What is the heat index? How humidity and the dew point can affect our bodies

As August kicks off, South Carolina residents are again under a heat advisory with temperatures that could feel like 108 degrees.

Like much of the Southeast, Upstate residents will see the heat index continue climbing into the weekend.

Heat advisory issued for most of SC

The National Weather Service has issued a heat advisory for almost the entire state , meaning temperatures could be physically dangerous. The NWS advises residents to be wary of temperatures in unattended vehicles, which could turn lethal in minutes. The advisory is active until 7 p.m. Thursday, but temperatures are expected to rise again on Friday. The NWS predicts Friday’s heat index could be 105.

So, why does it feel hotter than it actually is?

What is a heat index? How is it different from temperature?

The heat index is the “apparent temperature,” or what the air feels to the human body. It’s a combination of the temperature in the air and the relative humidity , which measures water vapor in the air.

To understand the heat index, here are some important terms to know:

• Temperature: A measure of how hot or cold something is using a scale (in U.S. measurements, that scale is degrees Fahrenheit)
• Absolute humidity: The actual amount of water vapor in the air
• Relative humidity: The percentage of water vapor in the air when compared to the maximum amount of water vapor the air can hold at its current temperature
• Dew point : The temperature of the air must be cooled to for water vapor to become liquid. At the dew point, the air cannot hold any more water vapor
• Evaporation : The process of liquid becoming gas – for example, water becoming water vapor

Heat overwhelming because of dew point

Heat can feel overwhelming because of the dew point. The higher the dew point, the more moisture in the air. That’s why, said NWS Meteorologist Thomas Winesett, it can better gauge how the air outside will feel than relative humidity.

“The higher the dew point, the more moisture that's available in the atmosphere,” Winesett said. “That's not necessarily true with a higher relative humidity. In the wintertime, you can have a relative humidity value of 95% on a cold, rainy day when it’s 40 degrees and raining out. Your relative humidity is high, but the atmosphere, when it's colder like that, can't hold as much moisture. But when the atmosphere is warmer, it can hold more moisture.”

The dew point is the temperature the air would have to cool to reach 100% relative humidity when the air cannot hold any more water vapor and condensation begins to form.

The dew point will never be higher than the temperature outside, so when the temperature is cooler, so is the dew point. That’s why a rainy day in the winter won’t feel as muggy as a summer afternoon.

Generally, in the summer, a dew point between 55 and 65 degrees can feel “sticky.” A dew point that is higher than 65 degrees can make the air feel overwhelming.

“When it's really hot outside, like that and you also have that humid air with those higher dew points, it's harder for the human body to cool. The evaporation process through sweating isn’t as efficient,” Winesett said. “It just gives you that really warm, muggy kind of feeling outside, where it’s that oppressive heat and humidity. That's what results in those heat index values being well above the actual air temperature.”

How hot is it in Greenville, SC?

Greenville’s temperature Thursday afternoon was 93 degrees. Meanwhile, the relative humidity in the air is 47%. The dew point is 70 degrees. Using the NWS calculator , the heat index is 99.

Is it getting hotter in SC?

Across the globe, scientists agree that the earth is getting warmer . In 2023, the planet experienced its warmest year on record.

According to the United States Environmental Protection Agency , heat waves, or multiple days of extreme heat, have increased in frequency nationwide since the 1960s.

In the 1960s, the country experienced an average of two heat waves each year. In the 2020s, the country is averaging six heat waves each year.

Average maximum temperature

Using data from the Southeast Regional Climate Center , the average maximum temperature from June 1-July 31 at the Greenville-Spartanburg International Airport dating back to 1963 is about 87.4 degrees. In 2024, the average maximum temperature for the same period was 90.6 degrees.

So far in the summer of 2024, the Upstate has experienced a heat dome effect accompanied by high heat indices as well as a statewide flash drought .

In addition to the NWS heat advisory, the Upstate also faces potential thunderstorms. Information on how to protect yourself during a heat wave is available through the NWS website , along with updated forecasts by zip code .

Sarah Swetlik covers climate change and environmental issues in South Carolina's Upstate for The Greenville News. Reach her at sswetlik@gannett.com or on X at @sarahgswetlik .

This article originally appeared on Greenville News: What is the heat index? How humidity and the dew point can affect our bodies