What are the chances (of rain)?

December 21, 2006 7:20 AM

Why do weather forecasters say there's an 80% chance of rain when it's raining right now?

This morning, when I was driving to work in the rain, I was listening to the radio and the forecaster said there was an "80% chance of rain today." It was raining everywhere in the city, so why don't they just say there's a 100% chance of rain today?

posted by pyjammy to Science & Nature (25 answers total)

Usually when they are talking about "today's" weather, they are talking about midday. Otherwise they would say "this morning" or "this evening". So, I'd assume if they say there is an 80% chance of rain, they are referring to later on in the day.

Either that or they just really, really suck at statistics.
posted by tastybrains at 7:30 AM on December 21, 2006

my guess (and it's only a guess, because IANAMeteorologist) is that what the forecast is actually saying is "80% of whatever region i am forecasting for will get rained on today". that forecast region may include areas outside of your city.
posted by the painkiller at 7:36 AM on December 21, 2006

Because, it might not be raining where they are at the moment, which might be in, like, Atlanta.

However, if said weather forecaster were standing right next to you (like you could reach out and knock his hat off), and it is raining, and he says "there's an 80% chance of rain", you then have the right to reach out and knock his hat off and call him an idiot.

Once we understand this, someone please explain the difference between "partly sunny" and "partly cloudy"....
posted by HuronBob at 7:36 AM on December 21, 2006

Best answer: It's a prediction - one that was done possibly hours earlier - on the likelihood of rain given the atmospheric weather conditions. The fact that it is raining doesn't really change the probability that it would rain in the same way that winning the lottery doesn't change the very small probability that you would. Rain and winning are both outcomes that have no effect on the original calculation of probability.

So basically, what you're seeing is an explanatory gap. The weather forecasters are not including the information that 'the meteorological service, at 7:15 this morning predicted that' whenever they give you the numbers.

Plus, it's still useful information. Raining at this moment on a day with a 10% chance of rain? Not likely to be raining in an hour. Raining at this moment on a day with an 80% chance of rain? Still likely to be raining in an hour. I don't think that's an explicit part of the calculation, but anecdotally and logically, it seems true that higher percentages equal more/longer/harder rain than lower percentages.
posted by jacquilynne at 7:37 AM on December 21, 2006

Best answer: Actually, they are very good at statistics. That's how they arrive at the for forecast. When they say "there's an 80% chance of rain today", what they are really saying is that out of all the past days in which we have collected data about the weather conditions, and in which the conditions have largely matched today's conditions, it has rained on 80% of those days.
posted by Lokheed at 7:37 AM on December 21, 2006

I wrote a handful of letters to the weather.com people asking this and they never answered it. I finally got an answer from a weathercaster I know. The percent is the measure that any given location will get rain that day. So any part of the land in your zip code has an 80 percents of getting wet in what ever time period they are measuring. Its kind of a screwed up system of measurement.
posted by stormygrey at 7:37 AM on December 21, 2006

I had a similar question about what 50% chance meant and this is how someone explained it to me. Their stats cover an area that probably extends past where you were right then. 80% chance means that when the conditions in the past matched current conditions, then 80% of the all those times - it rained.
posted by dog food sugar at 7:40 AM on December 21, 2006

Weather is a very dynamic thing, of course. The "80%" prediction is based on many variables. Mostly it means "We will definitely have rain...unless this variable tracks differently than expected. Then, we won't have rain. But the chances of it tracking differently are very small."

It may be raining now. But it may not be on the other side of town. To them, they are enjoying the 20% side of the prediction...for now.
posted by Thorzdad at 7:40 AM on December 21, 2006

...how they arrive at the for forecast...
posted by Lokheed at 7:40 AM on December 21, 2006

oops. Looks like Lokheed got it. I should have checked preview.
posted by dog food sugar at 7:41 AM on December 21, 2006

someone please explain the difference between "partly sunny" and "partly cloudy"

That's easy. In the Northwest, "partly sunny" is summer, and "partly cloudy" is winter.
posted by pdb at 7:57 AM on December 21, 2006 [1 favorite]

My understanding has always been that an n% chance of rain means that there is an n% probability that it is raining at a particular place and time -- ie, an 80% chance of rain means it will probably rain 80% of the day. I would be curious if anyone knows this understanding to be wrong.
posted by raf at 8:03 AM on December 21, 2006

Once we understand this, someone please explain the difference between "partly sunny" and "partly cloudy"

There isn't any, according to the NOAA:

Partly Cloudy
When the predominant/average sky condition is covered 3/8 to 4/8 with opaque (not transparent) clouds. Same as Partly Sunny.

Partly Sunny
When the predominant/average sky condition is covered 3/8 to 4/8 with opaque (not transparent) clouds. Same as Partly Cloudy.
posted by cmonkey at 8:17 AM on December 21, 2006

Previously.
posted by ninjew at 8:18 AM on December 21, 2006

dammit! Previously.
posted by ninjew at 8:18 AM on December 21, 2006

By definition, weather refers to atmospheric events that happen in a given area over a short period of time. It might be raining right now, but in X minutes / hours, statistically speaking there is a 20% chance of no rain.

I've seen one side of a street get completely drenched with rain and the other side stay bright and sunny. Predictions on this small scale are impossible. So another way to look at it would be (as others have said) using area, 80% of your area will get rain, but some parts may not.
posted by maxpower at 8:24 AM on December 21, 2006

Our local weatherman says that the 80% does _not_ mean that 80% of the area will get rain. He says it means that the models indicate the weather pattern will produce rain 80% of the time.

That is, if that weather patten came through your area again and again, it would produce rain 80% of the times it came through.
posted by tayknight at 8:38 AM on December 21, 2006

Response by poster: Hmm. There seems to be several ways of interpreting what "80% chance of rain" means, but I think I like jacquilynne and Lokheed's answers the best. Even though I still think it's dumb.

(ninjew: I saw that, but that's not exactly what I asked.)
posted by pyjammy at 8:44 AM on December 21, 2006

Partly Sunny and Partly Cloudy always seemed identifiably different to me. It surprises me NOAA calls them the same.

To me, Partly Sunny is the companion of Mostly Cloudy.

Partly Cloudy is the companion of Mostly Sunny.

I consider Mostly to mean "more than 50%" and Partly to mean "less than 50%".

And the most straightforward explanation I've heard about the "X% chance of rain" matches what was listed above, that given the current weather conditions, on days like this in the past, 80% of the days produced rain, 20% did not. Alternatively, you could say the conditions are 80% favorable to producing rain.

The "80% of the area can expect rain" is simply not correct.
posted by Ynoxas at 9:32 AM on December 21, 2006

As an extra question, when they say there's been "x inches of rain" over what area has there been x inches?
posted by knapah at 9:59 AM on December 21, 2006

Also, broadcasting stations may have a listening area a few hundred miles in diameter. A storm system may take 3-4 hours to travel across the area, and summer storm systems can be narrow, with highly variable precipitation patterns. With cable franchises your "local" news could be a city 50-100 miles away.
posted by KirkJobSluder at 10:03 AM on December 21, 2006

over what area has there been x inches

The region being discussed, generally. An area large enough to smooth out the noise, but small enough that you're still talking about a single weather event.
posted by dmd at 10:30 AM on December 21, 2006

The Straight Dope on "What does it mean when 'X percent chance of rain' is predicted?"

(Basically, this question has been answered, but the link has a little more detail.)
posted by shoseph at 10:39 AM on December 21, 2006

The fact that it's raining right now has no bearing on the probability that it will rain. The probability is still 80%.
posted by electroboy at 11:20 AM on December 21, 2006

No, no, no. They mean that there is an 80% chance of rain in any one spot in the area they cover, which can sometimes be many square miles.
posted by nintendo at 10:11 PM on December 21, 2006