SolriseWhat does a 40% chance of rain mean?
A 40% chance of rain means there is a 40% chance that at least 0.01 inch of rain falls at any given point in the forecast area during the forecast period. It does not mean it will rain 40% of the day, or over 40% of the area.
What the chance of rain is
Forecasters call it the probability of precipitation, or PoP. It is the forecast probability that measurable rain, snow, or other precipitation falls at a location during the period shown. "Measurable" has a precise meaning: at least 0.01 inch of liquid, or the water equivalent of snow or ice. The period can be anywhere from 1 to 12 hours, so always check which hours a percentage covers.
The National Weather Service is clear about what the number is not. It is not the percent of the area that will get wet, not the percent of time it will rain, and not a measure of how heavy the rain will be. A 100% chance means you will get wet at some point in the period, but it may rain all day or for just a minute.
How forecasters word each chance
| Chance | Forecast wording | What it means for you |
|---|---|---|
| 10% | (no rain mentioned) | No rain expected. |
| 20% | Slight chance | A 1-in-5 chance of getting wet. |
| 30 to 50% | Chance | Worth a backup plan. |
| 60 to 70% | Likely | Rain is likely. Plan around it. |
| 80 to 100% | Rain, stated plainly | Expect rain. |
Source: National Weather Service probability of precipitation wording (NWS Albany fact sheet, weather.gov/aly).
Why the chance of rain changes
The chance rises as a storm system, front, or moisture plume gets closer or more organized, and falls as it clears out or weakens. Forecasts for the next few hours can be more confident than forecasts for days out, so the number for tomorrow afternoon often firms up as tomorrow gets closer.
Thunder works the same way. A thunder chance is the hour-by-hour chance that a storm happens close enough to hear thunder, and it climbs on warm, humid afternoons when rising air can build tall storm clouds.
What the chance of rain means for you
Everyday
Above 60 to 70% is when it is worth planning around if you are heading outside: bring a jacket or umbrella, or shift plans indoors. A 30 to 40% chance often just means a passing shower, so check the hourly breakdown rather than skipping plans outright.
Outdoors
Pack a light rain shell whenever the chance is above about 30% on a long outing. Wet rock, roots, and painted road lines get slick fast, so ease off on descents and corners after rain starts.
Flying
Most consumer drones are not waterproof, so treat any rain as a no-fly. For pilots, rain showers can quickly lower visibility and ceilings below VFR minimums.
Common questions
Does a 40% chance of rain mean it will rain 40% of the day?
No. It is the chance that at least 0.01 inch of rain falls at any given point in the forecast area during the period. It says nothing about how long it will rain.
Does a higher chance of rain mean heavier rain?
No. According to the National Weather Service, a higher probability of precipitation is not an indicator of how heavy the rain will be.
What does a 20% chance of rain mean?
It is a 1-in-5 chance that you get at least 0.01 inch of rain at your location during the forecast period. Forecasters call it a slight chance.
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