Florida’s silent storm: Inside new science exposing how humidity can escalate a heat wave

By Kathryn Pizzurro, UF College of Liberal Arts and Sciences

When Floridians talk about extreme weather, hurricanes dominate the conversation. Each season brings updates on storm tracks, cone predictions and wind speeds, all in the hopes of predicting the unpredictable. But a quieter, more deceptive threat is already reshaping the way people live and work in the Sunshine State: extreme heat.

“Heat waves actually kill more people in the U.S. than hurricanes, floods, tornadoes, or any other form of extreme weather,” said David Keellings, associate professor of geography at the University of Florida. “The Centers for Disease Control attribute over a thousand deaths annually to hyperthermia, but that number is probably really underestimated considering subsequent complications of dehydration, heat exhaustion and heat stroke.”

Keellings, who has been studying the intensification of heat waves for more than a decade, is part of a UF team that has developed a groundbreaking way of measuring these events. Led by Ph.D. students Anuska Narayanan and Mostafa Rezaali, their findings, published in Science of the Total Environment, highlight how Florida and the broader Southeast are uniquely vulnerable.

Traditionally, scientists defined heat waves using only temperature and duration. But Keellings and his research team saw that measure as incomplete and developed a new framework called the Heat Severity and Coverage Index. The HSCI does not just consider how hot it gets, but how long the heat lasts and how far it spreads geographically.

In their most recent study, however, they went further and factored in humidity. That change revealed just how oppressive Florida’s climate can be.

“When we include humidity, we see a much bigger and sharper increase in the severity of heat in the eastern U.S.,” he said.

Humidity makes hot air harder for the body to cope with, slowing down its natural ability to cool itself. As a result, some events once considered moderate heat waves are among the most dangerous on record. Florida’s position between the Atlantic Ocean and the Gulf keeps a steady flow of moisture in the air, and the flat landscape lets the moisture linger, fueling the state’s hot, humid climate.

“Florida stands out in the Southeast as a place where heat waves are becoming more frequent, higher in magnitude and longer lasting,” Keellings said. “We’re seeing more frequent passages of warm, humid air masses over Florida than in the past, which is more hot, sticky weather that isn’t going away.”

The effects reach far beyond discomfort. Extreme heat strains power grids, disrupts agriculture, reduces worker productivity and disproportionately harms vulnerable populations such as the elderly and those without reliable access to air conditioning.

The study’s methods draw on machine learning tools to analyze decades of weather data. Algorithms identify heat waves, track their movement and measure their coverage. It’s a data-driven approach designed not just for scientific journals, but for the people most at risk.

“We’re even exploring the idea of a rating scale for heat waves, like the Saffir-Simpson scale for hurricanes, to make them more visible to people,” he added.

Just as Floridians know to prepare differently for a Category 1 storm versus a Category 5, a heat wave index could help communities understand when it’s time to check on neighbors, open cooling centers, or issue public health alerts.

But developing a rating system brings a new challenge to the equation, according to Keellings. “Heat is a very difficult animal to work with,” he said. “What is a heat wave in New York is not a heat wave in Miami, which is not a heat wave in Phoenix.”

Keellings and his colleagues are also expanding their research to look at the chronic health impacts of heat exposure. Instead of focusing only on hospitalizations during acute heat waves, they are studying whether long-term exposure to oppressive heat is tied to respiratory problems and cardiovascular disease.

By reframing how we measure and communicate heat waves, UF researchers are equipping communities, health officials and policymakers with better tools to protect people. And in a state where summer is already synonymous with sweat, that research could save lives.

Hurricanes may grab the headlines, but heat waves are the quiet danger unfolding across Florida. But with continued investment in climate research, Floridians don’t have to be caught off guard.

This piece was previously published at https://news.clas.ufl.edu/floridas-silent-storm/. Banner photo: Grass and a beach fence are silhouetted by the setting sun on Okaloosa Island (iStock image).

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