Pretend you had three marbles in a bag, one red and two green. If this sounds illogical and you’re unconvinced, consider that 30% is roughly a one-in-three chance. Thus, a 66% chance that it will rain or snow during the period.Now you multiply 0.7 x 0.7 x 0.7 which equals 0.343 which means a 34% percent chance of no precipitation falling.In this case it’s 70% for each of the three periods in question. First you determine the odds that it won’t rain.Say the Weather Service predicts only a 30% chance of rain or snow today, and again tonight and again tomorrow. Roughly speaking, there’s an 80% chance it will rain. Finally you subtract this from one to get the chance that it will rain: 78.4%.Bingo: those are the odds that it will not rain during the entire three periods in question. So you grab your calculator and punch in 0.6 x 0.6 x 0.6 and this equals 0.216.In this case it’s 60% for each of the three periods in question. Stick with me a few minutes, this is fun, as long as you don’t totally hate math. What do you think? With a 40% chance tomorrow, tomorrow night, and then again the next day, what’s the likelihood we’ll get rain any time during that entire time span?
The question is simple: If the Weather Service’s forecasts are accurate, then what are the chances that it will rain sometime during the period? Or, perhaps you have a stargazing session or outdoor event planned and desperately want it NOT to rain. Now maybe you’re a gardener and really want it to rain. So let’s say the forecast goes like this: Sunday 40% chance of rain Sunday night 40% chance of rain Monday 40% chance of rain. Rain refers to 0.01 inch or more.Ĭonfused? If you want to understand this further, read on. The answer? An “80% chance of rain” means that there is an 80% chance that rain will fall somewhere within the forecasted area. Does it mean that there’s a chance of rain across 80% of the forecasted area? Nope. That’s because weather forecasts give their “percentage chance” in probabilities. Many of us would assume that an ”80% chance of rain’ means you have an 80% chance of getting rained on? This sounds logical but is incorrect.
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But relatively few of us know how to make sense of it. We probably all check the forecast from time to time. If you’re a sky watcher like yours truly, the first step in observing the sky is having decent weather.
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