As Hurricane Matthew barreled toward the Southeast, millions of Americans fixated on a single measure of its destructive power: wind speed. While they tracked miles per hour like adrenaline junkies watching a speedometer, they took their eyes off a far more important factor. It was rain, not wind, that dealt the most damage as Matthew rumbled ashore.

Hurricane Matthew upended lives in the Southeast over the weekend, killing at least 33 people in Florida, Georgia and the Carolinas, while floods cut power to hundreds of thousands of homes and businesses. Earlier in the week, the storm pummeled Haiti, killing an estimated 1,000 people, according to Reuters.

Source: NOAA

For all its destructive power, Matthew was curiously underrated. By the time it reached North Carolina, it barely qualified as a Category 1 hurricane, a confusing designation for a storm that produced record floods.

Our current system of rating hurricanes is based on wind speed. Category 1 hurricanes produce winds of 75 to 95 miles per hour. Category 5 hurricanes can produce winds of more than 157 mph. Wind speed is a good proxy for the lethal force of a hurricane. Powerful winds can tear the roofs off houses, knock down trees and power lines, and drive storm surges that inundate coastal towns. But wind speed doesn’t tell the whole story. There’s also rainfall.

Even a mild-mannered Category 1 or 2 hurricane can prove catastrophic if it produces enough rain. Hurricane Matthew dumped 18 inches on parts of North Carolina — more rain than Louisiana and Mississippi saw during Hurricane Katrina. Floods in the Tar Heel State destroyed 7,000 homes. More than 2,000 people needed to be rescued.

Time and again, we see that water — not wind — wreaks the greatest havoc during severe storms. Just ask New York. Hurricane Sandy registered as a Category 1 storm, but it proved the second-costliest hurricane in U.S. history. Hurricane Matthew followed a similar pattern, prompting weather experts to criticize the wind-based system of classification.

To be fair, it would be difficult to design a scale that encapsulates every threat in one number. Some hurricanes boast high-speed winds. Others churn out massive storm surges. Columbia University atmospheric scientist Adam Sobel believes experts should warn the public about the specific hazards associated with any individual hurricane rather than offering up a single number.

This debate will likely heat up in the years ahead. Climate change is fueling more powerful hurricanes that spin up faster and deliver more rain. When it comes to extreme precipitation, the human fingerprint is clear. As the atmosphere heats up, it holds more water and produces heavier downfalls. That was evident this week. Parts of the Southeast saw record-high levels of atmospheric moisture as Matthew passed over the region.

Sediment-rich floodwaters pour from rivers in North and South Carolina. Source: NASA

While Matthew didn’t produce many iconic scenes of palm trees felled by high-speed winds, it proved immensely destructive, robbing people of their lives and homes.

The biggest hazards often come in small doses—a million tiny drops that add up to a devastating flood. As weather grows more extreme, meteorologists will need to find a better way to communicate the risks to the public.

Jeremy Deaton writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture. You can follow him at @deaton_jeremy.