What Does a 40 Percent Chance of Rain Actually Mean?
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.
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.
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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About the Author
Renny VandewegeFeatured Contributor Mississippi
Meteorologist Renny Vandewege has worked as a television meteorologist at WTOK in Meridian, MS, and KCTV in Kansas City, MO. He's an instructor of broadcast meteorology at Mississippi State University. He covered Hurricane Katrina and Hurricane Rita on television, and as a storm chaser, he has witnessed more than 40 tornadoes.