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What Does a 40 Percent Chance of Rain Actually Mean?

By Renny Vandewege

(WESHTV/Youtube)

Weather forecasts often call for a 40 percent chance of rain, but what does that really mean?

Despite all of our technological advances, the truth is that we still can't say with certainty whether one particular spot will get rain, so we use percentages.

But here's the thing: Most people don't understand how forecasters arrive at the chance of rain - the "40 percent" part. It's not too complicated, so let me explain.

The equation used to determine the probability of precipitation uses two variables: the confidence that precipitation will form and the areal coverage of precipitation if it does form. These two variables are multiplied together and the product is the percent chance of precipitation.

For example, if a meteorologist is 100 percent confident that rain will form, but only 40 percent of the people in the area will receive rain, the probability of precipitation is 40 percent. At the same time, if the forecaster is only 40 percent confident that storms will form but they will cover 100 percent of the area if they do form, the probability of precipitation is also 40 percent.

Meteorologists have determined that probability of precipitation occurring in one particular spot provides the most scientifically accurate way to produce a forecast. Technology has yet to advance far enough to give meteorologists total confidence in precipitation occurring in one spot at one given time.

Though probabilities are the most accurate way to forecast - meteorologists have become roughly 90 percent accurate - they are also largely responsible for the general idea that meteorologists are often wrong. That's because probability forecasts are often misinterpreted by the general public. There are two common scenarios in which the public misunderstands the forecast.

Scenario 1

There is a 40 percent chance of rain in the forecast.

Person A believes that every single day there is a 50 percent chance of rain. Either it rains or it doesn't rain.

Person B believes that a certain location might receive rain one out of every 10 days over a long period of time, so that any given day's chance of rain is 10 percent.

Person A is likely to view a 40 percent chance of rain as lower than normal and may not expect much rain. Person B is likely to view a 40 percent chance of rain as higher than normal and may even change their plans because of this threat.

If there is a 40 percent chance of rain and it has rained in 40 percent of the area on that day, Person A thinks the forecast was wrong and Person B thinks the forecast was right. It is a no-win situation for meteorologists when a large segment of their viewers think they were wrong even when their probabilities were right.

Scenario 2

The second problem forecasters face is the notion that "if it didn't happen to me, it didn't happen at all."

In a scenario in which there is a 70 percent chance of rain in the forecast, those who live in the 30 percent area that didn't receive rain are likely to believe the forecast was completely wrong without realizing that 70 percent of the forecast area did receive rain and the forecast was right.

Some meteorologists have decided to use word descriptors to better communicate their forecast. Such words as "isolated," "scattered," or "numerous" have replaced the traditional percentage-based forecasts. Other meteorologists use phrases such as "splash-and-dash storms," "pop-up storms," or "hit-and-miss storms" to indicate the uncertainty in the exact location where storms will form.

Forecasting has come a long way in recent years with advances in technology, but not far enough to pinpoint the exact location where precipitation will form on a given day. Probabilities provide the most accurate way to deliver a forecast - as long as the end user understands its meaning.

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Belgarath

I've never understood why people are so indignant when the weathermen is wrong about a forecast. The job literally requires them to predict the future, it's not easy to see how every factor will play out. Obviously they're going to be wrong some of the time. There are plenty of occupations where people are paid for their predictions on future events in areas such as sports, politics, business, and so on. In every one of these there is an accepted margin of error because people realize that some information is better than none.

December 09 2013 at 5:26 AM Report abuse rate up rate down Reply
Presh

Its not just the NYC weather people ..South African weather people have the same virus ..they always say the same statement.. Agggrrhh.. Its so annoying

August 14 2013 at 11:32 PM Report abuse rate up rate down Reply
Sharon

I go to the weather page and look at the radar map. Then I look at the predictions, and say, "Well, that isn't going to happen." I'm almost always right. (no % available on that last statement)

July 13 2013 at 2:20 PM Report abuse rate up rate down Reply
Boopelybee

pay meteo's when they are correct only, you will see they will do a better job!

March 02 2013 at 7:48 PM Report abuse rate up rate down Reply
markruck

This comes from a meteorologist...

A 40% chance of rain means they run computer models and, 40% of the time the models indicate that it'll rain.

I'd like to come back as a weatherman in my next life--screw up half the time, change my seven-day forecast ten times and get paid handsomely for it.

March 02 2013 at 5:17 PM Report abuse rate up rate down Reply
Brian Stanaway

I stopped reading your article at paragraph number 6 ! Meteorologists are 90 % accurate. RANDY VANDEWEGE, please tell all of us exactly where you found this percentage from, and what are the facts to back up this claim. I am in a weather related business (CAR WASH) and I can state as fact that your 90% number is 1 of 2 things, #1 incorrect facts, or #2 biased facts !The long range forecasts are a complete waste of time for both the viewer and the meteorologist. I can say with 100% certainty day #7 in a 7 day forecast is always incorrect ! The men and women in this field should stick to a 36 -48 hour forecast only. If you watch any forecast tonight before you go to bed, look at tomorrows weather. When you wake up first thing in the morning look at the current predictions for the weather. I will guarantee that you will see at least a 30% change in that forecast during the course of 8-10 hours that you have slept. For you to suggest that there is a 90% accuracy in forecasting, you must be talking about those forecasts which supply weather for only the upcoming 8-10 hours. Now onto your 40% claim, why not say that there is a 60% chance you will not see any precipitation. Why must all meteorologist focus on the NEGATIVE ? Do you guys realize how detrimental your forecasting is to any weather related business ? For example your forecast may say mostly cloudy with a 30% chance of rain (a typical NYC/NJ summer day forecast). That day most of the time it is a mostly sunny day with little or NO PRECIPITATION, but do to your industry being obsessed with focusing on the NEGATIVE, my business is basically nill for the day you predicted to be mostly cloudy and a 30% chance of precipitation. I am not trying to bash you or your field, but please provide some facts within the article. Why not try sticking to a much more obtainable forecast of 36-48hrs, instead of changing the forecast for day number 4-5-6 and 7 every few hours. Why not promote the POSITIVE when forecasting instead of 20% chance of rain, how about saying, folks go out and enjoy the day, there is an 80% chance we will see NO RAIN TODAY ! I know weather forecasting is not an exact science and is very difficult to do, but please think of all the different industries, all the employees and all the potential customers you are negatively affecting by trying to forecast the impossible 5-6-and 7 day forecast and also by constantly promoting the NEGATIVE side of the weather. I am very sure that most of us would like to hear that we have a 70% chance of having nice weather today, as opposed to the typical, "Well folks, get ready because you have a 30% chance of having a lousy day today, so try and enjoy your day anyway folks".Promoting the positive is always a better way to start your day and if you meteorologist promoted the positive maybe you guys would not be thought of in such a NEGATIVE way.Hopefully 5% disagree with my thoughts on your article

March 02 2013 at 5:04 PM Report abuse +1 rate up rate down Reply
Niky

All jokes aside. Don't ever believe a long range forecast. Tomorrow's weather sure. seven days away? Not going to happen and be accurate. My complaint is the way the weather reports get hyped up to try and panic people. The TV broadcasters in the NYC area do it constantly and when they are wrong? They expand their range to include someplace with weather similar to their prediction. Case in point. The last "blizzard" happened in New England but the forecast was for NYC. My north Jersey yard got less than 8 inches of snow with light wind. Blizzard?? You can't yell fire in a theatre to panic people and the weather people have to be prosecuted when they overhype the weather for ratings.

March 02 2013 at 2:55 PM Report abuse rate up rate down Reply
1 reply to Niky's comment
cathy

All jokes aside, the predictions of the blizzard weren't just for your limited area. They were for the entire area covered by the TV broadcasters in NYC. That includes further upstate in NY and CT. People in those areas also watch NYC news and they DID get snow.

March 02 2013 at 3:47 PM Report abuse rate up rate down Reply
sgtmike132

I had Accuweather on my Droid and their meteorologists must be the summer school people. The app. said sunny on Saturday and partly cloudy on Sunday. We hopped on the bike and it rained most of the day and Sunday was all clouds.. Two weeks later, it said partly cloudy for Saturday. We went out for a local hike and it drizzled rain all day. So, I checked the zip code on my droid and it was correct. I wrote down the seven day forecast and they were wrong 5 out of 7 days. I wrote accuweather numerous letters and on their F.B. page and never heard back..

March 02 2013 at 11:03 AM Report abuse +1 rate up rate down Reply
mn81262

there is a 99% chance that the Weatherman is wrong on any given day.
slight snow cover= we get a blizzard
cloudy= sunny
sunny=cloudy
drizzle=down pour
etc etc

March 02 2013 at 10:21 AM Report abuse +1 rate up rate down Reply
superponty

I think Florida has the worst weathermen in the country. Even someone taking a wild swing in the dark gets it right more then they do. I remember on many occasions watching the report where they were calling for a clear sunny day only to look out the window and see it raining. All i could think of was doesn't someone at least look outside before making the call.

March 02 2013 at 9:30 AM Report abuse rate up rate down Reply
1 reply to superponty's comment
jbnb63

"more THEN they do" makes you look less than intelligent...it is "more THAN they do"

March 02 2013 at 9:51 AM Report abuse -1 rate up rate down Reply
1 reply to jbnb63's comment
waagtod

Less than intelligent? Like someone who trolls comment boards and corrects slightly wrong spelling?

March 02 2013 at 10:17 AM Report abuse +1 rate up rate down
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