# 60% chance of rainApril 8, 2014 7:04 AM   Subscribe

What does it mean when the weather forecast says that there is a 60% chance of rain?

If the weather forecast shows a 60% chance of rain for today, does that mean that in all of (say) London, there is a 60% chance that there will be a drop of rain today? or 5 mm of rain, or how much? Or maybe 60% of London will see rain today? Or is the duration of the rainfall important?
posted by devnull to Science & Nature (11 answers total) 7 users marked this as a favorite

It means that, on days in the past when conditions were similar to conditions today, it rained on about 60% of those days.
posted by box at 7:08 AM on April 8, 2014 [2 favorites]

According to NOAA:

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point you select in the area.

How do forecasters arrive at this value?

Mathematically, PoP is defined as follows:
PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)

But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.

posted by inturnaround at 7:08 AM on April 8, 2014 [9 favorites]

Having wondered the same thing, I just Googled "interpreting weather forecasts" and found this:
PRECIPITATION PROBABILITY

The probability of precipitation forecast is one of the most least understood elements of the weather forecast. The probability of precipitation has the following features:
..... The likelihood of occurrence of precipitation is stated as a percentage
..... A measurable amount is defined as 0.01" (one hundredth of an inch) or more
(usually produces enough runoff for puddles to form)
..... The measurement is of liquid precipitation or the water equivalent of frozen precipitation
..... The probability is for a specified time period (i.e., today, this afternoon, tonight, Thursday)
..... The probability forecast is for any given point in the forecast area
posted by jon1270 at 7:10 AM on April 8, 2014 [2 favorites]

Another way of thinking about it (for once-a-day announcements) is that forecasters believe it will rain on 60% of the days that that statement is made. People are very confused about it.
posted by oliverburkeman at 7:38 AM on April 8, 2014 [1 favorite]

This has always bugged me, that you could take a day that had 60% chance of rain, look at the hourly forecast, and see 10%, 20%, 50%, 60%, 60%, 30%, 0%. My math brain couldn't let that add up to 60%. But the thing with the area, as well as time segments, makes a lot of sense.

posted by aimedwander at 7:39 AM on April 8, 2014

Wow we could really do with a better metric for this! A big city could surely have a higher chance of rain than somewhere else just because it's bigger, and the percentage doesn't tell us anything about the amount or duration of rain, be it 15 minutes of drizzle or 1 minute of downpour (if you can call 0.01" of rain a downpour).

Always difficult to give a single number to describe lots of things, I guess!
posted by devnull at 11:32 AM on April 8, 2014

A big city could surely have a higher chance of rain than somewhere else just because it's bigger.

Size does not matter. A 50% chance of rain in a small city is exactly the same as a 50% chance of rain in a large city.

The 50% chance is for any single spot within the area. It does not mean that that there is a 50% chance that there will be rain somewhere in the area.
posted by JackFlash at 2:26 PM on April 8, 2014 [1 favorite]

In my experience: if it says 60% today, it will probably rain today. If it says 20% today, it probably won't unless things get really freaky.
posted by jenfullmoon at 5:46 PM on April 8, 2014

In my experience unless it says anything above 75% chance of rain, it doesn't.
Inturnaround told you the correct answer.
posted by QueerAngel28 at 5:53 PM on April 8, 2014

devnull: "Wow we could really do with a better metric for this! A big city could surely have a higher chance of rain than somewhere else just because it's bigger, and the percentage doesn't tell us anything about the amount or duration of rain, be it 15 minutes of drizzle or 1 minute of downpour (if you can call 0.01" of rain a downpour).

Always difficult to give a single number to describe lots of things, I guess!
"

The metric is fine it is just the granularity of the coverage area is too large. A friend is a meteorologist working for a private forecasting company. As an example they provide forecasts for (amongst many other clients) paving companies. Those companies are willing to pay for very precise forecasts and he can say to them: "Your paving project in the NNE corner of your city will see intermittent rain all day so tell those crews to stay home but your ESE location is going to get an inch starting at 7AM and ending at 10AM and then it'll be cloudy but you won't get precipitation for the rest of the day". That kind of specificity might be forecast by the local news as "60% chance of showers" even though some places are going to be dry, some places intermittent showers and some places rain. The public forecast is an average.
posted by Mitheral at 7:13 PM on April 8, 2014

Mitheral: this sounds great. Sometimes I'd pay for this kind of information, but I expect a lot, lot less than a paving company would pay. Is there a special for this precise form of forecasting, I'd like to google it. Thanks,
posted by devnull at 2:05 AM on April 9, 2014

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