# What does it mean when "X percent chance of rain" is predicted? An Update??

**URL:** <https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740>\
**Category:** Cecil's Columns/Staff Reports\
**Created:** [August 31, 2011, 1:34pm UTC](https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740 "2011-08-31T13:34:29Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![drewtwo99](https://avatars.discourse-cdn.com/v4/letter/d/d07c76/32.png) [@drewtwo99](https://boards.straightdope.com/u/drewtwo99)\
**Post date:** [August 31, 2011, 1:34pm UTC](https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740/1 "2011-08-31T13:34:29Z")

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I was just listening to my local radio station here in Houston, TX, where they do “Dumb question amnesty” at the end of each month. A caller asked the question “What does 30% chance rain really mean? Is it a probability or is it that 30% of the area will get some amount of rain?”

The radio station called our local channel 11 meteorologist, and he said that, in no unclear terms, 30% chance means that meteorologists believe 30% of the area will recieve some measurable amount of rain, and that it did NOT refer to an overall probability…

_sigh_ So what’s the SD? Have they updated what X chance of rain means since the original column was written, or did our local guy screw it up?

I saw this column was discussed in '04, but have things changed since then? Or has there been some massive misunderstanding?

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**Author:** ![drewtwo99](https://avatars.discourse-cdn.com/v4/letter/d/d07c76/32.png) [@drewtwo99](https://boards.straightdope.com/u/drewtwo99)\
**Post date:** [August 31, 2011, 4:31pm UTC](https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740/2 "2011-08-31T16:31:05Z")

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[http://www.straightdope.com/columns/read/854/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted](http://www.straightdope.com/columns/read/854/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted) \<— link to the original column.

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**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\
**Post date:** [August 31, 2011, 5:44pm UTC](https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740/3 "2011-08-31T17:44:19Z")

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It means that under similar meteorological conditions in the past, it rained 30% of the time.

Another thing is that forecasts are only really useful for about 3 days out; after that, there’s too much variation to effectively forecast.

So if it says you have a 50% chance of rain tomorrow, it’s as likely to rain as not. If it says you have a 50% chance 5 days from now, I wouldn’t make plans one way or the other based on that.

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**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\
**Post date:** [August 31, 2011, 6:50pm UTC](https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740/4 "2011-08-31T18:50:20Z")

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But the time factor is still not covered. Does a 50% chance of rain from 2pm to 3pm mean that there is a 50% chance that rain will occur at some moment during that hour, or does it mean that at every moment of the hour there will be a 50% chance of rain, _i.e.,_ that an approximate total of 30 minutes of rain can be anticipated?

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**Author:** ![janeslogin](https://avatars.discourse-cdn.com/v4/letter/j/df788c/32.png) [@janeslogin](https://boards.straightdope.com/u/janeslogin)\
**Post date:** [August 31, 2011, 7:24pm UTC](https://boards.straightdope.com/t/what-does-it-mean-when-x-percent-chance-of-rain-is-predicted-an-update/594740/5 "2011-08-31T19:24:31Z")

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[FAQ - NOAA says:](http://www.srh.noaa.gov/ffc/?n=pop)

> [@](#):
>
> [probability of precipitation] 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.