Do you ever wonder how much rain constitutes heavy rainfall and what the largest recorded rainfall totals in the United States are for a single hour or minute?
What defines heavy rain? According to the American Meteorological Society, a rainfall rate of 0.30 inches or more per hour is considered heavy rain. When meteorologists report rainfall at an inch or more per hour, this is a sign of potential flash flooding, especially if the ground is already saturated. Such heavy rainfall rates are typically depicted in orange or red on Doppler radar.
While heavy rainfall rates are usually under an inch per hour, extreme cases can see rates of two, three, or even four inches per hour, often associated with tropical storms, hurricanes, or slow-moving thunderstorms. For example, Hurricane Harvey deposited an incredible 6.8 inches of rain in a single hour on a part of Houston in August 2017.
The highest one-hour rainfall record in the U.S. stands at 12 inches, recorded in Holt, Missouri on June 22, 1947. Similarly, the highest one-minute rainfall record is 1.23 inches in Unionville, Maryland on July 4, 1956. It’s worth noting that these are the highest known records, with some debate around rainfall rate records.
A study conducted by Climate Central in 2023 revealed that hourly rainfall intensity has increased in 136 out of 150 U.S. cities examined from 1970 to 2022, with rainfall hours becoming 13% wetter on average across the cities studied. This increase is attributed to climate change, which is intensifying extreme precipitation events due to the atmosphere’s increased moisture-holding capacity in warmer conditions.
Chris Dolce, a senior digital meteorologist with weather.com, has been reporting on weather for nearly 15 years, initially starting his career with The Weather Channel in the early 2000s.
