2025 Summer Package

KEY FACTS

• Meteorological summer starts June 1, and the season is heating up across the U.S. 

• Summers have warmed in 97% of 242 U.S. cities analyzed — by an average of 2.6°F since 1970.

• One in every five cities now experience at least an additional month of hotter-than-normal summer days than in 1970.

• Summers are heating up in every region — especially the Southwest and Northwest.

• Hotter summer temperatures lead to heat-related illness and worsen air quality — putting health at risk. 

This Climate Matters analysis is based on open access data from the National Oceanic and Atmospheric Administration (NOAA). See Methodology for details.

Summer heat ahead

Levels of planet-warming CO2 in the atmosphere surged in 2024, growing at the fastest annual rate on record. 

As heat-trapping pollution continues to warm the planet, summer temperatures are arriving earlier and getting hotter — and dangerous heat extremes are becoming more frequent and intense. 

Global temperatures in 2025 so far have been at least as warm as the two hottest years on record (2023 and 2024), and 2025 is virtually certain to rank in the top-five warmest years on record. 

In the U.S., summer 2025 is likely to be hotter than normal across most of the country, especially in a band extending from the Northwest through the Southwest, into the Gulf Coast states, and across the Northeast. 

Click the downloadable graphic: Average Summer Temperatures Map

Summer temperature trends since 1970

To understand how the summer season has changed locally as the planet has warmed, Climate Central analyzed the last 55 years (1970-2024) of summer (June-August) average temperature data in 242 major U.S. cities (see Methodology).

Click the downloadable graphic: Average Summer Temperatures

Summers are getting hotter in 234 U.S. cities

• Since 1970, average summer temperatures have increased in 234 (97%) of the cities analyzed. 

• Summers warmed by 2.6°F on average across these 234 cities. 

• Nearly one-third (77) of these cities have warmed by 3°F or more since 1970.

• The top five summer warming cities were: Reno, Nev. (11.3°F); Boise, Idaho (6.3°F); El Paso, Texas (6.2°F); Las Vegas, Nev. (6.1°F); and Salt Lake City, Utah (5.9°F). 

Summers are warming in every region — especially the Southwest and Northwest

• Summers have been heating up since 1970 in every region and 99% of the 3,142 contiguous U.S. counties analyzed. 

• Several regions experienced levels of summer warming above the national average of 2.6°F: the Southwest (3.7°F), Northwest (3.6°F), and South (3.0°F). 

Click the downloadable graphic: Summer Days Above Normal

Unusually hot summer days now happen more often

The warming season is also reflected in the growing number of summer days above the 1991-2020 normal summer temperature in each city. 

• Since 1970, 97% (234) of the cities analyzed have seen an increase in the annual number of hotter-than-normal summer days. 

• About 63% (152) of the locations analyzed now experience at least 14 more hotter-than-normal summer days than in 1970. 

• One in every five cities (48) now experience at least 30 more hotter-than-normal summer days than in 1970. 

• Nine cities now experience at least 50 more days above normal than they did in 1970. These cities were in: Nevada (Reno); Texas (Houston, McAllen, Tyler, Austin, and El Paso); Georgia (Albany); New Mexico (Las Cruces); and Louisiana (New Orleans).

Extreme heat is a serious health hazard

Extreme heat is a growing health risk in our warming climate. 

Excessive heat is the leading cause of weather-related deaths in the U.S. In 2023, a record 2,325 people died from heat in the U.S. alone. 

Climate change is increasing exposure to dangerous extreme heat for billions of people across the globe.

Exposure to extreme heat makes it difficult for our bodies to cool off, resulting in heat-related illnesses including heat cramps, heat exhaustion, and even fatal medical emergencies such as heat stroke. 

Summer heat can also worsen air quality by trapping harmful pollutants close to the Earth’s surface and fueling the formation of ground-level ozone. These pollutants can exacerbate respiratory health issues in people with asthma and other lung diseases. 

Heat risks unequally shared

Extreme heat can affect everyone, but some face greater risks of heat-related illness and mortality than others.

Children, adults over 65, pregnant people, and people living with illness are among those most at risk to heat-related illness. 

• Extreme Heat Risks for Children reviews how climate change is driving more frequent and intense extreme heat that is especially dangerous for children.

• Climate Change Nearly Doubled Pregnancy Heat Risk Days in U.S. shows that human-caused climate change has at least doubled the average annual number of pregnancy heat-risk days in 222 countries including the U.S. in recent years. 

Humid heat is especially dangerous for weather-exposed workers — presenting health and safety hazards, causing heat-related illness, injury, or death, and impacting livelihoods through lost labor hours and wages. 

• Rising Heat Risks for Workers shows that risky humid heat is becoming more frequent in cities across the U.S.

Structural inequities can also lead to higher extreme heat exposure in some communities. According to a 2021 study, people of color and people living below the poverty line are disproportionately exposed to urban heat island intensity in 169 of the largest U.S. cities.

Urban heat burdens are also linked to a history of racially biased housing policy. Recent analysis from Columbia University’s Grades of Heat project shows that historically redlined areas currently experience hotter summers than non-redlined areas in 150 (84%) of 179 major U.S. cities.

• Urban Heat Hot Spots in 65 Cities analyzes how and where the built environment boosts temperatures within major U.S. cities that are home to 50 million people, or 15% of the total U.S. population.

LOCAL STORY ANGLES

Is climate change influencing daily heat extremes in your local area?

Climate Central’s Climate Shift Index system provides tools, data, custom maps, and local alerts to answer this question in real-time. Here are three ways to use the Climate Shift Index this summer:

1. Use the tools. Climate Central’s Climate Shift Index map tool shows which parts of the world are experiencing high Climate Shift Index levels, every day. 

2. Access KML files to create custom Climate Shift Index maps. Fill out this form to receive CSI maps in KML format, and create custom Climate Shift Index maps. 

3. Sign up for alerts. Sign up here to receive custom email alerts when strong Climate Shift Index levels are detected in your local area.

How is extreme heat affecting public health in your local area?

The National Integrated Heat Health Information System HEAT.gov is a hub for information to support heat resilience in the U.S. Check out HeatRisk, an interactive map tool from the National Weather Service now available for the contiguous U.S. This color-numeric index shows current and forecast risk of daily local heat-related impacts. The CDC’s Heat and Health Tracker maps daily rates of heat-related illnesses across the U.S. 

What local measures are in place to protect people from heat?

The EPA maintains a Heat Island Community Actions Database of measures that communities are taking to mitigate local heat island effects. Reports from NOAA’s urban heat island mapping campaigns cover local risk reduction and adaptation strategies in 60+ U.S. cities. Track local climate-related hazards in real-time in the Climate Mapping for Resilience and Adaptation Portal, which also includes federal resources for long-term resilience planning. The NIHHIS provides heat preparedness and planning resources, including many resources in Spanish. 

CONTACT EXPERTS

Jeffrey Howard, Ph.D.
Associate Professor, Public Health
The University of Texas at San Antonio
Related expertise: Heat-related illness and morbidity in the U.S.
Contact: jeffrey.howard@utsa.edu

FIND EXPERTS

Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts. 

Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.  

Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices. 

Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area. 

METHODOLOGY

Average temperatures and days above normal were calculated for each meteorological summer (June, July, August) from 1970 to 2024 using data obtained from the Applied Climate Information System, which is developed, maintained, and operated by NOAA’s Regional Climate Centers. Summer days above normal were defined as the number of days with average temperatures above the 1991-2020 NOAA/NCEI climate normal.