The NWS definition in one sentence
The number in a US forecast is the probability of precipitation, PoP for short. The National Weather Service (NWS) glossary defines it as "the probability that precipitation will be reported at a certain location during a specified period of time."
The forecast-term pages of the NWS offices in Binghamton, New York, and Juneau, Alaska, spell it out in full. They define PoP as the likelihood of occurrence, expressed as a percent, of a measurable amount of liquid precipitation, or the water equivalent of frozen precipitation, during a specified period of time at any given point in the forecast area. Three parts of that sentence do the work:
- Measurable. At least 0.01 inch. NWS Paducah adds that this much usually produces enough runoff for puddles to form.
- A period of time. Usually 12 hours, such as "today" or "tonight", sometimes shorter, such as "this afternoon". More on that below.
- Any given point. Your house, your field, your ballpark. Not the whole county at once.
So a "40 percent chance of rain this afternoon" means a 40 percent chance that at least 0.01 inch falls at your spot between the start and end of the afternoon period. NWS Atlanta/Peachtree City sums up its own example the same way: a 40 percent probability for at least 0.01" of rain at the specific forecast point.
What it does not mean
NWS Paducah names the two common misreadings. A 40 percent chance does not mean that rain will cover 40 percent of the area at a given time. And it does not mean that you will see rain 40 percent of the time during the period. The number also says nothing about how hard it will rain. A 90 percent chance can bring a light sprinkle, and a 30 percent chance can bring a downpour to the spots it hits.
Confidence times area: PoP = C × A
NWS Paducah explains how the forecaster arrives at the number. It is the product of two factors: the forecaster's certainty that precipitation will form or move into the area, and the areal coverage of precipitation that is expected. Written as a formula:
A = share of the area expected to get it, if it does (in percent)
Multiply the two as fractions, or multiply the percentages and move the decimal point two places to the left, as the Paducah explainer puts it. The percentage calculator does the "what is X percent of Y" step if you want to check one.
Worked examples
The first two rows are the examples NWS Paducah gives. The others use the same arithmetic with different weather. The last column is the forecast word for that percentage, from the NWS Juneau quick reference covered further down.
| Situation | C | A | PoP | Wording |
|---|---|---|---|---|
| Rain is quite likely to form, but only over half the area | 80% | 50% | 40% | chance |
| A solid band of rain may or may not arrive | 40% | 100% | 40% | chance |
| Showers will form for sure, hitting 40% of the area | 100% | 40% | 40% | chance |
| A front is very likely, with wide coverage | 90% | 70% | 63% | likely |
| Maybe a few afternoon showers | 60% | 30% | 18% | slight chance |
| Steady rain everywhere, no doubt about it | 100% | 100% | 100% | rain or periods of rain |
Look at the first three rows. They are three different days: a likely storm that will miss half the area, an all-or-nothing rain band that may never arrive, and showers that are certain to pop up but will only hit some places. All three come out at 40 percent. For you at one point, that is the right answer each time: on such days you would expect rain at your spot about 4 times in 10.
The math also explains why high numbers need both factors. With 90 percent confidence and 70 percent coverage the result is 63 percent, which is "likely", not a sure thing. And a modest 60 percent confidence in showers covering 30 percent of the area gives only 18 percent.
Point, not county
Text forecasts are issued for a forecast area, and NWS Binghamton notes that the forecast area, or zone, is generally considered to be a county. NWS Paducah's example: if a county forecast says 40 percent this afternoon, there is a 40 percent chance of rain at any point in the county from noon to 6 PM. On the NWS map forecasts the number is tied to the spot you click. NWS Louisville explains that its forecasters draw probabilities across the region, so each click gives a forecast for that location, and a different click usually gives a slightly different number. In its example map, the same 12-hour period had 20 percent at Bowling Green, 41 percent at Louisville and 43 percent at Lexington.
What 40% looks like over many days
A single day cannot prove a percentage right or wrong: at your spot it either rains or it does not. The number only shows its meaning over many forecasts. The Paducah explainer calls PoP "simply a statistical probability". If forecasts with a given number are accurate, the share of those periods with measurable rain at your spot should match the number.
| PoP | Chance of staying dry | Rainy periods out of 10 | Out of 30 |
|---|---|---|---|
| 10% | 90% | 1 | 3 |
| 20% | 80% | 2 | 6 |
| 30% | 70% | 3 | 9 |
| 40% | 60% | 4 | 12 |
| 50% | 50% | 5 | 15 |
| 60% | 40% | 6 | 18 |
| 70% | 30% | 7 | 21 |
| 80% | 20% | 8 | 24 |
| 90% | 10% | 9 | 27 |
Two things follow. A 30 percent chance that ends up dry was not a wrong forecast, because dry is what happens 7 times in 10. And a 20 percent chance that ends up wet was not wrong either. Over 30 such afternoons, you would expect about 6 wet ones.
Forecast periods: today, tonight, this afternoon
The percentage always belongs to a period. NWS Binghamton and NWS Juneau both say that unless specified otherwise, the time period is normally 12 hours. They list these period definitions in local standard time:
| Forecast period | Hours (local standard time) |
|---|---|
| Today | 6 AM to 6 PM |
| This morning | 6 AM to noon |
| This afternoon | Noon to 6 PM |
| This evening | 6 PM to midnight |
| Overnight | Midnight to 6 AM |
| Tonight | 6 PM to 6 AM |
Offices can differ in the exact hours. NWS Louisville writes that its periods are typically 12 hours long and that its "Tuesday" runs from 7 AM to 7 PM EST, or 8 AM to 8 PM EDT, with "Tuesday night" running to 7 AM EST the next morning. The NWS Juneau brochure says its graphical forecast uses a period that is normally 6 hours. NWS Louisville also points to the hourly weather graph, linked from each point forecast page, which shows the forecast, precipitation probabilities included, on a graph.
Why periods matter
A longer period gives the rain more time to reach you, so 40 percent over 12 hours is not the same as 40 percent over 6 hours. If you will be outside only from 2 to 4 PM, the afternoon number covers your two hours and four more. Your own chance for those two hours is lower or the same, never higher.
Two periods do not add up
Planning around a whole day and night? The chance of rain in at least one of two periods is at least the larger of the two numbers and at most their sum, capped at 100. Where it lands depends on how linked the two periods are. If they were completely unrelated, you could multiply the chances of staying dry:
| Period 1 | Period 2 | Rain in at least one: range | If unrelated |
|---|---|---|---|
| 20% | 20% | 20 to 40% | 36% |
| 40% | 40% | 40 to 80% | 64% |
| 30% | 60% | 60 to 90% | 72% |
| 50% | 50% | 50 to 100% | 75% |
| 60% | 70% | 70 to 100% | 88% |
The same math is why a whole week of small chances adds up. Seven days at 20 percent each give, if the days were unrelated, a 79 percent chance of measurable rain on at least one of them, and only 21 percent for a fully dry week. A forecast gives one number per period, so this step is up to you.
Slight chance, chance, likely: the words
Text forecasts often use a word instead of, or next to, the number. The NWS glossary defines them this way: "slight chance" is usually equivalent to a 20 percent chance, "chance" is used for 30, 40 or 50 percent, and "likely" is the equivalent of a 60 or 70 percent chance. The forecast-terms page of NWS Binghamton adds the full scale, and the NWS Juneau page has nearly the same table:
| PoP | Word for uncertainty | Word for showers |
|---|---|---|
| 10% | None used | Isolated or few |
| 20% | Slight chance | Widely scattered |
| 30, 40, 50% | Chance | Scattered |
| 60, 70% | Likely | Numerous, or none used |
| 80, 90, 100% | None used | Occasional, periods of, or none used |
At 80 percent and above no uncertainty word is used. Both pages note that NWS forecasters use terms such as occasional, intermittent or periods of for precipitation with a high probability, 80 percent or more, that is expected to come and go.
The NWS Juneau brochure gives a quick reference with ranges instead of 10 percent steps: 15 to 24 percent is slight chance, 25 to 54 is chance, 55 to 74 is likely, and 75 to 100 is rain or periods of rain. Below 15 percent it uses no rain wording at all. Its shower column runs isolated, scattered, numerous, then showers or widespread showers, one step off from the forecast-term pages at the low end. The words are a guide to the number, not a separate forecast.
The example forecast on the NWS Atlanta/Peachtree City page shows the words at work: "this afternoon" had a 40 percent chance of showers and thunderstorms, and "tonight" had a chance in the evening, then a slight chance after midnight. That is a drop from the 30 to 50 percent band into the 20 percent band over one night.
Scattered, isolated, numerous: words for showers
The shower words belong to a different kind of rain. The NWS glossary describes a shower as precipitation marked by the suddenness with which it starts and stops, rapid changes of intensity, and rapid changes in the appearance of the sky. The NWS Juneau brochure ties the difference to the atmosphere: stable air brings stratus clouds and steady rain or snow, unstable air brings cumulus clouds and showers.
- Isolated and few: the glossary calls each a convective precipitation descriptor for a 10 percent chance of measurable precipitation, used interchangeably.
- Scattered: the glossary's entry for "chance" says the term scattered is used when the precipitation is convective in nature. The glossary entry for scattered describes area coverage of convective weather affecting 30 to 50 percent of a forecast zone.
- Numerous: a convective precipitation descriptor for a 60 or 70 percent chance.
Scattered is the one place where the area idea shows through. When the forecaster is sure that showers will form and the open question is where they land, confidence is near 100 percent and coverage drives the number. That is the third row in the worked examples: certain showers over 40 percent of the area give a 40 percent chance for any one point, which the glossary would call scattered.
"Scattered" also has a second meaning in the glossary. For sky cover, it means 3/8 to 4/8 of the sky covered by clouds. "Scattered clouds" is not a rain forecast at all.
0.01 inch and real rain amounts
The threshold is tiny on purpose: it answers "will anything measurable fall?", not "how much?". The amount is a separate forecast. The NWS glossary calls it the Quantitative Precipitation Forecast, or QPF, which predicts the potential amount of future precipitation for a specified region. Look for it when a forecast gives an expected rainfall total.
To get a feel for the numbers, here are common rain depths in metric, and the volume of water that falls on a patch of ground. One millimeter of water spread over one square meter is exactly one liter, so the millimeter column doubles as liters per square meter.
| Rain | Millimeters (= liters per m²) | US gallons on 1,000 ft² |
|---|---|---|
| 0.01 in | 0.254 | 6.23 |
| 0.1 in | 2.54 | 62 |
| 0.25 in | 6.35 | 156 |
| 0.5 in | 12.7 | 312 |
| 1 in | 25.4 | 623 |
| 2 in | 50.8 | 1,247 |
So the measurable threshold of 0.01 inch is 0.254 mm, or about 6.23 gallons on a 1,000 square foot roof or lawn. A period counts as wet for the forecast with no more than that. The length converter turns any rainfall total between inches and millimeters, and the volume converter handles cubic feet, gallons and liters.
The other numbers in the same forecast
The chance of rain sits next to other figures that people often mix up with it.
- Humidity and dew point. These describe the moisture in the air, a different question from whether rain will fall. The dew point calculator explains what the dew point tells you and finds it from temperature and humidity.
- Feels-like temperature. The heat index and the wind chill are covered in the guide feels-like temperature: heat index, wind chill, dew point. For a cold, windy day, the wind chill calculator works it out with the NWS formula and frostbite times.
Reading a forecast for your plans
Put together, a rain forecast answers a few separate questions. Read them in this order:
- Which period? Find the period your plans fall in, and check whether it is 12 hours, 6 hours or one hour on an hourly graph. A short outing inside a long period carries no more risk than the number, and often less.
- What number? Read it as "rain at my spot on about this many days in 10 like this one", using the table above.
- Which word? In the NWS Juneau brochure, rain wording goes with relatively continuous, uniform precipitation, and shower wording (isolated, scattered, numerous) with precipitation that starts and stops suddenly and changes intensity quickly.
- How much? Check the expected rainfall amount, if one is given, before deciding whether rain would matter for your plans.
- Several periods? Remember that the chance of rain in at least one period is at least the larger number and at most the sum.
Then decide what a wet outcome would cost you. A 30 percent chance is a good bet for a walk with an umbrella in the bag and a poor bet for laying out a picnic for 50 with no shelter. The number does not make that decision; it only tells you how often the wet outcome comes up. For warnings and dangerous weather, always follow the alerts from your local NWS office.
Frequently asked questions
Does 40% chance of rain mean it will rain 40% of the day?
No. The National Weather Service explains that the number is the probability of at least 0.01 inch of rain at a given point during the forecast period. It says nothing about how many hours it will rain. NWS Paducah lists "40% of the time" as one of the two common misreadings.
Does 40% chance of rain mean 40% of the area will get rain?
Not by itself. Area coverage is one of two ingredients, and the forecaster multiplies it by their confidence that rain forms or arrives at all. A 40 percent forecast can come from 80 percent confidence and 50 percent coverage, or from 40 percent confidence in rain that would cover the whole area. Both give you the same 40 percent chance at your spot.
Is a 100% chance of rain a promise of heavy rain?
No. It is a forecast that measurable precipitation, at least 0.01 inch, will fall at your point in that period. That can be a brief light shower. How much rain is expected is a separate forecast, the Quantitative Precipitation Forecast, or QPF.
Does the chance of rain include snow?
Yes. NWS forecast-term pages define the probability of precipitation as the chance of a measurable amount of liquid precipitation or the water equivalent of frozen precipitation. Snow counts when its melted water reaches 0.01 inch.
Why is the chance different for my neighbor across town?
Because the number belongs to a point. NWS Louisville explains that its map forecasts give each spot its own probability, so clicking a different location usually gives a slightly different forecast. In its example, the same 12 hours had 20 percent at Bowling Green, 41 percent at Louisville and 43 percent at Lexington.
If today and tonight both say 40%, is the chance for the whole day 80%?
No. Chances for two periods do not simply add. The chance of rain in at least one of them is somewhere between 40 and 80 percent. If the two periods were completely unrelated, it would be 64 percent.
Sources
- NWS Glossary, Probability of Precipitation
- NWS Paducah, Precipitation Probability (PDF)
- NWS Atlanta/Peachtree City, Explaining "Probability of Precipitation"
- NWS Louisville, What Does Probability of Precipitation Mean?
- NWS Binghamton, Forecast Terms
- NWS Juneau, Forecast Terms
- NWS Juneau, Showers vs. Rain or Snow and Probability of Precipitation (PDF)
- NWS Glossary, Chance and Slight Chance
- NWS Glossary, Likely
- NWS Glossary, Scattered
- NWS Glossary, Isolated
- NWS Glossary, Few
- NWS Glossary, Numerous
- NWS Glossary, Shower
- NWS Glossary, QPF