Report•September 3, 2025
Monthly Attribution Overview – August 2025
An analysis of how climate change boosted United States temperatures in August 2025
Using Climate Central’s Climate Shift Index (CSI) tool to measure the impact of climate change on daily temperatures across the United States, as well as NOAA’s Applied Climate Information System (ACIS) to find daily temperature information, we have compiled a high-level overview of how climate change has affected temperature trends in August in cities across the United States. (Dataset downloadable as Excel workbook here.)
1. High-Level Findings
The U.S. saw cooler temperatures in August — an average of -0.5°F cooler than normal across 193 cities.
Ten cities experienced one of their top five warmest August temperatures on record. On average, stations have data reaching back to 1893.
Elevated Climate Shift Index values were observed across the Northeast, Southeast, South, Ohio Valley, Upper Midwest, Northern Rockies & Plains, Northwest, Southwest, and West United States.
Twenty-seven cities experienced at least two weeks' worth of CSI values of 2 or higher, indicating a strong climate change influence.
Although August 2025 was cooler than average, long-term August warming trends show that nearly all cities analyzed have warmed since 1970, consistent with findings from Climate Central’s 2025 Summer Package.
Figure 1. Threaded ACIS temperature anomalies (differences from normal) for August 2025 relative to the 1991-2020 normal. Analysis based on ACIS data.
Figure 2. Days with a CSI of 2 or higher for August 2025 for ACIS threaded stations. Analysis based on ERA5 data (Aug.1-31) and GFS data (July 31).
2. Local Temperature Anomaly Analysis
The average temperature anomaly — or difference from normal — across the 196 cities analyzed was -0.5°F. The majority (124 cities) recorded cooler-than-average temperatures.
Temperatures were especially down across the eastern half of the U.S., where nearly every city experienced below-average temperatures. The most unusually cold region was the Northeast — which includes Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, and Pennsylvania — with an average temperature anomaly of -2.1°F.
The most unusually cold city in August was Roanoke, Virginia, which was -4.4°F cooler than usual.
The most unusually hot city in August was Eugene, Oregon, which was 3.9°F warmer than usual.
Although August 2025 was cooler than normal, of the 196 ACIS stations analyzed, 189 cities showed long-term warming trends for August, indicating that these cities have been warming on average since 1970.
Reno, Nevada, has seen the greatest warming since 1970 — the average August there is now 11.1°F warmer than it was in 1970.
Table 1. Top 10 ACIS stations with the highest August 2025 temperature anomaly.
City | State | Temperature Anomaly or Difference from Normal (°F) | Average Temperature (°F) | Warming Since 1970 (°F) |
|---|---|---|---|---|
Eugene | OR | 3.9 | 71.8 | 3.8 |
El Paso | TX | 3.8 | 86.7 | 6.5 |
Medford | OR | 3.8 | 78.2 | 5.7 |
Phoenix | AZ | 3.8 | 98.2 | 4.8 |
Tucson | AZ | 3.7 | 90.6 | 4.7 |
Albuquerque | NM | 3.4 | 80.2 | 3.1 |
Spokane | WA | 3.3 | 73.5 | 4.2 |
Lubbock | TX | 3.0 | 82.8 | 5.4 |
Portland | OR | 3.0 | 73.6 | 4.6 |
Flagstaff | AZ | 2.9 | 67.9 | 2.0 |
Table 2. Top 10 ACIS stations with the fastest warming August since 1970.
City | State | Warming Since 1970 (°F) | Temperature Anomaly or Difference from Normal (°F) | Average Temperature (°F) |
|---|---|---|---|---|
Reno | NV | 11.1 | 0.6 | 75.7 |
El Paso | TX | 6.5 | 3.8 | 86.7 |
Odessa | TX | 6.4 | 2.7 | 86.0 |
Boise | ID | 6.3 | 0.9 | 76.7 |
Las Vegas | NV | 6.3 | 1.8 | 93.5 |
Helena | MT | 5.8 | 0.9 | 69.7 |
Fresno | CA | 5.8 | 2.1 | 84.3 |
Medford | OR | 5.7 | 3.8 | 78.2 |
Houston | TX | 5.6 | 1.7 | 86.9 |
Lubbock | TX | 5.4 | 3.0 | 82.8 |
3. Local Climate Shift Index Analysis
San Juan, Puerto Rico, experienced 28 days at CSI 5 in August, indicating that temperatures on those days were made at least five times more likely because of climate change.
Twenty-seven cities across the country experienced at least two weeks' worth of days at or above a CSI 2.
On average, cities in the Southwest experienced the highest number of days (20) with a CSI of 2 or above.
Table 3. Top 10 ACIS stations with the highest number of days at or above a CSI 2 during August 2025.
City | State | Days at CSI 2 or higher | Days at CSI 5 | Average Temperature (°F) | Temperature Anomaly or Difference from Normal (°F) |
|---|---|---|---|---|---|
Honolulu | HI | 29 | 20 | 82.9 | 0.7 |
San Juan | PR | 29 | 28 | 84.4 | 0.9 |
West Palm Beach | FL | 26 | 22 | 85.6 | 2.4 |
Albuquerque | NM | 26 | 12 | 80.2 | 3.4 |
Flagstaff | AZ | 24 | 20 | 67.9 | 2.9 |
Miami | FL | 24 | 21 | 85.3 | 1.1 |
Tucson | AZ | 24 | 20 | 90.6 | 3.7 |
Phoenix | AZ | 22 | 17 | 98.2 | 3.8 |
Fort Myers | FL | 22 | 15 | 84.8 | 1.4 |
Orlando | FL | 22 | 12 | 83.8 | 1.2 |
Tampa | FL | 21 | 15 | 85.3 | 1.3 |
METHODOLOGY
Calculating the Climate Shift Index
All Climate Shift Index (CSI) levels reported in this brief are based on daily average temperatures and ERA5 data. See the frequently asked questions for details on computing the Climate Shift Index, including a summary of the multi-model approach described in Gilford et al. (2022).
City Analysis
We analyzed 191 Applied Climate Information System (ACIS) stations associated with U.S. cities. For each city, we found the CSI time series from the nearest 0.25° grid cell. We calculated the number of days at CSI levels 2, 3, 4, and 5. We used ACIS data to find the average monthly temperatures, temperature anomalies (compared to 1991-2020 normals), and precipitation information, and to derive average monthly warming trends for each city.
Regions
Regions are defined by the National Centers for Environmental Information climate regions.