Quick answer
A 30% chance of rain means there is a 30% probability of measurable precipitation at your location during the specified forecast period.
It does not describe how long, how widely, or how heavily it will rain. [1]
What it really means
Start with two things: your location and the time period shown. A forecast for this afternoon applies to that interval, not automatically the next 24 hours. An hourly percentage applies to its stated hour.
“Measurable” has a defined threshold. The US National Weather Service generally uses at least 0.01 inch (about 0.25 mm) of precipitation. Providers can use different thresholds, so compare like with like. [2]
Probability expresses uncertainty. A 30% forecast leaves both rain and no rain as possible outcomes. A dry afternoon does not, by itself, prove the probability was wrong.
Common misunderstanding
A 30% chance of rain does NOT mean:
- Rain for 30% of the day.
- Rain over exactly 30% of the city or region.
- Rain will be “30% intense,” or necessarily light.
Rainfall amount and intensity are separate forecast information. A lower chance is not a promise that any rain will be gentle. [3]
Simple example
Think of many similar forecast situations. A well-calibrated 30% forecast would produce measurable precipitation at the location in roughly 30 out of 100 such situations. It is not a schedule for this one day.
10% chance
Rain is less likely, but still possible. Check the timing if staying dry matters.
30% chance
A real possibility of rain. A small umbrella may be useful for an outdoor plan.
80% chance
Rain is more likely. Plan for wet conditions, while checking expected amounts.
These examples compare likelihood only. None tells you the duration or intensity of the rain.
Why the percentage can change
Forecasts are updated as observations and models give a clearer picture. A shift from 30% to 60% means the estimated likelihood has increased; it does not mean the rain will be twice as heavy. [4]
New weather data
Radar, satellite, and surface observations help refine where systems are developing.
Different forecast models
Models can place showers in different locations or at different times.
Changing conditions
A weather system may shift, strengthen, or weaken as it approaches.
Forecast time frame
Small timing differences matter. Check that two percentages cover the same interval.
How to use this forecast
Use the percentage alongside your plans. A short walk near shelter and an all-day outdoor event call for different levels of preparation.
Be prepared
Consider a rain jacket or umbrella, and have a sheltered alternative if getting wet would disrupt your plans.
Look at the bigger picture
Check temperature, wind, expected rain amounts, and official alerts—not just the percentage.
Check the timing
Use the Hourly Forecast to compare the hours that matter. Do not add hourly percentages together.
Watch the latest conditions
Open the Weather Map for available radar coverage and check its timestamp before heading out.
How SkyAndTime displays rain probability
SkyAndTime uses Open-Meteo forecast data. Its hourly precipitation-probability field uses a threshold of more than 0.1 mm for the preceding hour. The daily rain-chance value shown on SkyAndTime is the highest hourly probability in that day, not a sum of hourly chances or a separate all-day probability. Use the hourly view for a specific outing. Open-Meteo forecast documentation →
Sources & further reading
Checked September 27, 2026.
