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ninjacolin's avatar

Does it rain P% of the time that meteorologists predict P% chance of showers?

Asked by ninjacolin (13728 points ) July 18th, 2010

I’ll use 60% chance of showers as an example:

Out of all the days that meteorologists have predicted 60% chance of showers, shouldn’t it also be true that it actually rained 60% of those days over the past, say, 20 years?

Are there records for this?

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8 Answers

downtide's avatar

There are records of the actual rainfall but I think it would be much harder to find past records of meteorologists predictions. Plus, when and how do you count it? As a case in point, today (Monday) they may say “50% chance of rain on Wednesday” and then the weather patterns change overnight and on Tuesday they say “40% chance of rain on Wednesday”. So then, what is the chance of rain on Wednesday? 50%? 40%? 45%?

jazmina88's avatar

Let’s look at 20% chance…..only 20% chance of being correct? i think with weather models and the amount of change made in forecasts, especially snow. would make this a very unlikely science.

Austinlad's avatar

The whole idea of “percent chance” is generally misunderstood. As it’s been explained to me, if the forecast is for 60% chance of rain, it does not mean 60% of the forecast area WILL get rain. Nor does it mean that it WILL rain for 60% of the day in at all points in the forecast area. What it DOES mean is that any point, say, right where you’re sitting, at any time during the forecast period, there is a 60% chance of rain. It might rain. This means that 6 out of 10 times when these exact current weather conditions existed previously, it rained. It also means that with a 60% chance of rain, there’s also a 40% chance it won’t rain. Anyway… that’s how I understand it. (If there’s a 60% chance of rain, take an umbrella. Or don’t. ;-)

wundayatta's avatar

@Austinlad has it right. And, @ninjacolin, the forecast is based on past experience. So, in the past, when the conditions were approximately the same as expected today, it rained 60% of the time.

ninjacolin's avatar

Perfect, thanks wundayatta. I’ll try to wiki this stuff but let me know if you know a specific link where I can learn more about how this is calculated.

MaryW's avatar

Here in Oklahoma, it means that 20% of the viewing area might get rain if he says 20% chance.

ninjacolin's avatar

complaint: “might get rain” isn’t very informative in any of these cases.

If it rained 60% of the time under similar conditions in the past, does that mean that it “will” rain today in the present or that it “won’t” rain today in the present??

The truth is: We still have no clue! So, what good is a 60% chance of rain suppose to do for me?

MaryW's avatar

Really rereading your question:
“Out of all the days that meteorologists have predicted 60% chance of showers, shouldn’t it also be true that it actually rained 60% of those days over the past, say, 20 years?”
The answer is No.
Since the 60% is a prediction (for chance of showers on each of those days over a certain area) the law of averages would not make the prediction 100% correct 60% of those days. Yes, there are records to see what the prediction was, say the day before, and what the actual amount of rain and coverage most places had 20 years back.
60 % correct is statistically a large percentage above probability of 50% by luck.

Just for fun here is the answer to the question I thought you asked: http://www.srh.noaa.gov/ffc/?n=pop what does % chance mean

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