Tampa has never hit 100 degrees. The heat index has. Here’s why that matters.

Despite its hot and steamy reputation, the city of Tampa has never cracked 100 degrees.

We’ve come close, hitting 99 degrees as recently as 2020, and high temperatures in inland spots like Plant City have reached 100 in years past. Friday afternoon temperatures were expected to reach 97 degrees, adding to a prolonged dry spell and heat wave that has scorched the South.

The reason Tampa has never hit 100 is its close proximity to water. Cooler breezes from the Gulf of Mexico tend to roll in during the afternoons, regulating temperatures. The process works the same in most coastal areas, said Austen Flannery, a meteorologist with the National Weather Service’s Tampa Bay office.

Flannery said it’s unlikely a person would be able to feel the difference between 99-degree weather versus 100-degree weather. It’s going to be hot, regardless.

However, the heat index — an important indicator of how temperature feels to the human body — routinely surges past 100 in the Tampa Bay area in the summer, and forecasters say that, more than the standard temperature mark, is an indicator of dangerous conditions.

Temperatures this week were in the upper 90s across Tampa Bay. However, inland portions of the area were under a heat advisory because the heat index stretched as high as 110 degrees.

Forecasters advised people to stay indoors, but if you had to go outside, you would have felt the unholy mixture of heat and humidity bearing down on you. The heat index, or the “feels like,” refers to how the temperature feels to the human body when combined with air temperature and humidity.

Human bodies release heat by sweating, and as the water evaporates it transfers the body’s heat to air, according to the Massachusetts Institute of Technology. When the relative humidity is high, evaporation slows down.

With climate change, the dew point — the temperature at which air must be cooled to reach saturation — is likely to go up, said T.H. Culhane, director of the climate mitigation and adaptation program at the University of South Florida. With higher dew points, more humidity can accrue, he said.

“The trend is going to be that because the global temperature average is rising ... our dew point is going to rise and our relative humidity is therefore going to rise,” Culhane said. “I think we’ve got to get the public to understand that this fantasy figure of 100 doesn’t mean anything.”

Last year was the warmest year on record for Tampa. It’s a consistent trend. Nine out of the 10 warmest years on record for the city have occurred in the last decade.

The average annual temperature in Tampa has increased by 2.5 degrees since records began in 1891, according to a recently released Climate Action and Equity Plan. The city currently experiences 73 days each year when the “feels like” temperatures reach 100 degrees. In 30 years, that number is expected to reach 103 days a year.

First Street Foundation, a nonprofit that analyzes the country’s climate risk, released a study in 2022 that shows how heat will change in the next 30 years.

Of the 20 counties in the United States that will see the largest increase in the number of days when the heat index will reach or exceed 100 degrees, 18 are in Florida. Manatee County will see an increase of 36 days, the study stated, while Hillsborough and Polk counties will see an increase of 34.

Extreme heat is the top weather-related cause of death in the United States, according to reporting from Scientific American. Heat kills more people than hurricanes, floods and tornadoes combined in most years, the publication said.

A 2021 study by the Tampa Bay Regional Planning Council predicts there will be 242 heat-related deaths in the Tampa Bay area in 2030. That number will rise by 164% in 2050 to 639 deaths, the study stated.

In Tampa, and across areas of Tampa Bay, temperatures will remain in the upper 90s for the next few days. By next week, temperatures mellow a bit to the mid 90s. On Friday, the heat index was likely to reach 103 in Tampa. On Sunday, the heat index could reach 106, according to the weather service.

While temperatures fluctuate throughout the summer months, Flannery says to keep heat safety in mind. It could mean spending time in the shade when needed or including plans to stay cool while spending time outdoors.