What does it mean when “X percent chance of rain” is predicted?

Dear Cecil: When the weather service predicts some percentage chance of rain for the next day, what does this number mean? I always figured they compiled certain parameters about current conditions and compared them to analogous days in the past. The percentage of those days in which it rained is then used as the predicted chance of rain now. In other words, they base the percentages on historical data. However, a friend who is a pilot (and who presumably should know about these things) claims that the figure is determined by looking at the current conditions of a storm system which is expected to pass over us. The number represents the percentage of the area the storm is now passing over in which it is actually raining. This strikes me as complete bullshit. Anyway, what’s the straight dope on the predicted chance of rain? How can you ever have 100 percent chance of rain? How reliable are these figures? Ted F., Chicago

Cecil replies:

You are one shrewd hombre, Ted, no doubt as a result of having been exposed to this column regularly during your formative years. Your guess about what the percentages mean is pretty much on the money.

Here’s what happens. The National Weather Service (and various other weather services around the world, under the guidance of the World Meteorological Organization, a UN agency) sends up measuring devices called radiosondes in helium balloons twice a day. These collect weather data continuously as they rise through the atmosphere — wind speed and direction, temperature, barometric pressure, and humidity. The information is radioed to the ground and eventually ends up at the National Meteorological Center near Washington, D.C., where it’s fed into a computer. (There are other centers in other parts of the world that do the same thing.) Other data from satellites and ground reporting stations are also fed in, and what you end up with, metaphorically speaking, is a complete three-dimensional picture of atmospheric conditions around the world. The computer program then applies various laws of fluid mechanics to predict future conditions.

Unfortunately, there is only so far you can go with this. While it’s possible to predict the temperature, say, with a reasonable degree of certainty, precipitation is much chancier. The best forecasters can do is to give the probabilities, which they do by having the computer compare present conditions with historical data. When you hear there’s a 10 percent chance of rain, that means that out of the last 100 times the weather conditions were just like they are now, it rained 10 times. (More or less — I’m obliged to oversimplify a bit.)

The weather service has been calculating precipitation probabilities for as long as anybody can remember, but it’s only been in the last decade or two that the nation’s cadre of broadcast weather beings has deigned to convey this to the masses. Why the belated fascination with numbers I don’t know; I suppose they feel it gives an aura of precision to a business that, let’s face it, is still about one jump ahead of tea-leaf reading. But who can say?

As for your last couple of questions: a 100 percent chance of rain means that out of the last x number of times things were just like they are now, it rained every time. (It does not, incidentally, mean it’s raining right this second.) On the question of how reliable the figures are, the amazing truth is that they are absolutely 100 percent reliable all the time — that is, presuming the raw data were fed in properly and the calculations done correctly. Remember, we’re just talking about probabilities here. A probability isn’t “wrong” if it tells you there’s only a 10 percent chance of rain and it rains anyway; it’s only wrong if a series of such predictions doesn’t pan out over the long haul. Not much comfort if your dermis gets damp next time you’re out on a picnic, but it’ll have to do.

Another view

Dear Cecil:

In Washington, Oregon, and British Columbia, we have a different interpretation of precipitation forecasts. If the weatherman says there’s a 20 percent chance of rain, that means it will rain 20 percent of the day. If there’s a 50 percent chance, it will rain 50 percent of the time, etc., up to 100 percent, which means, of course, a typical January day. This interpretation seems to be quite accurate.

— Joyce K., Seattle

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.