When weather presenters say there’s a 30 percent chance of rain, most of us would think that means there’s a 30 percent chance it’s going to rain.
But in North America, the meaning is drastically different – and people on the internet are only just finding out, leaving them completely baffled.
In the US and Canada, “chance of rain” refers to the results of a calculation that takes into account both the likelihood that it will rain and how widespread the rain is likely to be.
The result of this calculation, known as ‘probability of precipitation’ or ‘PoP’, is given by weather presenters as a percentage – but the meaning is routinely misinterpreted.
A woman in one TikTok video shared her disbelief when she discovered that another definition exists – but confusingly, it is not used by all meteorologists worldwide.
When weather presenters say “there’s a 30 percent chance of rain,” many of us think that means it’s 30 percent likely to rain — but this isn’t always the case (file photo)
The TikTok video has received over two million likes since its posting in 2021 and is still circulating on the popular app.
The woman in the video says, “I thought when they said there’s a 30 percent chance of rain, that meant there was a 30 percent chance it’s going to rain.
“I never knew that meant there’s a 100 percent[chance]it’s going to rain and it’s going to happen in 30 percent of your area.”
Unfortunately, the woman’s explanation of what “30 percent chance of rain” actually means isn’t quite right—although it isn’t completely wrong either.
MailOnline goes into more detail.
WHAT DOES A 30 PERCENT CHANCE OF RAIN MEAN?
When meteorologists in North America report the “chance of rain” as a percentage, they are actually reporting the so-called “chance of precipitation” (or PoP).
PoP is calculated by multiplying two numbers – expressed as ‘C’ (representing how ‘certain’ a meteorologist is that it will rain) and ‘A’ (the ‘amount’ of an area that will receive this rain) .
Both A and C are expressed as percentages, but in decimals (for example, 0.2 for 20 percent, 0.5 for 50 percent, and 1 for 100 percent).
So let’s imagine that a meteorologist is working out a PoP so that he or she can provide a televised weather update about rainfall in a particular region.
Confusion about the definition is once again circulating the internet after the popular Twitter account @wildtiktoks posted a screenshot that read: ‘Am I the only one who thought 30% chance of rain meant it was 30% chance it’s going to rain’
They can be 100 percent sure (C) that 30 percent of a given area (A) will receive rain.
To present this to the viewers as a single comprehensible figure, they would work out a PoP by converting their percentages to decimals (i.e. 1.0 for C and 0.3 for A) before multiplying them together.
If they did, they would get a PoP of 0.3 or 30 percent, which is why they would proclaim on TV that “the chance of rain is 30 percent.”
The point is that a 30 percent PoP can also have different values for confidence (C) and area (A).
For example, a meteorologist could be 50 percent sure that 60 percent of an area will receive rain, which would be 0.5 x 0.6 — which also gives a PoP of 0.3 or 30 percent.
In other words, a meteorologist might say there’s a 30 percent chance of rain, but that doesn’t necessarily tell us how certain he is it will rain (C), or how much of a given area will rain (A). just give a total grade for both A and C together.
The point of the North American definition is to provide a single number that takes into account two factors: the probability of rain and how widespread the rain will be.
But unfortunately it is often completely misunderstood, hence the confusion of the woman in the TikTok video.
In the US, probability of rain actually refers to the results of a calculation that takes into account both the likelihood of rain and how widespread the rain is likely to be. The result of this calculation, known as PoP, is given as a percentage and is what North American meteorologists use to represent the ‘chance of rain’.
Partly because of this confusion, many meteorologists will completely avoid saying that there is a certain percentage chance of rain.
Kelsey McEwen, a meteorologist for Toronto’s CTV Your Morning, says she doesn’t use the “30 percent chance of rain” stage (or whatever the percentage might be) because the meaning is often misinterpreted by the public.
Meteorologists in Britain tend not to use the PoP system either, opting instead for the more literal meaning.
According to the British Met Office, by ’30 per cent chance of rain’ the experts will mean what any sensible person would expect: the chance of rain.
For example, if you look at the forecast for Greater London today, the chance of rain at 2pm is shown as 10 percent, meaning there’s a 10 percent chance that Greater London could see some rain at 2pm.
In response to videos circulating on TikTok about the PoP system, one of the Met Office’s in-house experts took to the app to explain how the proprietary predictions work.
The Metoffice expert says: ‘The percentage of rain means the probability of rain at that moment for that location.
‘So 60 percent means 60 percent chance of rain, 40 percent chance of drought.
‘There are different ways of expressing percentages, and in the US they use area one, but here in Britain they usually don’t.’