When Charleston residents and visitors opened their weather app over Fourth of July weekend, the first numbers they saw didn’t tell the whole story.

While meteorologists predicted a high of 94 degrees on Independence Day, the heat index, known as the “feels like” temperature, was projected to exceed 100. That’s what forecasters use to determine the actual heat risk and to issue advisories, watches and warnings.

The heat index is what stepping outside actually feels like to the human body when the temperature and humidity are combined. In Charleston, bouts of extreme heat indexes are hotter and longer than the region previously experienced.

Between 1979 and 2023, Charleston saw an increase of 19 days annually with a heat index over 80 degrees, according to Climate Central.

The National Weather Service office in Charleston uses the heat index to decide when to issue extreme heat warnings, watches and advisories. The determination is made from records maintained by the S.C. Department of Public Health, as well as the equivalent agency in Georgia, regarding EMS calls for heat-related emergencies.

The days with the highest number of calls were compared to conditions such as daily temperature and heat index. There were several cases where call volumes for heat-related illness were high, despite the heat index sitting below the advisory threshold at the time. Ultimately, the National Weather Service office lowered the requirements for a heat advisory to be issued.

Today, heat alerts fall into one of three categories:

• Heat Advisory: Issued when the heat index is between 108 and 112 degrees for at least two hours. During this time, people are encouraged to reschedule outdoor activities during the heat of the day. If being outside is necessary, people are encouraged to stay in the shade and drink lots of water.

• Extreme Heat Watch: Issued when the heat index is 113 degrees or higher for 24 to 72 hours. When this alert is issued, people should adjust their outdoor plans. In this case, the conditions for an extreme heat event are all there, but the timing and exact outcome are uncertain.

• Extreme Heat Warning: Issued when the heat index exceeds 113 degrees for at least two hours.
  When this alert is issued, people should avoid outdoor activities during the heat of the day and stay in air conditioned spaces as much as possible.

In South Carolina, heat-related emergency room visits tend to peak during July. The most-recent data available from the state Department of Public Health indicates that Charleston County saw 713 emergency department visits for heat-related illness in July 2024.

Those most at risk, according to the state, are construction workers and landscapers, followed by postal carriers, recreation workers, farmers and lifeguards, among others.

Meteorologists expect a hot summer this year, as an “extremely strong” El Niño has formed in the Pacific Ocean. The global weather phenomenon is known to cause wetter conditions and a less-active hurricane season in the Southeast, as well as higher temperatures.

Climate scientists fear that the El Niño could thrust the world over the 1.5 degree Celsius, or 2.7 degree Fahrenheit, threshold of increased global surface temperatures above pre-industrial levels. Numerous scientific and political documents use that threshold when considering the turning point for climate change’s most intense and potentially irreversible impacts.