What Is Heavy Rain? Here's How Much Rain Can Fall In One Hour, Including The US Record

What Is Considered Heavy Rain? What's The Record?

Extreme rainfall rates often contribute to flash flooding, but have you ever wondered how much rain qualifies as heavy as well as the largest total ever recorded in a single hour or minute in the United States?

What's Considered Heavy Rain?

A rainfall rate of 0.30 inches or more per hour is considered heavy rain, according to the American Meteorological Society.

When meteorologists say rainfall is falling at an inch or more per hour, that's when your ears should start to perk up for the potential of flash flooding, especially when the ground is soaked from recent heavy rain.

That doesn't mean rainfall will total up to an inch in a given spot, just that the rate if it remained the same for an hour would add up to an inch.

Those heavier rainfall rates generally show up in orange or red on Doppler radar, as the example below shows.

Rainfall Rate Extremes

We sometimes see rainfall rates far greater than an inch per hour.

In some cases, rainfall rates can be two, three or four inches per hour (or even higher). Tropical storms and hurricanes are notorious for producing these high-end rainfall rates, as well as slow-moving thunderstorms.

In August 2017, Hurricane Harvey dropped an astounding 6.8 inches of rain in a single hour on the southeast side of Houston.

One Hour U.S. Record

An incredible 12 inches in a single hour was reported in Holt, Missouri, on June 22, 1947. That's the highest known one hour rainfall total in the United States and around the world, according to the World Meteorological Organization.

The highest known one minute rainfall total is also held by a location in the United States. Unionville, Maryland, reportedly picked up 1.23 inches in a single minute during a downpour on July 4, 1956.

Notice that for both of these we said "highest known" since there is some debate when it comes to rainfall rate records, as weather historian Christopher Burt has detailed before.

Hourly Rainfall Intensity Has Increased

A 2023 study by Climate Central found that 136 of 150 U.S. cities they examined saw an increase in hourly rainfall intensity from 1970 to 2022.

Rainfall hours became 13% wetter on average across the 150 cities that were studied. Climate Central said every region saw an increase, with the biggest jumps occurring in the central and southwestern regions.

That's because climate change is increasing the intensity of extreme precipitation events since warmer air can hold more moisture. For every 1 degree Fahrenheit of warming, the atmosphere can hold 4% more moisture.

Chris Dolce has been a senior digital meteorologist with weather.com for nearly 15 years after beginning his career with The Weather Channel in the early 2000s.