I think I may have blogged this before, but...

I am sorely puzzled by the percentages that weather forecasters give to the chance of precipitation.

They say, for instance, that there is a 30% chance of rain in Milford on Monday.

What does this mean? There is a 30% chance that a single drop of rain will fall somewhere within the borders of Milford on Monday? That 30% of the day it will be raining and 70% of the day it will not? That it is likely to rain in 30% of the area of Milford, but the other 70% will be dry?

Not a single one of the above formulations of a "30% chance of rain" strikes me as plausible. Yet if none of those are correct, just what does it mean to say that there is a 30% chance of rain in Milford on Monday?

To add to my confusion, when the Weather Channel predicts a 30% chance of rain, for that day they show a rain icon along with the prediction. But doesn't this prediction mean that there is a 70% chance that it won't rain? Why, on a day when the odds are 7-to-3 against there being rain, is a rain icon the correct choice for symbolizing what will be going on that day?


  1. Google-fu: http://www.srh.noaa.gov/ffc/?n=pop

    1. Good link, John, but it really doesn't address my concern. i am a cultivator of the land; What I need to know is "Should I water my plants today?"

      In the page you linked to, a 100% chance of rain equally describes the situations In which it is 100% certain that for one minute on Monday few drops of rain will fall on my property and that is absolutely certain that rain will fall On my property in abundant amounts for 24 straight hours. How does such a forecast Enable me to know Whether I should water my plants or not?

      Again, I apologize for Siri's stupid capitalization scheme.

  2. Gene, Gene, Gene. This has been asked before! Bob Murphy can explain to you why that means it's no longer pertinent.
    So much for your crushing argument that weather forecasts are vague bafflegab based on loose and unclear terms.

    The technical answer is that it's the chance anyone in the zone of the prediction will get rained on, which could be that 30% of the zone gets rain or that all the zone gets rain 3/10 days. These of course are very different things, but that's just part of the fun.


  3. The rain symbol should be taken as a precaution to prepare. Whether you should water or not has as much to do with how dry it has been as whether some rain might fall.

  4. To address part of your question, the cutoff for precipitation according to the weather folks is 1/100th of an inch. So if they weatherman says there is a 100% chance of rain and there are only a few drops he can't claim vindication.


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