Climate Matters•June 28, 2023
Sweltering Summer Nights
KEY CONCEPTS
Sweltering summer nights limit our ability to cool off from scorching summer days, prolonging heat stress and related health risks.
Since 1970, average summer minimum (nighttime) temperatures have warmed in 230 U.S. locations analyzed by Climate Central—by 3°F on average.
Use Climate Central’s Climate Shift Index tool, now available in KML format, to quantify the influence of climate change on sweltering nights in your local area this summer.
Scorching days, sweltering nights
As our climate warms, summer is heating up across the U.S. But summer warming isn’t all about scorching days—it’s also about sweltering nights.
Summer temperatures are also rising after the sun goes down. And when nights don’t cool off enough relative to peak daytime temperatures, people also have a harder time cooling off.
Sweltering nights can prolong heat stress and related health risks during the hottest time of year. Heat is the deadliest weather-related hazard in the U.S., and warming nights worsen heat-related health risks.
Summer nights warming faster than days
Summer minimum (nighttime) temperatures across the U.S. have warmed on average at a rate of 1.64°F per century since records began in 1895. That’s nearly twice as fast as the warming rate observed for average U.S. summer daytime highs over the same period.
Recent summer records are consistent with this long-term trend.
Average summer nights in 2021 and 2022 rank as the two warmest on record for the contiguous U.S.
During 2022, 66% of the contiguous U.S. (by area) experienced extremely warm summer nighttime temperatures. That’s more than double the area exposed to extremely warm summer days (28%). Here, extremely warm summer nights and days are those in the upper tenth percentile of the local summer temperature distribution over the period of record.
Also in 2022, 102 counties across the contiguous U.S. set records for hottest summer nights, while 19 counties set record summer daytime temperatures.
And since the start of summer 2023 (June 1), the U.S. has already set nearly twice as many records for extremely hot nights than for extremely hot days.
Summer nights heating up across the U.S.
Climate Central analyzed average summer minimum (nighttime) temperatures from 1970 to 2022 in 247 U.S. locations. Summaries below are based on 241 locations (see Methodology).
Summer nights have warmed by 2.7°F across the contiguous U.S. since 1970.
Since 1970, average summer minimum (nighttime) temperatures have warmed in 230 (95% of 241) U.S. locations analyzed by Climate Central.
The average nighttime warming across these 230 locations was 3°F.
Summer nights have warmed by 4°F or more in one-quarter of these 230 locations.
Grouping all 241 locations by region, the greatest summer nighttime warming has occurred across the western U.S:
Southwest: summer nights warmed 3.8°F on average, across nine locations
West: summer nights warmed 3.5°F on average, across 17 stations
Northwest: summer nights warmed 3.5°F on average, across 16 stations
Locations that experienced the most summer nighttime warming since 1970 were: Reno, Nev. (17.4°F); Las Vegas, Nev. (9.6°F); El Paso, Texas (8.2°F); Salt Lake City, Utah (7.7°F); Boise, Idaho (7.1°F); and Medford, Ore. (7.1°F).
Sweltering nights in a warming world
Our warming climate makes heat extremes more frequent and intense in general—but this is happening especially at night.
Warming increases evaporation from the land surface, and a warmer atmosphere can hold more of that evaporated moisture, leading to higher humidity and more cloud cover.
The resulting clouds can reduce daytime warming by reflecting some incoming heat from the sun back into space.
But after the sun goes down, clouds can amplify nighttime warming by absorbing heat from the land surface and re-emitting that heat back down toward the ground.
Parts of the globe that have experienced faster nighttime warming have also experienced increased cloud cover, specific humidity, and precipitation, likely due to the intensification of the water cycle in our warming climate.
Fingerprints of climate change on hot summer nights
Scorching days followed by sweltering nights limit the body’s ability to efficiently cool off without costly and energy-intensive air conditioning.
But according to a 2020 study, extremely hot summer days followed by extremely hot nights have become significantly more frequent and intense across the Northern Hemisphere from 1960 to 2012, mainly due to carbon pollution.
The same study found that co-occurring extremely hot summer days and nights are likely to happen four to eight times more often by 2100 (relative to 2012 levels) if moderate to very high levels of carbon pollution continue.
Climate Central’s Climate Shift Index tool provides real-time estimates of the influence of carbon pollution on daily high and low temperatures across the U.S.
Risks and unequal burdens of nighttime heat
Sweltering nights limit our ability to cool off and recover from extremely hot summer days. This can lead to greater heat stress and related health risks—especially among people with chronic illness, young children, the elderly, outdoor workers, and other heat-vulnerable populations.
These risks compound when extremely hot days and nights occur in historically cooler climates where people are not accustomed to these events, in urban heat islands, and in neighborhoods with lower-income and more non-white residents, which face disproportionate exposure to urban heat extremes across the U.S.
More warm nights also mean higher demand for air conditioning, which is less accessible to low-income households. A 2021 Climate Central analysis found that 235 U.S. locations have experienced rising cooling demands since 1970 due to rising temperatures.
Higher cooling demand means higher energy bills and higher risks among households that lack or cannot afford to run air conditioning systems. Running air conditioners can further increase cooling demand both in the short term (as waste heat increases outdoor air temperatures) and in the long term (due to additional carbon pollution from running a rapidly-growing number of air conditioners globally).
LOCAL STORY ANGLES
How is climate change influencing nighttime temperatures today?
Climate Central’s Climate Shift Index tool, based on peer-reviewed research, provides real-time estimates of the influence of climate change daily high and low temperatures across the U.S. Access the free tool to see how much climate change has altered current and forecasted daytime and nighttime temperatures in your local area.
How is extreme heat affecting public health near you?
The Centers for Disease Control and Prevention Heat and Health Tracker maps heat-related illnesses at the census-tract level in real time. Use the National Integrated Heat Health Information System (NIHHIS) Heat Equity Mapper to find census tracts in your area with the highest heat burden. Check the U.S. Department of Health and Human Services monthly Climate and Health Outlook for extreme heat outlooks in your region throughout the summer.
What measures are local officials taking to protect people from heat?
Reports from NOAA’s urban heat island mapping campaigns cover local risk reduction and adaptation strategies in 60+ U.S. cities. The NIHHIS provides heat preparedness and planning resources. Find your state's emergency management agency to learn more about heat action plans in your area. Check with local officials for available cooling centers. The Low Income Home Energy Assistance Program (LIHEAP) aims to alleviate extreme heat stress for vulnerable populations. Explore LIHEAP data and mapping tools to see how and where historical funding has been allocated.
CONTACT EXPERTS
Rachel Morello-Frosch PhD, MPH
Professor
University of California, Berkeley
Related expertise: Environmental justice, environmental health disparities
Contact: rmf@berkeley.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
Summer (June, July, August) average minimum temperature trends were computed using data from the Applied Climate Information System and applying a mathematical linear regression. The national graphic used contiguous U.S. data from NOAA/NCEI Climate at a Glance. Climate Central's local analyses include 247 stations. Data summaries based on linear trends include 241 stations due to large data gaps in Dothan, Ala.; Hazard, Ky.; Jefferson City, Mo.; Terre Haute, Ind.; Twin Falls, Idaho; and Wheeling, W.Va.