How much rain is a lot?
Updated
A day that brings an inch of rain counts as a lot. In 145 measured U.S. cities, 9.2 days a year is the median count for that much rain; Hilo, Hawaii has the most at 28 days, while Boise, Idaho almost never gets it.
Daily rain totals, from light to extreme
| Amount (in) | Days a year | What it looks like |
|---|---|---|
| 0.1 | 71.3 | Wets surfaces but barely soaks into the ground. |
| 0.25 | 47.2 | Steady rain that leaves puddles on pavement. |
| 0.5 | 24.2 | Soaks lawns and gardens; gutters start to run. |
| 1 | 9.2 | Heavy; water ponds in places, and dry soil gets a thorough soaking. |
| 2 | 1.6 | A serious downpour; roads and low areas may hold water. |
| 4 | 0.1 | Extreme; flooding likely, rare almost anywhere. |
The days column is the median for the 145 cities in the table below. A 3-inch day sits between the 2-inch and 4-inch rows; 5 inches is above the last row.
Rain rates by the hour
The American Meteorological Society's Glossary of Meteorology defines light rain as a trace to 0.10 inches per hour. Moderate rain is 0.11 to 0.30 inches per hour. Heavy rain is any rate above 0.30 inches per hour.
An inch of rain as snow
A long-standing rule of thumb, used by the Weather Bureau since 1875, is 10 inches of snow for every inch of water. A 30-year study of National Weather Service cooperative observer data by Baxter, Graves and Moore (2005) put the national average closer to 13 inches. The actual ratio shifts with storm type and region: wet snow yields less, dry powder yields more.
An inch of rain in gallons
According to the U.S. Geological Survey Water Science School, one inch of rain on one acre equals 27,154 gallons. That same inch falling on a 1,000-square-foot roof adds up to about 623 gallons.
How much rain causes flooding
There is no single rainfall total that guarantees flooding; the rate, how wet or frozen the ground already is, pavement cover, and terrain all play a part. A Flash Flood Warning from the National Weather Service means flooding is happening or about to happen.
Annual heavy rain days by city
Averages for 1991-2020 come from the U.S. Climate Normals published by NOAA's National Centers for Environmental Information, using one airport or city station per city.
Under 17 U.S.C. 105, no copyright attaches to the government-produced figures on this page, including reports, alerts, station observations, and normals, which were created by the National Weather Service and other NOAA offices. The notice that 17 U.S.C. 403 requires appears in this sentence. It does not imply endorsement by or affiliation with those agencies. See: Data sources and attribution.