# People are amazed after finding out what a “30 percent chance of rain” really means

When weather presenters say, “There’s a 30 percent chance it will rain,” most of us would think there’s a 30 percent chance that it will rain.

But in North America, the meaning is completely different – and people on the internet are just now figuring that out and are stumped.

In the United States and Canada, “rain probability” refers to the results of a calculation that takes into account both the probability of rain and the projected amount of precipitation.

The result of this calculation, known as the ‘precipitation probability’ or ‘PoP’, is given as a percentage by weather moderators – but the meaning is regularly misinterpreted.

A woman has expressed disbelief that there’s another definition in a TikTok video – but, confusingly, not all meteorologists around the world use it.

When weather presenters 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 its release in 2021 and is still making the rounds on the popular app.

The woman in the video says, “I thought when they said there was a 30 percent chance of rain, they meant there was a 30 percent chance of rain.”

“I never knew it meant there was 100 percent.” [chance] It’s going to rain in 30 percent of your area.’

Unfortunately, the woman’s explanation of what “30 percent chance of rain” actually means is not entirely correct – but not entirely wrong either.

MailOnline takes a closer look.

WHAT DOES A 30 PERCENT CHANCE OF RAIN MEAN?

When meteorologists in North America report the “probability of rain” as a percentage, they are actually reporting what is known as the “probability of precipitation” (PoP).

PoP is calculated by multiplying two numbers together – expressed as “C” (which represents how “confident” a weather forecaster is that it will rain) and “A” (the “amount” of an area where it rains). becomes).

Both A and C are expressed as percentages, but in decimal places (for example, 0.2 for 20 percent, 0.5 for 50 percent, and 1 for 100 percent).

So let’s imagine a weather forecaster develops a PoP so that he can broadcast a weather update about the rain in a certain region on TV.

Internet confusion has reignited over the definition after popular Twitter account @wildtiktoks posted a screenshot that read: “Am I the only one who thought that a 30% chance of rain meant there was a 30% chance of rain?” % gives?”

You could be 100 percent confident (C) that 30 percent of a given area (A) would get rain.

To give viewers this as a single, digestible number, they would calculate a PoP by converting their percentages to decimals (i.e. 1.0 for C and 0.3 for A) before multiplying them together.

This would give them a PoP of 0.3 or 30 percent and they would therefore announce on TV that “the chance of rain is 30 percent”.

In fact, a PoP of 30 percent could also have different values for confidence (C) and area (A).

For example, a weather forecaster could be 50 percent sure that it would rain on 60 percent of an area, which is 0.5 x 0.6 – which is also a PoP of 0.3, or 30 percent.

In other words, a weather forecaster might say there’s a 30 percent chance of rain, but that wouldn’t necessarily tell us how certain he is that it will rain (C) or how much rain will fall in a given area (A). – that would be the case. Just give a total for A and C together.

The purpose of the North American definition is to provide a single number that takes into account two factors – the likelihood of rain and the spread of rain.

Unfortunately, there are often complete misunderstandings, hence the confusion of the woman in the TikTok video.

In the US, the probability of rain actually refers to the results of a calculation that takes into account both the probability of rain and the projected amount of precipitation. The result of this calculation, known as the PoP, is expressed as a percentage and is what North American meteorologists use to describe the “probability of rain.”

Partly because of this confusion, many meteorologists avoid saying that there is any percentage chance of rain.

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

Meteorologists in the UK also avoid using the PoP system, opting instead for the more literal meaning.

According to the UK Met Office, when they say “30% chance of rain” their experts mean what any sane person might expect – the chance of rain.

For example, if you look at the Greater London weather forecast today, the chance of rain at 2:00 p.m. is given as 10 percent, meaning that there is a 10 percent chance that it could rain in Greater London at 2:00 p.m lies.

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

The Met Office expert says: “The percentage of rain means the chance of rain at that time for that location.”

“So 60 percent means a 60 percent chance of rain and a 40 percent chance of dryness.”

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