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Percent Chance of Rain

Percent Chance of Rain
Elginb
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Posted Jun 15, 2010 - 8:13 AM:
Subject: Percent Chance of Rain
Sometimes I'll wake up, look out my window, and see that it's raining. Then I'll flip on my radio and hear a weather forecast saying that there's a 40% chance of rain today. I shake my head and go "Buh-what? If it's raining now, isn't there a 100% chance of rain today?" Anyway, it gets me thinking that I don't really understand what meteorologists mean when they say there's a particular chance of rain.

So, if it's raining within the target area of the forecast, but that doesn't raise the perecentage chance of rain to 100%, then it must not be that they're forecasting a 40% chance of ANY rain AT ALL within the target area, but maybe that AT ANY POINT within the target area, there's a 40% chance of rain?

But if that's true, when they say that there's a 40% chance of rain in the target area, are they actually saying that there's a 100% chance that 40% of the target ares is going to experience rain?

Anyway, I'm hoping that there's a meteorologist or a statistician who can clear this up for me. Thanks.
meye1105
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Posted Jun 15, 2010 - 8:19 AM:

Well, I am neither a meteorologist, nor a statistician, but what I can say is that meterologists, when they say there is a certain chance of rain, compile certain parameters about current conditions and compared them to analogous days in the past. The percentage of those days in which it rained is then used as the predicted chance of rain now. If that percentage of those days in which it has rained is 40%, then there is a "40% chance of rain".
DJPavel
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Posted Jun 23, 2010 - 5:30 PM:

Elginb wrote:
Anyway, it gets me thinking that I don't really understand what meteorologists mean when they say there's a particular chance of rain...


40% chance of rain (or anything for that matter) means that if you construct an ensemble of identical universes and let them evolve, in 40% of them it will rain tomorrow and in 60% it will not.
meye1105
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Posted Jun 24, 2010 - 12:31 AM:

DJPavel wrote:
40% chance of rain (or anything for that matter) means that if you construct an ensemble of identical universes and let them evolve, in 40% of them it will rain tomorrow and in 60% it will not.



That could be a punchline to a joke about philosophy
wuliheron
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Posted Jun 24, 2010 - 7:43 AM:

Elginb wrote:
Sometimes I'll wake up, look out my window, and see that it's raining. Then I'll flip on my radio and hear a weather forecast saying that there's a 40% chance of rain today. I shake my head and go "Buh-what? If it's raining now, isn't there a 100% chance of rain today?" Anyway, it gets me thinking that I don't really understand what meteorologists mean when they say there's a particular chance of rain.

So, if it's raining within the target area of the forecast, but that doesn't raise the perecentage chance of rain to 100%, then it must not be that they're forecasting a 40% chance of ANY rain AT ALL within the target area, but maybe that AT ANY POINT within the target area, there's a 40% chance of rain?

But if that's true, when they say that there's a 40% chance of rain in the target area, are they actually saying that there's a 100% chance that 40% of the target ares is going to experience rain?

Anyway, I'm hoping that there's a meteorologist or a statistician who can clear this up for me. Thanks.



It means their best guestimate is that 40% of the area will get rain.

I lived in Hawaii for six years which is a good example of why they do it this way. Because of the prevailing trade winds half the island is a tropical forest, while the other half just 15 miles away is a desert. It rains like clockwork every day for one hour on one side; a good soaking rain. It's so dependable that you'll see people look at their watches and take off running because they know to within a minute or so when it will rain. Still, the other side of the island is a desert so it would be utterly useless for a weather person to say "100% chance of rain today" since everyone knows where it will definitely rain on one side.

The question is where will it rain that it doesn't always rain? If you happen to live where it seldom rains then when you hear 100% chance of rain you know you will get rain.
perseus
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Posted Aug 1, 2010 - 11:33 PM:

I have often thought about this, and it is remarkably difficult to present a useful forecast irrespective of the predictive difficulties.

For example, what if it is forecast to rain within the area covered by the prediction, but not all places, should the prediction be geographically weighted? This might depend whether you are static or driving.

What about the strength of rainfall. If the forecast is 90% certain it will rain just a few drops which wouldn't bother anyone, should this be weighted in excess of a 10% chance of a thunderstorm which might wreck an event?

Then there is the complication of what is rain. What about sleet, dew etc.

I generally favour some sort of plot depicting the mean predicted precipitation in any specific location verses time with with confidence levels on either side. However, what happens in the 10% thunderstorm case? The plot shows only 10% of the strength if the thunderstorm actually occurs, with a much larger 95% confidence limit higher up the chart. Obviously this distribution will be skewed as well.
zarko90
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Posted Aug 20, 2010 - 5:17 AM:

Ignoring the fact that meteorologists tend to be wrong (haha), you can think of it as simply a probability. If we consider their percentage prediction to be true, then like someone said, take 100 identical universes, and a 40% chance translates into 40 of them producing rain under those same conditions. They use certain data to make predictions about what will happen and then infuse probability into the equation. It sounds like you're really asking about probability rather than weather per se, but its a good example.
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