What does weather percentage mean?

Answer 1

"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.)" "As for your last couple of questions: a 100 percent chance of rain means that out of the last xnumber 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." --CECIL ADAMS answer from: http://www.straightdope.com/classics/a2_115b.html