weatherWow!  What a dream of a forecast! At last, a Sunday that will live up to its name, Weather Underground promised when I consulted it on Thursday. Okay, a high of 56 suggests that it’s not quite picnic weather. But layer a sweater and light jacket and you can stroll the city all day long hunting for crocus and daffodil shoots. Because it’s absolutely, positively not going to rain. Weather Underground said so: “0% chance of precipitation.” Please note that the goose egg appears in boldface—their emphasis, not mine. Not a single arrant droplet, not a hint of mist. Umbrellas begone!

So why am I steaming?  Why (oh, forgive me) am I hoping for just a wee drop of rain?

It’s that oxymoronic, unbearably arrogant phrase “0% chance of precipitation.”  If we’re talking about chance, how can it ever be zero or one hundred percent? 

The beautiful math of probability teaches us that any fair coin is equally likely to come up heads or tails, no matter how many times it has come up one or the other way (or how fervently you wish for a certain outcome).  In other words, there’s no such thing as 100% probability that it will come up heads, even if it came up heads or tails in a thousand previous tosses.

Weather forecasting is based primarily on mathematical modeling and the laws of persistence (while banks and brokers are obligated to remind you that past performance is no guarantee of future success, yesterday’s weather offers valid information about tomorrow’s).   But all predictions, however well informed, are a kind of coin tossing, which is to say that randomness rules. Over the past 10 years, I’ve sporadically compared weather forecasts with astrological predictions for Aquarius, and the astrologers have been right roughly as often as the meteorologists—even allowing for my bias and the usual statistical slippage.

Hey, I don’t want to rain on your Sunday crocus hunt.  But if perchance a droplet falls, you should wear a smile as your umbrella.

  • Lucy swy April 9, 2014 at 8:32 am

    I love this!

  • Nancy Weber April 5, 2014 at 10:49 pm

    O, I like the comparison of weather forecasters with bookmakers. Both professions require flair–natty dressing, slick patter–because it’s show business, isn’t it? They want you to feel entertained (and thus to return to the same TV channel or betting parlor): sun or rain, win or lose. The difference is that bookmakers don’t pretend to be scientists. Probability of Precipitation (PoP) is the only equation I know in which one of the factors is blatantly subjective. According to the National Weather Service, “PoP = C x A where ‘C” = the confidence that precipitation will occur somewhere in the forecast area, and where ‘A’ = the percent of the area that will receive measureable precipitation, if it occurs at all.” No matter how well informed the forecaster is, “confidence” is fallible. The percentage offered may well be some human being’s “best effort,” but that doesn’t confer a license for haughtiness. I really enjoyed the brilliant sunlight today (Saturday). If Sunday brings more of the same, I’ll be as happy as everyone else.

  • Paul April 5, 2014 at 5:15 pm

    Where there is an infinity of possible outcomes — that would seem to describe the weather — an event ascribed a probability of 0% could still occur. It has always struck me that weather forecasters are more like bookmakers than probability mathematicians. Saying the odds are 1,000-1 against rain on a sunny day or 5-4 on on a grey and overcast one would probably be a more accurate forecast.

  • bob April 5, 2014 at 1:53 pm

    Why fight 0%? It’s only humans doing their best.
    Don’t be the Grunch who stole picnics.
    Bob M.


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