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(NPR)   What does a 20 percent chance of rain REALLY mean?   (npr.org) divider line 134
    More: Interesting  
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13751 clicks; posted to Main » on 23 Jul 2014 at 1:50 PM (31 weeks ago)   |  Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»



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2014-07-23 08:34:39 AM  
They have precisely the correct answer in the story:

The 20 percent chance of rain forecast, he says, could mean a few things. Imagine you have a database of days where the weather conditions were similar. If you find 1,000 days and notice that it rained the next day on 200 occasions, you might say there's a 20 percent chance of rain.

^^^^^^^^^^^^^^
It means that.

Any other answer is incorrect.  In essence, it's like rolling a 10 sided die and having the probability of a 1 or a 2 come up.

The problem is that we don't really teach basic probability in grammar school.  You don't have to get fancy, just experiments with coin flips and dice rolls.   That way kids will intuitively understand things like this when they are older.
 
2014-07-23 08:37:51 AM  
It means that if I buy kippers it will not rain and that frogs live in trees.

/Spouse of a professional logician.
 
2014-07-23 08:38:28 AM  
BTW, heard this on the way home from work, and I was kind of shocked that anyone could interpret it differently.
 
2014-07-23 08:53:32 AM  
It means it is Wednesday, the chance of it raining go up as you get closer to the weekend.
 
2014-07-23 09:25:47 AM  
It means there's a slightly better than no chance at all of it raining. It means that the BBQ is still on. It means I'm still going to the baseball game. It means that I'm still going to turn on the lawn sprinkler.

It means they should do away with the silly "percent chance" thing as most folks are too stupid to grasp it, and instead go with the same sort of scale they use for chance of sun (i.e. "sunny", "mostly sunny", "partly sunny", "partly cloudy", "mostly cloudy", "cloudy").

/And now, here's Ollie with the weather...
 
2014-07-23 09:26:27 AM  

Angry Drunk Bureaucrat: /Spouse of a professional logician.


Arguments over how the toilet seat should be left are fun in your house, I bet.
 
2014-07-23 09:31:34 AM  
I'm not sure I understand how anyone finds this confusing...
 
2014-07-23 09:32:35 AM  

Angry Drunk Bureaucrat: It means that if I buy kippers it will not rain and that frogs live in trees.

/Spouse of a professional logician.


Some frogs do indeed live in trees.

http://en.wikipedia.org/wiki/Tree_frog :
As the name implies, these frogs are typically found in trees or other high-growing vegetation. They do not normally descend to the ground, except to mate and spawn, though some build foam nests on leaves and rarely leave the trees at all as adults.
 
2014-07-23 09:45:31 AM  
See they never include important info that is necessary in order to understand the question. Like what does a 20% chance of rain mean? They failed to include an absolutely essential bit of info you need to answer that .... Is it my day off work?
 
ZAZ [TotalFark]
2014-07-23 09:49:24 AM  
What does it mean if the chance of rain in an hourly forecast is 20% for each of 6 hours, but the chance of rain for the afternoon is 20%?
 
2014-07-23 11:49:58 AM  

ZAZ: What does it mean if the chance of rain in an hourly forecast is 20% for each of 6 hours, but the chance of rain for the afternoon is 20%?


That means there is a 140% chance of rain.
 
2014-07-23 12:47:52 PM  
Woo-hoo!  I got them right, I got them right.  I never get them all right!

Today is the day that everything is coming up brap!

*gets struck by lightning*
 
2014-07-23 12:57:22 PM  
If you leave your car windows open it means Jim Cantore will personally urinate in each cupholder.
 
2014-07-23 01:12:12 PM  
This made it a true Fark moment:
'And in case you're dying to know, the technically correct answers to the first two polls above are option "C"'
 
2014-07-23 01:19:35 PM  

brap: Today is the day that everything is coming up brap!


I had a day like that.  Too much broccoli and baked beans.
 
2014-07-23 01:30:25 PM  

dittybopper: brap: Today is the day that everything is coming up brap!

I had a day like that.  Too much broccoli and baked beans.


Would it kill you to have your bopper cleaned?  I can practically smell your cheesy turtleneck through the interwebby tubes.
 
2014-07-23 01:53:54 PM  
"On 20% of the days statistically that have the same conditions, it rained."

Gecko Gingrich: It means there's a slightly better than no chance at all of it raining. It means that the BBQ is still on. It means I'm still going to the baseball game. It means that I'm still going to turn on the lawn sprinkler.

It means they should do away with the silly "percent chance" thing as most folks are too stupid to grasp it, and instead go with the same sort of scale they use for chance of sun (i.e. "sunny", "mostly sunny", "partly sunny", "partly cloudy", "mostly cloudy", "cloudy").

/And now, here's Ollie with the weather...


It's gon rain!
 
2014-07-23 01:55:47 PM  
I live in Washington State, so a 20 percent chance of rain means it'll rain.
 
2014-07-23 01:56:30 PM  
I play softball with a meteorologist and his batting average isn't any better than his forecasts.
 
2014-07-23 01:57:04 PM  

Shostie: ZAZ: What does it mean if the chance of rain in an hourly forecast is 20% for each of 6 hours, but the chance of rain for the afternoon is 20%?

That means there is a 140% chance of rain.


UK met office does that all the time. Confuses me
 
2014-07-23 01:57:06 PM  
Means you are not going to get laid?
//DNRTFA
 
2014-07-23 01:57:37 PM  
It means there is an 80% chance the groom is going to run off with a stripper and leave you at the alter.
 
M-G
2014-07-23 01:58:09 PM  
I can never understand how people get confused with watch and warning.  They go so far as to say 'a watch means conditions are favorable for the development of...' in the watch statement.
 
2014-07-23 01:59:26 PM  

dittybopper: The problem is that we don't really teach basic probability in grammar school.


No, it's just a matter of definition.  You could imagine, for example, that numerical weather forecasting wasn't effective and "twenty percent of weather forecasters believe it will rain tomorrow" is the best you could do; this would be a Bayesian subjective prior.

Or, you could define "20% chance of rain" to mean "20% of the land area in the forecast region is expected to experience rain at some point in the day", and therefore a person randomly distributed within that region would have a 20% chance of seeing rain.

That's not how it happens to be defined by weather forecasters, but "20% chance of rain" really is an ambiguous phrase which has a precise meaning within the meteorological community that not everyone knows.
 
2014-07-23 01:59:27 PM  
It means it isn't going to rain but the reporting agency says 20% as a CYA measure.
 
2014-07-23 02:00:14 PM  
Forget figuring out what the 20% means, I want to know what this "rain" thing is.

/ easiest job in the world: Arizona weather forecaster.
// Today's high is 112. Tomorrow is 112. The next day is 112.
/// It isn't forecasted yet, but I'll tell you that next week is going to be hot and sunny.
 
2014-07-23 02:00:47 PM  
I head heard the following:
"It means that there is 100% chance of rain, but only in 20% of the coverage area."

I always believed that to be true, as it makes perfect sense.

DNRTFA.  Is that the correct answer?
 
2014-07-23 02:01:47 PM  
It means that as long as I leave the top down on the Jeep, it WILL rain. If I go ahead and button up, drought conditions prevail.

/It's a lot of meteorologic responsibility
//I get wet, so your gardens and flowers can, too
///growing three slashies in the rain
 
2014-07-23 02:01:51 PM  

ZAZ: What does it mean if the chance of rain in an hourly forecast is 20% for each of 6 hours, but the chance of rain for the afternoon is 20%?


This is what I've always been confused about. Is the "daily/afternoon" % chance of rain simply the max of any hour or is it something else.
 
2014-07-23 02:02:15 PM  
It means they really have no farking clue what it's going to do, so they're hedging their bets.
 
2014-07-23 02:03:09 PM  

durbnpoisn: I head heard the following:
"It means that there is 100% chance of rain, but only in 20% of the coverage area."

I always believed that to be true, as it makes perfect sense.

DNRTFA.  Is that the correct answer?


I had always heard and believed this as well, but according to TFA, false. Always pick C.
 
2014-07-23 02:03:22 PM  

dittybopper: They have precisely the correct answer in the story:

The 20 percent chance of rain forecast, he says, could mean a few things. Imagine you have a database of days where the weather conditions were similar. If you find 1,000 days and notice that it rained the next day on 200 occasions, you might say there's a 20 percent chance of rain.

^^^^^^^^^^^^^^
It means that.

Any other answer is incorrect.  In essence, it's like rolling a 10 sided die and having the probability of a 1 or a 2 come up.

The problem is that we don't really teach basic probability in grammar school.  You don't have to get fancy, just experiments with coin flips and dice rolls.   That way kids will intuitively understand things like this when they are older.


C is the correct answer. A 20% chance of rain doesn't mean that 200/1000 days had rain and so tomorrow will have a 20% change of rain. It means that comparing forecasted condition to of the tens- or hundreds-of-thousands available past observations, in which the meteorological parameters match tomorrow's within a 95% confidence interval, a weighted average of a 20% of the time, it rained. The models for making this prediction are quite complex and well beyond anything a kid in grammar school could understand.
 
2014-07-23 02:04:07 PM  
Tonight's forecast is dark, followed by partly light in the morning.
 
2014-07-23 02:04:52 PM  
c.   the answer is always c.

(And in case you're dying to know, the technically correct answers to the first two polls above are option "C," according to the survey creators. The last one is truly subjective.)

/and 'technically correct' i've been told is the best kind of correct.
 
2014-07-23 02:06:07 PM  
Dunno, but I always love when they say "50% chance of rain."

Might rain, might not, fark, we don't know.
 
2014-07-23 02:07:16 PM  
When I lived in Biloxi there was a 20% chance every day. It rained every day I was there.
 
2014-07-23 02:07:42 PM  
Lies
Damned lies
Extended forecasts
 
2014-07-23 02:08:26 PM  

grokca: It means it is Wednesday, the chance of it raining go up as you get closer to the weekend.


The weather around here (Denver) is pretty unpredictable and rain storms can be very localized. While your definition might be technically correct my anecdotal observation tells me that when they call for a 20% chance of rain, they know it is going to be raining somewhere between Colorado Springs and Fort Collins, but damned if they can tell you which cities.
 
2014-07-23 02:08:53 PM  
so (tangent early on here...why must EVERYONE interviewed, or speaking on a panel, on npr have to start all their sentences "so..."?)...i didn't rtfa, but i can tell by all you farkers in this thread that what i was originally taught is true. statistically, based on data derived by studying similar days with similar weather patterns, today there is a 20% chance of rain.

a few years ago i began to be told that "20% chance of rain" means that approx. 20% of the viewing/listening area of said weather report would experience rain. this never made sense to me. the statistic thing does
 
2014-07-23 02:09:34 PM  
I was always told "20% chance of rain" means that on that day in the last 20 years, it rained on 5 of those days. That's what I was told, and it makes perfect sense.
 
2014-07-23 02:09:52 PM  

ZAZ: What does it mean if the chance of rain in an hourly forecast is 20% for each of 6 hours, but the chance of rain for the afternoon is 20%?


In each individual hour, there is a 20% chance of rain occurring at some point within that hour. Over the course of the day, there is a 20% chance of rain at some point.
 
2014-07-23 02:09:55 PM  
My prediction?

3.bp.blogspot.com

Pain.
 
2014-07-23 02:11:12 PM  
We put the question above to a few folks across the country, and many of them came to different conclusions. Some thought a 20 percent chance of rain means you should definitely bring an umbrella, while others said they would be surprised if it even drizzled. And at least one person looked at the question the other way: There was an 80 percent chance it wouldn't rain.

This paragraph gives me the distinct feeling that the author is trying to portray the question as deeper and more complex than it really is.
 
2014-07-23 02:11:57 PM  
All I know is you can plan a pretty picnic but you can't predict the weather.
 
2014-07-23 02:12:00 PM  
You said there'd be no math meth. Yaaaa-buddy.
 
2014-07-23 02:12:18 PM  

Dow Jones and the Temple of Doom: I was always told "20% chance of rain" means that on that day in the last 20 years, it rained on 5 of those days. That's what I was told, and it makes perfect sense.


It makes absolutely no sense at all as it completely ignores current conditions. And that is ignoring the fact that 5/20 would be a 25% chance of rain.
 
2014-07-23 02:14:23 PM  

Gecko Gingrich: Angry Drunk Bureaucrat: /Spouse of a professional logician.

Arguments over how the toilet seat should be left are fun in your house, I bet.


a game theoretic approach to the toilet seat problem/
 
2014-07-23 02:14:53 PM  
Pop Quiz: 20 Percent Chance Of Rain. Do You Need An Umbrella?

No.

/headline in the form of a yes-no question
/no one needs an umbrella
/except maybe the Witch from Oz
 
2014-07-23 02:16:13 PM  
I always read it as 20% of the broadcast area is likely to see some rain.  It's not like they're every spot-on correct anyway.
 
2014-07-23 02:16:18 PM  
Most of the time I view it like this under most of my local weather conditions.

20-30% - pop up showers/storms - lots of areas see nothing a few areas see a lot but generally not a lot of time raining
30-40% - fast front coming through, everyone gets drenched for an hour or less
50-60% - slow front coming through, everyone gets rain for a few hours
70-100% - stationary front, near if not total washout day

I know it's far from perfect but those percentages aren't really designed to tell you how much or how long the rain is.  I think it's a big problem with the system.
 
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