Extreme heat — defined as days where the temperature exceeds 90°F — is one of the most challenging aspects of American climate. For residents of the hottest counties, extreme heat is not an occasional inconvenience but a persistent feature of daily life that shapes everything from energy bills to outdoor activity patterns.
We analyzed NOAA 30-year climate normals to find the counties that endure the most extreme heat days annually. The results reveal a clear geographic pattern concentrated in the desert Southwest, the Central Valley of California, and the Deep South.
The 25 Counties with the Most Extreme Heat Days
Starr County, TX tops the list with an average of 191 days per year above 90°F.
| Rank | County | State | Heat Days/Year | Avg July Temp (°F) | Avg Annual Temp (°F) |
|---|---|---|---|---|---|
| 1 | Starr County | TX | 191 | 88.25 | 75.5 |
| 2 | Yuma County | AZ | 180 | 93.17142857142858 | 74.05714285714286 |
| 3 | Imperial County | CA | 180 | 92.22 | 73.67999999999999 |
| 4 | La Paz County | AZ | 178 | 93.7 | 72.4 |
| 5 | Webb County | TX | 174 | 87.2 | 73.4 |
| 6 | Zapata County | TX | 172 | 87.4 | 74.4 |
| 7 | Hidalgo County | TX | 171 | 86.45555555555556 | 74.84444444444443 |
| 8 | Maricopa County | AZ | 167 | 91.67500000000001 | 71.775 |
| 9 | Brooks County | TX | 160 | 85.4 | 72.2 |
| 10 | Jim Hogg County | TX | 158 | 86.8 | 73.9 |
| 11 | Pinal County | AZ | 157 | 89.48181818181818 | 70.10909090909091 |
| 12 | La Salle County | TX | 157 | 86.3 | 72 |
| 13 | Duval County | TX | 155 | 85.45 | 71.94999999999999 |
| 14 | Kenedy County | TX | 155 | 84.25 | 72.65 |
| 15 | Zavala County | TX | 154 | 86.4 | 72.7 |
| 16 | Dimmit County | TX | 153 | 86.1 | 70.6 |
| 17 | Maverick County | TX | 151 | 85.6 | 70.2 |
| 18 | McMullen County | TX | 149 | 85.83333333333333 | 71.46666666666665 |
| 19 | Kleberg County | TX | 148 | 85.7 | 73.3 |
| 20 | Brewster County | TX | 144 | 83.4 | 67.93846153846152 |
| 21 | Jim Wells County | TX | 144 | 85.33333333333333 | 72.63333333333333 |
| 22 | Inyo County | CA | 142 | 89.46000000000001 | 66.64 |
| 23 | Atascosa County | TX | 141 | 85.39999999999999 | 71 |
| 24 | Frio County | TX | 140 | 85.4 | 71 |
| 25 | Riverside County | CA | 140 | 85.96666666666665 | 69.54444444444444 |
Important
Extreme heat is the leading weather-related cause of death in the United States. If you live in or are considering a move to a high heat-day county, ensure your home has reliable air conditioning and understand local heat emergency protocols.
Methodology
Extreme heat days are defined as days where the maximum temperature exceeds 90°F, based on NOAA NCEI 1991-2020 climate normals. Data represents 30-year averages, not individual year records. Approximately 89% of US counties have sufficient weather station coverage for reliable climate normals.
Data source: NOAA National Centers for Environmental Information (NCEI), U.S. Climate Normals 1991-2020. All figures represent 30-year averages and may differ from individual-year observations. Station-level data is aggregated to county level using spatial averaging.