Climate MattersJuly 27, 2022

Summer Air: Hot, Stagnant, Polluted

CM: Stagnant Air, National


  • Hot, still, dry weather can lead to a build-up of pollutants in the air we breathe—a process called stagnation. 

  • 83% of 241 U.S. cities analyzed had an increase in the number of summer stagnant days (6 more days on average) since 1973.

  • Summer high temperatures and stagnation are closely linked in 98% of the 241 U.S. cities analyzed.  

  • Rising summer temperatures due to climate change will likely bring more stagnation days—and more health risks.

Check out this article about the first 10 years of Climate Matters, published this week in Nature Climate Change

Sweltering summer days when the pavement bakes and the air is still—those are the days when air quality can plummet. Hot, still, dry weather can lead to a build-up of pollutants in the air we breathe. 

This phenomenon is called stagnation and it can have serious health consequences, especially during summer. New Climate Central analysis shows that hundreds of U.S. cities have already seen an increase in summer stagnation. 

CM: Stagnant Air, National
Change in Annual Stagnation

More summer stagnant days 
Climate Central looked at the change in summer stagnation using the NOAA/NCEI Air Stagnation Index—which incorporates upper atmospheric winds, surface winds, and precipitation to calculate the daily level of stagnation. 

  • From 1973 to 2021, 83% of the 241 U.S. cities analyzed experienced an increase in the number of summer stagnant days.

  • Across these 200 cities, the average increase was 6 more stagnant summer days since 1973.   

  • California is home to the top five cities for increasing stagnant summer days: San Francisco (+32 days), San Jose (+30 days), Monterey (+27 days), Sacramento (+26 days), and Stockton (+24 days).  

CM: Stagnant Air, Local
Summer Heat and Stagnation

Summer heat and stagnant days are closely linked
The links between summer high temperatures and summer stagnation are clear in Climate Central’s analysis. 

  • Since 1973, 98% of the 241 U.S. cities analyzed show a positive correlation between summer high temperatures and the number of summer stagnant days. 

  • The relationship between heat and stagnation was especially strong in Cincinnati, Ohio, Fresno, Calif., Fort Wayne, Ind., and Reno, Nev.

One consequence of climate change is more summer heat, which could mean more summer stagnation events and worse air quality in the future. 

More annual stagnant days
Stagnation isn’t only a summer issue. It’s also becoming more common throughout the year over most of the U.S. 

  • Since 1973, the number of annual stagnant days has increased in 81% of the 241 U.S. cities analyzed. 

  • Across these 197 cities, the average increase was 10 more stagnant days per year.

  • Southern Texas and coastal California are the largest national hotspots for more annual stagnation days, as are the areas around New Orleans, La. and Salt Lake City, Utah. 

  • The mountainous Northern Rockies and Plains, along with neighboring Idaho and Colorado have seen an overall decrease in annual stagnant days since 1973.

More stagnant days bring more health risks 
Rising summer temperatures will likely bring more stagnation days. These stationary domes of hot and dry air trap pollutants in the lower atmosphere, putting human health at risk. 

Although national air quality has improved dramatically since the Clean Air Act of 1970, the American Lung Association’s latest State of the Air report found that 4 in 10 people in the U.S. live in places with unhealthy levels of fine particulate pollution or ozone—some of the dangerous pollutants that stagnant air traps. 

Communities of color are disproportionately exposed to air pollution in 202 U.S. cities, where present pollution exposure disparities are associated with discriminatory housing policies from the 1930s. As the climate warms, these inequitable health risks could worsen. Stagnant days are projected to increase further, with up to 40 more days per year by late-century.


What’s the air quality in your local area, and how does it compare with other cities and states?
The EPA’s AirNow site includes tools to search current air quality by city, state, and zip code. The site also includes interactive air quality maps, extensive resources in English and Spanish, including those focused on air quality and health. The American Lung Association’s 2022 State of the Air page includes features to search air quality data by city and zip code. The site also includes report cards on the cleanest and most polluted cities as well as a tool to compare air quality between two cities. 

How does air pollution in the U.S. compare with other countries? 
A June 2022 review in The Lancet found that over 90% of pollution-related deaths occur in low-income and middle-income countries. The State of Global Air’s interactive features can be used to explore global air pollution data and create custom maps. The site includes global map views of fine particulate pollution and ozone exposure.


The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists who have expertise on air quality and climate change. The American Association of State Climatologists is a professional scientific organization composed of all state climatologists


Rupa Basu, PhD, MPH
Chief, Air and Climate Epidemiology Section
Cal EPA/Office of Environmental Health Hazard Assessment (OEHHA)
Related expertise: health effects from high temperatures and air pollution
*Available for interviews in English and Bengali

Beatriz Cardenas, PhD
Global Lead, Air Quality and Director, Air Quality WRI Mexico
Related expertise: air quality science and policy
*Available for interviews in English and Spanish

Jessica Seddon, PhD
Senior Fellow, Air Quality
WRI Ross Center for Sustainable Cities
Related expertise: climate and clean air


Climate Central analyzed gridded data from the NOAA/NCEI Air Stagnation Index, calculating trends in annual stagnation days since the dataset began in 1973. Local correlation plots were created using annual average summer (June, July, and August; 1973-2021) maximum temperature data from the Applied Climate Information System and summer stagnation data from the Air Stagnation Index. 

Climate Central’s local analyses typically include 247 stations, including 5 stations in Alaska, Hawaii, and Puerto Rico. However, stagnation data were only available for the 242 stations in the contiguous U.S.