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## What Does a 40% Chance of Rain Actually Mean?

by | Oct 6, 2020

Let’s say you want to plan an outdoor happy hour with friends tomorrow. You check the weather and it says that there is a 40% chance of rain during that time. What does 40% chance of rain actually mean? Should you go ahead with your happy hour plans?

## Looking Under the Hood

The chance of rain is actually called the “Probability of Precipitation” (“PoP”) and it is derived from two variables:

1. The confidence the forecaster has whether it will rain, and
2. How much of the forecast area likely will be affected

So, the chance of rain can be expressed by this formula: PoP = C x A where C = confidence and A = forecast area affected.

Here’s a few helpful examples from the National Weather Service that give the same statistical result of a 40% chance of rain:

(1) If the forecaster was 80% certain that rain would develop but only expected to cover 50% of
the forecast area
, then the forecast would read “a 40% chance of rain” for any given location (0.8 x 0.5 = 0.4).

(2) If the forecaster expected a widespread area of precipitation with 100% coverage to
approach, but he/she was only 40% certain that it would reach the forecast area, this would, as
well, result in a “40% chance of rain” at any given location in the forecast area (1.0 x 0.4 = 0.4).

These are very different situations but both expressed as a 40% chance of rain! In the first example, there is a very high chance (80%) that some of the forecast area (50%) will get some rain. The second example is a low chance (40%) that all of the forecast area will experience rain. Two very different things!

## How Much Rain?

Another key thing to know is that the Probability of Precipitation only provides the chance of rain and not how much rain will occur. 0.01 inches of rain isn’t very much! 100% chance of rain could mean barely a sprinkle or being drenched by inches of rain.

## Putting it All Together

So, what does a 40% chance of rain mean? According to the National Weather Service, a 40% chance of rain means that there is a 40 percent chance that rain will occur at any given point in the area. But without knowing the underlying variables, you don’t know if the forecaster is highly confident that it will rain but just over a part of the area, or if the forecaster has low confidence that it will rain but if it does it will be across the entire area. Knowing what the forecast area encompasses is key as well (is it multiple counties, just your county, just your town?). Chance of doesn’t tell you how much rain will occur.

Putting it all together — a 40% chance of rain suggests that you more likely than not can have your happy hour outdoors. But it may just sprinkle on you. Or you might get soaked. Or it might not rain at all because the rain didn’t materialize in the area. Or it rained, but in a different part of the forecast area.

Knowing what chance of rain means (and what it doesn’t mean) can help reduce our frustrations with precipitation forecasts. Realize that if there is a 80% chance of rain and you don’t get rained on it could mean that some other part of the forecast area got rain. Or it could mean that the 20% chance of no rain is what occurred across the forecast area.

Predicting the weather is complex and difficult. On a related note, here’s an IFOD from 2018 about how much more accurate weather forecasts have gotten over the years: Weather Forecasts Are Getting Much Better.

1. I’m confused, is happy hour on or off?

2. Hmm, dose this same kind of animus be applied to your chances of getting the Corona Virus?

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