Photo by: Getty Images

Getty Images

Here's What a Chance of Rain Really Means

By: Ashley Hamer

Forecasting rain involves lots of probabilities and complicated math.

August 01, 2019

Pop quiz: You're planning a picnic and the weather forecast calls for a 60 percent chance of rain. How likely is it that it'll rain on your picnic? If you don't know, you're not alone. "Chance of rain" is a much more complicated concept than you might think.

Under My Umbrella

In 2015, The Washington Post polled its readers on their knowledge of weather forecasts. Most readers believed that if the forecast calls for a 60 percent chance of rain, that means that it will rain across 60 percent of the area tomorrow. This is a common belief, but it's only half right. In reality, the chance of rain, or probability of precipitation (POP), is based on a mathematical formula that takes the forecaster's confidence into account. That formula is as follows:

POP = Coverage x Confidence

To use a very simple example, if a forecaster is 100 percent confident that 40 percent of a given area will see measurable rain, the POP is 40 percent. Of course, 100 percent confidence almost never happens in science, so the formula is often more complicated. What if a forecaster is 50 percent sure that rain will occur and expects that, if it does occur, 80 percent of the area will get that rain? 50 percent of 80 percent is 40 percent, so the POP is 40 percent.

An Easy Rule of Thumb

This might sound complicated, and that's because weather deals in probabilities and probabilities often require complicated math. The good news is that the average person doesn't need to get out a pencil and paper every time they read the forecast. According to the National Weather Service, if you see a 40 percent chance of rain, "there is a 40 percent chance that rain will occur at any given point in the area."

This article first appeared on Curiosity.com.

Next Up

Here's Why Smells Trigger Such Vivid Memories

Smells have a stronger link to memory and emotion than any of the other senses.

Here's Why Sound Carries Farther on Cold Days

It's not in your head—you hear better on cold days.

Here's Why You Unconsciously Copy Other People's Mannerisms

Get to know how the chameleon effect works with people.

Here's Why Static Shock Is Worse in Winter

The electric zap is caused by more than just dry air.

Tracking Hurricane Dorian: Here's Everything You Need to Know

Here is what you need to know about the tropical cyclone making its way to the U.S. mainland.

Here's the Real Reason Why Australia Has Bubblegum Pink Lakes

After years of suspecting salt or microalgae as the cause of Lake Hillier's pink waters, DNA analysis helped science discover the more likely reason. Read more at Discovery.com.

This Black Hole Ripped a Star to Shreds — Here’s How

Ohio State astronomers capture a black hole shredding a star — a rare tidal disruption event.

Food Coma? Here's Why You Get Sleepy After You Eat

You can reduce the need for nodding off after dinner with a few simple steps.

Here's How Little Exercise It Takes to Boost Your Mental Health

Exercise benefits more than just your physique.

Why Does Pluto Have Such a Weird Orbit?

Pluto is the black sheep of the planets in our solar system and it looks like astronomers aren’t sure how long Pluto will remain in its present orbit.