When weather announcers say “there is a 30 percent chance of rain,” most of us would think that this means there is a 30 percent chance of rain.

But in North America the meaning is drastically different, and people on the internet are only just discovering it, 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 probability of rain and its extent.

The result of this calculation, known as the “probability of precipitation” or “PoP”, is given by weather presenters as a percentage, but the meaning is often misinterpreted.

a woman in a tiktok video has shared his disbelief to discover that another definition exists, but confusingly, not all the world’s meteorologists use it.

When weather announcers say “there’s a 30 percent chance of rain,” many of us think that means there’s a 30 percent chance of rain, but that’s not always the case (file photo)

The TikTok video has received more than two million likes since it was posted in 2021 and is still circulating on the popular app.

The woman in the video says: ‘I thought when they said there was a 30 per cent chance of rain they meant there was a 30 per cent chance it was going to rain.

“I never knew what it meant that there is a 100 percent [chance] It’s going to rain and it will be in 30 percent of your area.’

Unfortunately, the woman’s explanation of what “30 percent chance of rain” really means isn’t entirely correct, though it’s not entirely wrong either.

MailOnline takes a closer look.

WHAT DOES A 30 PERCENT CHANCE OF RAIN MEAN?

When North American meteorologists give a “chance of rain” as a percentage, they are actually giving what is known as a “probability of precipitation” (or PoP).

The PoP is calculated by multiplying two figures, expressed as ‘C’ (meaning how ‘confident’ 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 to decimal points (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 putting together a PoP to be able to provide a TV weather update on rainfall in a certain region.

Confusion over the definition is swirling the internet once again after the popular Twitter account @wildtiktoks posted a screenshot that read: “Am I the only one who thought a 30% chance of rain meant a 30% chance of rain?” chance of rain?

They could be 100 percent sure (C) that it will rain in 30 percent of a given area (A).

To offer this to viewers as a single, digestible number, they would calculate a PoP by converting their percentages to decimal points (ie 1.0 for C and 0.3 for A) before multiplying them.

Doing so would give them a PoP of 0.3, or 30 percent, and thus they would proclaim on TV that “there’s a 30 percent chance of rain.”

The thing is, a 30 percent PoP could also have different confidence (C) and area (A) values.

For example, a meteorologist might be 50 percent confident that it will rain in 60 percent of an area, which would be 0.5 x 0.6, which would also give a PoP of 0.3 or 30 percent. .

In other words, a meteorologist might say there is a 30 percent chance of rain, but that doesn’t necessarily tell us how sure they are that it will rain (C), or how much of a given area it will rain (A); just give a general figure for A and C together.

The goal of the North American definition is to give a single figure that takes into account two factors: the probability of rain and the extent of the rain.

But sadly it tends to get completely misconstrued, hence the confusion of the woman in the TikTok video.

In the United States, the probability of rain actually refers to the results of a calculation that takes into account both the probability of rain and the probable extent of the rain. The result of this calculation, known as PoP, is given in percentage and is the one used by North American meteorologists to give the ‘probability of rain’

In part because of this confusion, many meteorologists will avoid saying that there is a certain percentage chance of rain altogether.

Kelsey McEwen, a meteorologist for CTV Your Morning in Toronto, says she doesn’t use the “30 percent chance of rain” phase (or whatever the percentage is) because the public often misinterprets the meaning.

UK meteorologists also tend to avoid using the PoP system, instead opting for a more literal meaning.

According to the UK Met Office, their experts will use “30 per cent chance of rain” to refer to what any sensible person might expect: the chance of rain.

So, for example, if you look at the forecast for Greater London today, the chance of rain is listed as 10 percent at 2pm, which means there’s a 10 percent chance that Greater London could see some rain at 2 pm

In response to videos circulating on TikTok about the PoP system, one of the Met Office’s own experts took to the app to explain how their own forecasts work.

The Met Office expert says: ‘Rain percentage means the probability of rain at that time for that location.

‘So 60 percent means a 60 percent chance of rain and a 40 percent chance of drought.

“There are different ways of expressing percentages, and in the US the area is used, but not generally here in the UK.”