Weather.Com and Percentages

WeatherLike most retirees, I follow the weather a lot.  Some watch the weather channel, some check the weather website, some do both.  The website is my choice.

But someone needs to explain percentages to them.

In their hourly forecast for today, they predicted a 30% chance of rain for each hour, all day long.  Doesn’t this mean it is almost certain to rain sometime today?   But switch to the full-day forecast, and the chance of rain they give for all day is—30%!  Which is it?

(Take a simpler example:  You buy a $10 grocery item with a 20% off store coupon and a 20% off manufacturer’s coupon.  How much will you pay for the item?  $6?  Wrong!  You will pay $6.40.  Successive percentages are not simply additive because the base changes.  Grocery stores know that.  There is no easy way you can calculate the total in your head, but you can be sure your discount will be less than you think.  In the weather example, 8 hours, each with a 30% chance of rain works out to over a 94% chance of rain for the entire day, but I needed a spreadsheet and a clear head to figure it out.)

Another percentage mistake:  A week ago, they predicted the chance of rain for the following day as 100%.  That’s pretty definite—in fact, totally definite.  Predicting just 99% would leave some wiggle room, but not 100%.  That evening, with the weather patterns changing, as we all know they do, they changed their prediction to only 10% chance of rain, meaning it was unlikely to rain at all.

But hold on a minute, I thought.  Earlier, you said it would absolutely, positively, 100%, not a shadow-of-a-doubt, take-it-to-the-bank, rain tomorrow.  Now you say it probably won’t?  Didn’t you know weather can change?

They need to avoid absolutes, like 100%.  The same goes for 0%.  But they don’t know how to combine percentages, anyway, so the problem goes deeper.  They are the ones confused, not you.

I suggest they drop percentages altogether.  Instead, use words like “maybe,”  “possibly,”  and “could.”   Even, “Ehh?”


About Roger Walck

My reasons for writing this blog are spelled out in the posting of 10/1/2012, Montaigne's Essays. They are probably not what you think.
This entry was posted in Uncategorized. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.