# What does 20% chance of rain really mean?

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What does 20% chance of rain really mean?

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Based on the data we currently have we have calculated 100 possible likely scenarios of what will happen and in 20 of them it is raining sometime between 3 and 4 o clock

Multiple super computers have crushed the numbers together after performing lots of simulations on what the weather might do based on some photographs taken by satelites…

This results are thrown together into a big pile of simulation soup… and found that in 20% of the simulations in the soup… it would rain!

That the weather forecasters expect 20% of people in the given area to have rain during the hour/day.

This is actually misinterpreted quite often. When you are given a chance of rain in an area, the probability that it will rain in at least some part of that area is close to 100%. So what the chance of rain actually tells us is the percentage of the area within that region that will receive rain (with some assumptions going into it).

As an example, if the weather report says there’s a 20% chance that it will rain in town A, then 20% of the area in town A will receive rain and the probability that at least some part of town A receives rain is about 100%

It means, that based on all records from the past, in 20% of all cases with comparable meteorological conditions there was rain.

It’s different in different areas, so check where you live.

As far as I know it’s either 1 in 5 chance of rain or the amount of that amount of the area should expect rain.

UK usually does the first and US usually does the second.

Environmental Science graduate with a little meteorology. I’ve been using/interpreting meteorology for 10+ years as an air traffic controller.

Using 20% chance of rain in this example, I like to explain it like this: of five days that are just like this one, on one of them it will rain.

According to National Weather Service rules, percent chance of rain is (what percent of your area will see rain) * (confidence in forecast.)

So, if half your area will see rain (.5) and you are 80% sure (.8) you would say 40% chance of rain.

The idea is to answer the question: If someone lives in this area should they bring an umbrella? (They calculate how risky they want to be given the percentage.)

I’ve been told that it means 80% of the area they are talking about will not get rain. So **20%** of the area has a **chance of** getting **rain**.

This has been brought up by meteorologists in my area recently, to the point that they took some time to detail what it means.

In short:

20% chance of rain = it may rain somewhere in the viewing area, and 20% of the area (by landmass) will experience it.