Probability of Precipitation
Also called: PoP, chance of rain
Probability of precipitation (PoP) is the chance that a given point receives measurable precipitation, at least 0.01 inch, during the forecast period. A 40% chance of rain means a 4 in 10 chance of at least 0.01 inch at your location. It does not mean rain over 40% of the area or for 40% of the time.
Why It Matters
PoP is the number behind every chance of rain in a forecast, and it answers one question: will it rain here at all? It says nothing about how much will fall, how long it will last, or how hard it will come down. A brief shower and an all-day soaking can carry the same PoP.
How to Read It
- The bar is low. Anything from 0.01 inch up counts as measurable, so a high PoP does not mean a washout.
- Check the period. National Weather Service forecast periods are typically 12 hours, so a daytime PoP covers the whole day, not a single hour.
- It is for a point, not a region. The value applies to the forecast grid square around your location, and nearby places can have different values.
What It Does Not Mean
- Not coverage: 40% does not mean 40% of the area gets rain.
- Not duration: 40% does not mean it rains for 40% of the day.
- Not intensity: PoP says nothing about amounts beyond the 0.01 inch threshold.
Frequently Asked Questions
What does a 30% chance of rain mean?
There is a 30% chance that at least 0.01 inch of rain falls at your location during the forecast period, and a 70% chance that it stays dry there.
Does a 100% chance of rain mean it will rain all day?
No. It means measurable rain is expected at your location at some point in the forecast period. It could be a short shower or steady rain for hours. The hourly forecast and the expected rainfall amount tell you which.
Is the chance of rain the same as how much rain will fall?
No. The chance of rain only covers whether at least 0.01 inch falls. The expected amount is a separate forecast.
Related Terms
Ensemble Forecast
A collection of multiple model runs with slightly different initial conditions, showing the range of possible outcomes. Ensemble forecasts help quantify uncertainty and are especially valuable for medium-range predictions.
Drizzle
Very light precipitation consisting of tiny water droplets less than 0.5mm in diameter that appear to float in the air. Drizzle typically falls from stratus clouds and reduces visibility more than light rain of the same intensity.
Virga
Precipitation that falls from clouds but evaporates before reaching the ground. Virga appears as wispy streaks extending from the base of clouds and is common in dry or desert climates where the air below clouds has low humidity.
GFS
Global Forecast System, the primary American global weather model run by NOAA. The GFS provides forecasts out to 16 days and is widely used for medium-range forecasting. It runs four times daily with global coverage.
HRRR
High-Resolution Rapid Refresh, a real-time 3km resolution weather model updated hourly. The HRRR excels at short-range (0-18 hour) forecasts of convection, precipitation, and aviation weather across the contiguous United States.
ECMWF
ECMWF is the European Centre for Medium-Range Weather Forecasts, an intergovernmental organization supported by 35 states, and the name commonly given to its global weather model, often called the European model or the Euro. Its physics-based Integrated Forecasting System (IFS) runs on a global grid of about 9 km and forecasts up to 15 days ahead.
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Sources
Explore Other Terms
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
Thundersnow
A rare phenomenon where thunder and lightning occur during a snowstorm. Thundersnow typically happens in intense winter storms and is often associated with heavy snowfall rates of 2-4 inches per hour.
Wind Chill
Wind chill is how cold the air feels on exposed skin once wind is taken into account. Wind strips heat from the body faster than still air does, so the wind chill temperature is lower than the air temperature. At 0°F with a 15 mph wind, the wind chill is -19°F, and exposed skin can freeze in 30 minutes.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
IFR
Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.
Post-Tropical Cyclone
A post-tropical cyclone is a former tropical cyclone that no longer has enough tropical characteristics to be classified as one. It can still bring heavy rain and strong winds. The two kinds are extratropical cyclones, which draw their energy from temperature contrasts between air masses, and remnant lows, which have winds below 34 knots (39 mph).
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In the WeatherAI App
WeatherAI's hourly and daily forecasts show precipitation probability, and where supported a next-hour precipitation graph shows minute-by-minute timing.
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