Tuesday, 30 September 2014

30% Chance of Rain?

I listen faithfully to the weather forecast for our area every morning, but I'm blowed if they are ever right about the rainfall.  The temperature is normally accurate - within a degree or two and they are pretty good about wind speed but 30% chance of rain?  Maybe I don't know how to interpret the language.  Does it mean that 70% of the day will be dry and that for 30% of the time it will rain  - or does it mean there is only a 30% chance that there will be any rain at all - or for those of you optimists that believe the glass is half full - that most of the day will be sunny?  What do the statisticians say?


  1. Percentage of precipitation unless zero or 100% can never be wrong. You have 99 numbers that allow you to be absolutely correct, they should never get precipitation wrong. If it rains well that is because there was a 30% chance that it would rain, if it doesn't, then that is because there was a 70% chance that it wouldn't. You can't go wrong unless you say it is zero or 100. Even 100% can be accurate if it is raining at the time of the forecast on the day of the forecast...zero? No because it may start raining later. As a guideline it is not bad though. To be honest I am kind of amazed the do as well as they do. But to answer your question, I believe that the percentage is the likelihood that it will or will not have precipitation, not the percentage of the day that will be affected.

    In process engineering, there is no such thing as optimism. A glass half full is only true if you are filling the glass, a glass half empty is true only if you are draining the glass. Stable liquid level would be a glass that automatically fills at less than 50% and automatically drains at greater than 50%.

    Further analysis of optimism or pessimism might depend on the medium in the glass. A glass of beer optimistically is half full. A glass of bitter medicine is optimistically half empty. It is sort of like relativity, ever notice how 10 minutes in the dentist chair takes a lot longer than a 10 minute roll in the hay? I have a theory that you could attain immortality in the dentist chair or at least it would seem that way.

    1. That engineering stuff is really fascinating - almost entering the realms of philosophy (from my viewpoint). I do get the part about the beer or the medicine though and as for the roll in the hay - sometimes even 10 minutes can seem awfully long (although preferable to the dentist's chair).