Ever hear of "wet bias"? It's the tendency of forecasters to over-estimate our chances of feeling the wrath of the weather gods. Why do they do it? Basically, we force them to.
Ever notice that when the local weather channel predicts a 15% chance of rain, you can plan on there being, essentially, no chance of rain? There's a consistent "wet bias" in weather reporting that causes forecasters to say that there will be a bit more of a chance of rain than the data suggests.
The specific configuration of the bias depends on the venue. Local stations will inflate their estimations of the chance of rain no matter how much of a chance there really is. So it will rain ten percent of the time on days when the local channel predicts a twenty percent chance of rain, and it will rain seventy percent of the time on days when the channel predicts an eighty percent chance of rain. Larger services, like The Weather Channel, will mark up the chance of rain on days when there's a low chance, but will accurately report high chances of rain. Everyone pays attention to a report of a seventy-five percent chance of rain, but no one gives credence to a ten percent chance, so they mark the ten percent up to twenty.
Nate Silver examined this in his books, The Signal and the Noise, and found that weather channels to this because, "People don't mind when a forecaster predicts rain and it turns out to be a nice day. But if it rains when it isn't supposed to, they curse the weatherman for ruining their picnic."
Yes, we're wrong to act out at forecasters, but is a little bias so bad? There's a reason people will assume that the worst-case scenario will always happen. Not only are they prepared if things go south, they get a little bump of happiness if things work out better than expected. If we expect rain on a day out, and we don't get it, we're happy. That means we've set up a nation-wide system in which we are bound to get a set percentage of nice surprises per year, and a system where we're rarely caught in the rain without an umbrella. As mental blind spots go, that isn't so bad.
If you prefer to get your facts straight, though, the National Weather Service will always tell it like it is.
[Via The Signal and the Noise, Icon Forecast Bias and Pleasant Surprises, American Meteorological Society.]