12 ms·

I actually learned that the rain percentage is not the percentage chance that it will rain but is in fact the percentage of the coverage area that they will exp

by tedajax 11y ago

I actually learned that the rain percentage is not the percentage chance that it will rain but is in fact the percentage of the coverage area that they will expect to get rain. So 50% rain meaning no rain at all or rain for the whole day is entirely reasonable.

rbritton 11y ago
That's funny. I had long suspected that was the case but never knew for sure. It's also important to know where the weather station is for the forecast area too -- here they can cover relatively large areas with significant elevation change, which can greatly affect things like temperature and wind speed.
rikkus 11y ago
Reasonable unless you want your forecast to be at all useful!
TheOtherHobbes 11y ago
How does that work when the percentage is supposed to cover a small area, like a town? FWIW I've found that combining the iPhone weather app with a rain radar app gives me usefully accurate predictions of rain. The Met Office is useful for coarser extreme weather warnings, but I don't pay much attention to its daily forecasts.
Al-Khwarizmi 11y ago
I've always interpreted it as the percentage of time it will rain, i.e. if it says 50% chance from 7 to 8, it will rain during about 30 minutes of that interval and not rain during the other 30 minutes. (equivalently, you could say it's the % chance that it will be raining if you choose a given instant X in the interval). I've never read an official interpretation but that seems to fit reality (where I live) quite well.
- nindalf 11y ago
  50% means that they ran the simulation say 10 times and in 5 of those simulations there was rain in the area. How to translate that to real life - you should probably carry an umbrella if you don't like getting wet. Also interesting to note that weather presenters usually bump up the probability of rain by 10-20%, because people complain about false negatives (rained when it was predicted not to) rather than false positives (didn't rain when it was predicted to). If you're interested in a good intro to this topic I recommend the chapter on weather prediction from The Signal and the Noise by Nate Silver. Its a fascinating read.
rockinghigh 11y ago
Yes, it's the probability of rain.
mikekchar 11y ago
Not quite: http://www.srh.noaa.gov/ffc/?n=pop http://www.srh.noaa.gov/ffc/?n=pop Interesting that it only measures the chance of "measurable rainfall" and does not attempt to predict the duration of the rain.

by tedajax 11y ago

rbritton 11y ago

That's funny. I had long suspected that was the case but never knew for sure. It's also important to know where the weather station is for the forecast area too -- here they can cover relatively large areas with significant elevation change, which can greatly affect things like temperature and wind speed.

rikkus 11y ago

Reasonable unless you want your forecast to be at all useful!

TheOtherHobbes 11y ago

How does that work when the percentage is supposed to cover a small area, like a town? FWIW I've found that combining the iPhone weather app with a rain radar app gives me usefully accurate predictions of rain. The Met Office is useful for coarser extreme weather warnings, but I don't pay much attention to its daily forecasts.

Al-Khwarizmi 11y ago

I've always interpreted it as the percentage of time it will rain, i.e. if it says 50% chance from 7 to 8, it will rain during about 30 minutes of that interval and not rain during the other 30 minutes. (equivalently, you could say it's the % chance that it will be raining if you choose a given instant X in the interval). I've never read an official interpretation but that seems to fit reality (where I live) quite well.

nindalf 11y ago
50% means that they ran the simulation say 10 times and in 5 of those simulations there was rain in the area. How to translate that to real life - you should probably carry an umbrella if you don't like getting wet. Also interesting to note that weather presenters usually bump up the probability of rain by 10-20%, because people complain about false negatives (rained when it was predicted not to) rather than false positives (didn't rain when it was predicted to). If you're interested in a good intro to this topic I recommend the chapter on weather prediction from The Signal and the Noise by Nate Silver. Its a fascinating read.

rockinghigh 11y ago

Yes, it's the probability of rain.

mikekchar 11y ago

Not quite: http://www.srh.noaa.gov/ffc/?n=pop http://www.srh.noaa.gov/ffc/?n=pop Interesting that it only measures the chance of "measurable rainfall" and does not attempt to predict the duration of the rain.