What are "Rain Chances"

When I first became aware of the Weather on the News, the Weatherman wore a suit and stood in front of a paper map of the US and drew on it with a magic marker, showing the location of various lows, highs, and fronts. He also spoke of a xx% chance of rain for the upcoming days.

Today, we have a ditzy weathergirl dressed in either a pantsuit (although jeans are not totally out of the question, it seems) or a skirt and blouse (on special days, I guess) who has trouble with the English language (although I suspect English is the only language she speaks), confusing prepositions, using “into” where “in” is called for, as “today it will reach 96 or 97, lower near the coast, with it only getting to 94 ***into ***Houston”. I mean, is there really enough difference between 96 or 97 and 94 to justify mangling the language?

Anyway, such weathergirl also mentions that the “Rain Chances today are only 20%”. Not “there is a 20% Chance of Rain” but “Rain Chances”. What is the difference between a 20% chance of rain and rain chances at 20%? Of course, I know that what she means is “it ain’t gonna rain, no way, no how” but people like to have hope, so I guess saying there is a small chance gets you more viewers. But, when the rain chances jump to 30%, she suggests you get your umbrellas out. Really, less than a 1 out of 3 chances of rain, when it has been so long since it rained that it could rain for 30 minutes without even getting the ground wet and she thinks anyone is going to bother with an umbrella?

Maybe I should have put this in the Pit, so I could rant with a bit more style, but I really would like a factual answer to what the difference between “a chance of rain” and “a rain chance”.

I don’t know if there’s a difference between a chance of rain and a rain chance (in point of fact, I’ve never once seen the weather be delivered by a ‘weathergirl’. Even when it’s a woman, they are described as a meteorologist, have a degree in meteorology and at least to my layman’s eye seem as sharp as any other weatherperson. If your news is using ‘weathergirls’ I have to wonder where else are they cutting corners?) But as far as I know the chance of rain, or probability of precipitation is the confidence that precipitation will occur somewhere in the forecasted area multiplied by the percent of the area that will receive measurable precipitation if it actually rains. So if the chance of rain is 30%, then maybe they’re 100% confident that 30% will get precipitation or 50% confident that 60% will or 75% confident that 40% will and so on. But regardless, at any given point in that forecasted area, you have a 30% chance of getting rained on.

I have asked weathermen this, and gotten evasive answers.

It is my understanding that the chance of rain applies to the day part specified (“this afternoon”) and is the chance that measurable rain will occur somewhere in the specified area (“metro Houston”). A 100% chance of rain does not mean that it will certainly rain on your front lawn, but that rain will certainly fall in the forecast area during the forecast day part.

By the way, supposing the chance of rain is 20% today, 50% tonight, and 10% tomorrow. What is the chance that there will be rain in the next 24 hours? To calculate this, you multiply the probabilities of NO rain, and subtract from one. In this case, .80 x .50 x .90. So a 36% chance that it will not rain during any of those dayparts, or a 64% chance that it will rain somewhere.

Seeing Weathergirls, however, implies that there is a high probability that it will shortly be raining men on your lawn.

this was Cecilized

I believe this statement conforms with The Master’s article:

If the weather forecaster says xx% chance of rain, then there’s an xx% chance that it will rain and a ( 100% - xx% ) chance it will not rain. For example, if it’s 30% chance of rain, then there’s a 70% chance of no rain.

In the NWS’s “Forecast Discussion”, sometimes the forecasters will speak about Precipitable Water Content of an air mass. This is about the best we can do right now to forecast how much it will rain.

But chances of rain is exactly that, the chances of rain.

Your summation does not address the question of whether it means 20% chance that rain will occur*** someplace*** in the forecast area (within the five county area), or some*** particular*** place in the forecast area (e.g., my front yard).

I guess I am not asking the question correctly.

About 10-15 years ago, all they talked about what the “chance of rain”, that is, “tomorrow, we except a 25% chance of rain”. Then, all of a sudden, they switched to “rain chances”, as in “tomorrow, we will put the rain chances at 25%”. It makes no sense that the two ways of phrasing the information mean the same, otherwise, why did they change? You will only confuse your audience if you keep changing how you present the data.

So, what is the difference between a “25% chance of rain” and “rain chances of 25%”? I know what a “25% chance of rain” can mean.

I’ve never heard “rain chances of x”. It might just be your news station’s policy.

What decade are you living in? No, we don’t. Calling them girls is sexist as hell to begin with, and secondly weather forecasters are professional scientists.

The forecaster may or may not be the person presenting the forecast, though. You can have a professional forecaster (who has a degree in meteorology) making the forecast and putting it up on a teleprompter, and a professional presenter (chosen for being telegenic) who’s reading it off of the 'prompter. While this means two people on the payroll, it also opens up your options, since you don’t need the same person to have both skillsets.

Or does a 25% chance of rain this afternoon mean that there is a 25% chance that it will rain at ANY point between (say) noon and five. Or that at any given time between noon and five (say 3:47pm) it will be raining.

I don’t know if this is a a function of time or market size, but I knew someone twenty-odd years ago who was a “weather girl” on a network affiliate in Harrisonburg, VA. She was a grad student in Communications at James Madison. Later she graduated and moved “up” to a station in Roanoke. She was absolutely positively not a scientist. She would read the forecasts from accuweather or something.

She was dating my roommate, who was a sports reporter at the same station. They both also sold advertising time.

For the last 20 years I’ve lived in major metro areas. All the stations have multiple meteorologists on staff.

Yay! I get to let my weather geek knowledge spill out a bit in just my second post! :stuck_out_tongue:

There is no difference in meaning between those two wordings. The difference between “a chance of rain” and “a rain chance” is just like the difference between “a computer that sits on a desk” and “a desktop computer”. The same meaning, just different wordings.

So, that said, there’s two different methods that meterologists use to come to that “chance of rain” percentage. One is what I call the “10 Like Days” calculation, and the other is the “areal coverage” calculation. In both methods, the percentages are always multiples of 10, from 0 to 100.

The “10 Like Days” calculation is generally used in every weather forecast outlet EXCEPT local and regional TV/radio newscasts and their newscasts’/stations’ websites.

Let’s say that the Intellicast website calls for a “40% chance of rain” for your ZIP/postal code. That means that, given 10 days with the exact same weather conditions in said ZIP/postal code (everything from temperature to front placement), a given location in the ZIP/postal code (your backyard, the grocery store, the park, whereever) would see rain on 4 out of the 10 days.

And, it’s almost always something hyperlocal like ZIP/postal codes these days, because forecasting tech has come to a point where it’s possible to predict weather on such a level. That means that, in sprawled-out cities like my very own Fort Worthless, Tex-ass, one ZIP code in the far east of the city can have a 30% chance of rain, and another in the west end can have a big fat goose egg. :wink:

The “areal coverage” calculation is pretty much always used by local radio/TV forecasts. It’s literally a prediction of what percentage of the radio/TV station’s viewing area will get rain.

For example, let’s use my preferred TV news/weather outlet I watch here in North Tex-ass, WFAA/8.

They use areal coverage. Their coverage area is basically everything that is north of Waco, west of Paris, south of the Red River, and east of Abilene.

In the summertime here, rain tends to fall by way of randomly scattered showers and thunderstorms. Because of their scattered nature, we typically have “areal coverage” percentages around the 30% range. By that, WFAA means that, literally, they’re predicting that 30% of their viewing area will get rain. On days where a big “blob” of widespread rain is forecast to crawl over the region and cause it to be cloudy and wet all day–how we typically get rain in the autumn and winter–the chance prediction from WFAA will be as high as 90-100%.

Both methods of calculation leave a lot of room for forecast “busts” (and pissed-off viewers/listeners), which makes the second method of calculation especially risky. A 90% chance of rain done by the 10-day method very rarely busts, but if the second method’s used, and that widespread rain doesn’t develop in the first place, you can easily bust a “100% chance of rain” forecast. :smiley:

I can possibly answer the time element. The NWS (and probably commercial forecasters) have a product called the hourly Weather Forecast:


This also is a point forecast, but if I move the point a mile it probably will not change
(well, we have bluffs - the forecast at the top may differ than the valley)


Ah, yes, forgot the time element in my earlier post. Both of those methods of calculation can also be used in day-part forecasts (this morning, this afternoon, this evening, tonight, et cetera) and in hourly forecasts. In both cases, the rain chances are calculated for the day-part or hour, rather than the full day.

The NWS point-forecasts are for each square meter of the Earth’s surface. No fucking shit, latitude and longitude are given to SEVEN significant digits. lon=-107.36688&lat=42.63474. Some years ago they used goddamn TWENTY significant digits, forecasts for every six molecules of silicon dioxide.

I think they’re compensating for something.

Those of us of a certain age can call women “girls” all we want to, they like it coming from us … especially the 40, 50 and 60-something girls. You know you’ve reached that age when little old ladies hold doors open for YOU.

The person reading the weather copy on screen is a reporter, and perhaps has a degree in Journalism. They call themselves “Meteorologists” but they are NOT Atmospheric Scientists. Most TV stations around here just read the NWS copy. The one station does have a guy who seems more informed about the subject, at least he’s able to run the public domain weather modeling software.

It’s a separate skill set to be able to face the camera, that’s why actors get paid so much. These people don’t giuve a rat’s ass about informing you about the future weather, they’re using weather data to sell you Ford F-150 4x4s because mother of Jesus, look how much it snowed here !!!

I think you’re confusing the accuracy of the map with the actual granularity of their forecasting models.

You can define a point to 7 decimal places in latitude and longitude, but it’s still within a forecast area- you can see the forecast area in that little map down and to the right on your link. It’s a green square roughly 1.5 miles to a side. Pick any point or address within that square and you get the forecast for the square. Move your point into an adjacent square, and your forecast may change slightly.

Well yes and no. There are science degrees in meteorology and there are degrees in broadcast meteorology (or other similar names). Both are usually B.S. degrees, but the former concentrate much more on the science of it, and the latter do what you exepct the name implies.

Technically speaking, a meteorologist is one who holds a Ph.D. in meteorology. Those TV weathermen may call themselves meteorologist, and most do have some education in meteorology (perhaps even a bachelor’s degree with a major in meteorology), but do not have doctorates, except for the head honchos on The Weather Channel.