Dear Answer Person: If a weather forecaster says there is a 10 percent chance of rain today, 20 percent tonight, and 30 percent tomorrow, what happens if it rains early today? Does it stay 20 percent chance for tonight or is that gone? If it doesn’t rain earlier today, do you add the 10 percent to the 20 percent, making it 30 percent chance of rain tonight? Or what ? — Jeffrey Michael Austin Foxmore, La Jolla

Put away the calculator and clear your mind of all extraneous thoughts. Weather predicting is not simple. Though you may be sorry you asked, ’cause you’ll know less about the likelihood of rain in your particular neighborhood after I explain this than you did before. But if you’re a faithful reader, maybe you’re used to that situation. Anyway, if the National Weather Service says there’s a 30 percent chance of rain in the San Diego area tomorrow, what they’re saying is the odds are three in ten that someplace (or places) in the area will get wet. In some large geographical areas, the probability is expressed slightly differently. If the meteorologist sees a 60 percent chance of rain over half the area, he’ll express the prediction as a 30 percent chance of rain. Doubly misleading, it seems to me. But weather predictors know the general public doesn’t understand the system anyway, so maybe it doesn’t matter.

The National Weather Service’s system of calculation is called the Precipitation Probability Program. It’s based on computer models of weather patterns, and they’ve been refining the models since the early 1970s. They now can predict weather for the next 72 hours with about 85 percent accuracy, though weather prediction is still a bit of an art as well as a science.