Wet bias is the phenomenon whereby some weather forecasters report an overestimated and exaggerated probability of

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

to increase the usefulness and actionability of their forecast.

The Weather Channel The Weather Channel (TWC) is an American pay television channel owned by Weather Group, LLC, a subsidiary of Allen Media Group. The channel's headquarters are in Atlanta, Georgia. Launched on May 2, 1982, the channel broadcasts weather forecas ...

has been empirically shown, and has also admitted, to having a wet bias in the case of low probability of precipitation (for instance, a 5% probability may be reported as a 20% probability) but not at high probabilities of precipitation (so a 60% probability will be reported as a 60% probability). Some local television stations have been shown as having significantly greater wet bias, often reporting a 100% probability of precipitation in cases where it rains only 70% of the time.

Discovery

In 2002, Eric Floehr, a computer science graduate of the

Ohio State University The Ohio State University, commonly called Ohio State or OSU, is a public land-grant research university in Columbus, Ohio. A member of the University System of Ohio, it has been ranked by major institutional rankings among the best publ ...

, started collecting historical data of weather forecasts made by the

National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...

(NWS),

(TWC), and

AccuWeather AccuWeather Inc. is an American media company that provides commercial weather forecasting services worldwide. AccuWeather was founded in 1962 by Joel N. Myers, then a Pennsylvania State University graduate student working on a master's degree i ...

for the United States, and collected the data on a website called ForecastWatch.com., Page 131-136 Floehr found that the commercial forecasts were biased: they consistently predicted a higher probability of precipitation than actually occurred. The NWS forecasts were unbiased, whereas those at The Weather Channel were biased for low probabilities of precipitation: when TWC predicted a 20% probability of precipitation, it had historically rained only 5% of the time, but a 70% probability of precipitation could be taken at face value. Blogger Dan Allan noted that The Weather Channel is also biased at the upper end: a probability of 90% or higher will be rounded up to 100%. On the other hand, local television stations tended to exaggerate the probability of precipitation throughout (except when they forecast a probability of 0%, in which case it still rained about 10% of the time). The findings on wet bias, though informally well known within the weather forecasting community for some time, were first popularized outside the weather forecasting community in

Nate Silver Nathaniel Read Silver (born January 13, 1978) is an American statistician, writer, and poker player who analyzes baseball (see sabermetrics), basketball, and elections (see psephology). He is the founder and editor-in-chief of ''FiveThirtyEight' ...

's 2012 book ''

The Signal and the Noise ''The Signal and the Noise: Why Most Predictions Fail – but Some Don't'' (alternatively stylized as ''The Signal and the Noise: Why So Many Predictions Fail – but Some Don't'') is a 2012 book by Nate Silver detailing the art of using ...

''. The term ''wet bias'' is used because this is a

systematic bias Systematic may refer to: Science * Short for systematic error * Systematic fault * Systematic bias, errors that are not determined by chance but are introduced by an inaccuracy (involving either the observation or measurement process) inheren ...

in the direction of the weather being wetter than it actually is.

Reasons for wet bias

According to Silver, The Weather Channel has openly admitted to deliberately exaggerating the probability of precipitation when it is low. This is because of biased incentives: if the correct low probability of precipitation is given, viewers may interpret the forecast as if there were no probability of rain, and then be upset if it does rain. In other words, The Weather Channel compensates for inaccurate perceptions of probabilities. Silver quotes Dr. Rose of The Weather Channel as saying, "If the forecast was objective, if it has zero bias in precipitation, we are in trouble."

References

{{Biases Bias Precipitation Weather forecasting