainty, and English is understood only in context.

Killing Time's comment is useful: https://sciencenotes.org/percent-chance-rain-mean/ You will see that meteorologists use the term 100% chance, and you can too.

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    Also, why deviate from the structure already in place of ... chance of rain?
    – Yosef Baskin
    Commented Jan 12, 2023 at 0:46
  • Some things are absolutely certain, and chance doesn't play in to it. I am going to die, it is not a matter of chance. Rain is always less than certain. Unless it is already raining, then calling it 100% chance seems as silly as chance of death, when you are at a funeral. Dithering around serves no purpose.
    – RudolphTheRedKnowsRainDear
    Commented Jan 12, 2023 at 2:37
  • This is outdated. Meteorologists now generally don't take area into account: tasteofhome.com/article/percent-chance-of-rain
    – alphabet
    Commented Jan 12, 2023 at 2:43
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Consider "X% chance of rain" as a set phrase used at the particular interface between the technical-language of meteorologists and the casual-language of laypeople.

Sure, the "0% chance" and "100% chance" edge-cases run somewhat afoul of some semantics, but that is just what happens when technical language meets casual language.

A 100% chance means it is certain, so it is not really 'chance' in that case. Someone I know said that rain is always uncertain, so why not say a 99% chance then?

Breaking the mathematical rigor to patch over some semantics is not a good fix. At best you're trading one kind of pedantry for another, and at worst you are outright misrepresenting the technical information.

Really, the better fix would be to say "near 100% chance" or "approaching 0% chance" or some other construction along those lines. But, again, the issue is that "X% chance of rain" is just such a common set phrase that these kinds of edge-case rules are quite likely to be glossed over and forgotten.

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This question and most of the answers and comments on this page confusingly combine two very different questions:

(1) Is rain ever really certain, and if so, in what way?

(2) Assuming that rain is certain, in whatever way is relevant, why don't weather forecasters just say so, rather than that there is 100% chance of rain.

The first question is a matter of science and philosophy. Only the second question is a matter of English language and usage.

Whether one will find it natural or strange to speak of certainty as 100% chance is a manifestation of a more general difference between people whose ways of speaking are shaped by an education or professional experience that revolves around analysing phenomena in quantitative terms, and those with other kinds of backgrounds. The differences that the latter group characterises as qualitative, may be quite spontaneously characterised as quantitative by the former.

Consider, for example, the difference between moving and standing still. To many people that is probably a very clear, definite, qualitative difference. Such people may speak of moving things as having this or that speed, but would never speak of the speed of an object that is standing still. A scientifically trained person, on the other hand, finds it quite natural to say that such an object has the speed that equals zero. In such a person’s conceptual framework, the difference between moving and standing still is merely quantitative; it’s the difference between having the speed of zero and having some other speed.

Similarly, the everyday framework for conceptualising probabilities has the concepts such as impossible, a chance of, likely, certain. The differences among these at first appear to be qualitative, and are spoken of as such. People who are trained to analyse probabilities in quantitative terms, however, think of them as a continuum between zero and one. In that framework, something that is impossible has the probability of zero, something that is certain has the probability of one, and everything that is possible but not certain has some probability that is between these extremes. A person who is accustomed to that framework thinks of 100% chance (i.e. certainty) as something that is only quantitatively different from 99% chance, and speaks accordingly; such a person feels no need to suddenly switch to using an entirely different term when the probability of something jumps from 99% to 100%.

Incidentally, the whole matter is not specific to English, as analogous differences between people of different educational/professional backgrounds can probably be found among speakers of any language.

(This answer is an a adaptation of the answer I posted some time ago to a different but related question.)

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  • I guess it would depend on who the forecast is for. In a mixed audience, it should be for the majority. Generally speaking, most people do not have a quantitative background.
    – RudolphTheRedKnowsRainDear
    Commented Jan 12, 2023 at 16:57
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A coin with two heads has a 100% chance of landing heads, right?

I'd argue it can land standing up or even doesn't land at all.

Think of rain as a cake and chance as the ingredients. You might have 100% chance of cake, unless:

  • The baker falls ill
  • The baker falls
  • A dog tips over the ingredients
  • The oven breaks
  • The over catches fire and you get a lump of coal
  • Etc...

You don't have cake until the cake is present.

Don't forget that rain in your area doesn't translate to rain on your head.

At the end of the day it's CYA nomenclature of that respective field because weather reporters know how pedantic people like to be about the weatherman being wrong.

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  • Yes. I'm just being pedantic about them not being right. Someone I know who used to read the weather forecast out to an Air Force base said that he was told: "Never give a forecast more than 6 hours in advance." We should not express or expect unreasonable certainty. When I see a 10 day forecast I just laugh.
    – RudolphTheRedKnowsRainDear
    Commented Jan 12, 2023 at 17:04
  • @RudolphTheRedKnowsRainDear But they are right, there is a 100% chance of rain until there isn't. Just wait till you learn about puffery. "100% of the 9 out of the 10 dentists who agree with this statement agree that our toothpaste is the best!" =)
    – MonkeyZeus
    Commented Jan 12, 2023 at 17:15
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A 100% chance of rain means that the chance of rain is over 99.5%. That is, typically, the probability is rounded to the nearest integer. It is not possible for the probability of rain for tomorrow to be exactly 100%, since there is a non-zero probability that the Earth could be destroyed today. But, 99% would imply the chance is less than 99.5%, and would be wrong if the chances were 99.9%. One exception is when it is raining, then the probability of rain presently is 100%.

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    Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
    – Community Bot
    Commented Jan 12, 2023 at 17:09
  • You need supporting evidence that the Earth could be destroyed today? Or that weather probabilities are given as integers? Or that to provide integers one needs to round?
    – eshaya
    Commented Feb 25, 2023 at 7:01
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Means in other words:

In historical weather collection it rained on this date every time over years.

1.10.1999 rain ... 1.10.2003 rain ... 1.10.2022 rain 1.10.2023 > there should be rain with a chance of 100%

If in one of these years no rain had occured the chance would be lower than 100%.

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    Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
    – Community Bot
    Commented Jan 12, 2023 at 12:08
  • The issue in the question is with putting "100%" and 'chance' right next to each other, it's not about the weather.
    – RudolphTheRedKnowsRainDear
    Commented Jan 12, 2023 at 12:56
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    This is not how weather prediction works. Historical data from previous years is nowhere near enough to tell what's likely to happen; weather is predicted using the movement of air in the surrounding area.
    – Hearth
    Commented Jan 12, 2023 at 15:07
  • @Hearth But longer forecasts, like the common 10 day ones, do simply 'predict' the average weather for that time period. What else could they do? So extending the forecast too far (more than about 3 days) is just pandering to people who don't know any better. They should cut the forecast off at a certain number of hours in the future and say, "We have no idea."
    – RudolphTheRedKnowsRainDear
    Commented Jan 12, 2023 at 17:19
  • @RudolphTheRedKnowsRainDear It depends on the region. In my area (which has pretty predictable weather), forecasts out to six or seven days are pretty reliable. As I understand it, historical averages aren't used until you go beyond the ten-day forecast, though predictions near the end of the ten-day forecast might not be much more accurate than historical averages.
    – Hearth
    Commented Jan 12, 2023 at 20:47

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