High Heat Index Days

A Climate Central analysis of 239 contiguous U.S. locations from 1979 to 2022 shows that 201 locations experienced an increase in annual days with a heat index of 90°F or higher, averaging 10 additional days. The most significant increases occurred in the South, Southeast, Northwest, and Southwest, with McAllen, Texas, seeing the highest increase of 32 more days. The report emphasizes that high humidity multiplies heat risks by limiting the body's ability to cool through sweating, leading to preventable but serious illnesses like heat exhaustion and heat stroke, which disproportionately affect vulnerable populations including children, seniors, and low-income communities.

• Between 1979 and 2022, the annual number of days with a heat index of 90°F or higher increased in 201 of the 239 contiguous U.S. locations analyzed, representing 84% of the sample. These locations saw an average increase of 10 high heat index days per year, with nearly one-quarter of them experiencing 14 or more additional days.
• The greatest increases in high heat index days occurred in the South, Southeast, Northwest, and Southwest regions. The highest increases were recorded in the South and Southeast, specifically in Texas, Florida, and Georgia, with McAllen, Texas, recording 32 more days, followed by Miami and West Palm Beach, Florida, with 30 and 27 more days respectively.
• High humidity increases health risks because it slows the evaporation of sweat, which is the body's primary cooling mechanism. When the heat index reaches 90°F or higher, risks of heat cramps, heat exhaustion, and life-threatening heat stroke increase, particularly for vulnerable groups such as children, adults over 65, pregnant people, and low-income communities.
• Structural inequities contribute to higher heat exposure, with people of color and those living below the poverty line being disproportionately exposed to urban heat island intensity in 169 of the largest U.S. cities.

APA

Climate Central (2023). High Heat Index Days. https://www.climatecentral.org/climate-matters/high-heat-index-days-2023

Chicago

Climate Central. High Heat Index Days. 2023. https://www.climatecentral.org/climate-matters/high-heat-index-days-2023.

Wikipedia

{{cite report |author=Climate Central |title=High Heat Index Days |date=12 July 2023 |url=https://www.climatecentral.org/climate-matters/high-heat-index-days-2023 |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{climatecentral2023high, author = {{Climate Central}}, title = {{High Heat Index Days}}, institution = {Climate Central}, year = {2023}, month = jul, url = {https://www.climatecentral.org/climate-matters/high-heat-index-days-2023}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }