Meteorologists on local television stations are overly pessimistic about the likelihood of rain, as the graph above shows. When they predict that the chances of rain are even, it rains less than 30 percent of the time. When they predict that rain is certain or nearly certain, it rains less than 70 percent of the time. In other words, if the weatherman on TV says that there is a 100 percent chance of rain tomorrow, the chances are about one in three that it will not actually rain. Presumably, this bias is a result of the fact that weathermen do not want to be held responsible when barbecues and baseball games are rained out.
The data above are from Nate Silver’s The Signal and the Noise. Many thanks, as always, to Randy Olson for sharing his work with Know More.