New Global Study: Dangerous Nighttime Heat Rising in 83% of Global Cities Analyzed, Becoming More Oppressive, And More Frequent 

Rio De Janeiro, 6 November 2025, — From the COP 30 Local Leaders Forum - a critical gathering of global mayors focused on climate action- a sweeping new scientific analysis was released of the most dangerous summer weather conditions across 100 major global cities. It revealed that while leaders have been focusing on avoiding breaching the +1.5°C threshold of the Paris Agreement, in many global cities minimum nighttime temperatures have been rising much more rapidly during oppressively hot weather situations than daytime average highs, up to 10 times faster in some cities.

​​

The study by Climate Resilience for All, Extreme Heat and the Shrinking Diurnal Range: A Global Evaluation of Oppressive Air Mass Character and Frequency, analyzed weather data over a 30-year period from 1994 to 2024, isolating the two types of weather conditions, or “air masses”, considered most dangerous for human health: “dry tropical” (DT) weather, which is hot and dry, and “moist tropical” (MT) weather which is hot and humid.  

​

Heat exposure has traditionally been measured by exposure to daytime high temperatures and increasing “average” temperatures. This study points clearly to the urgent need for preventative and responsive actions on extreme heat—including early warnings, public health measures, and adaptation planning—to explicitly account for and address the rapidly rising threat of hotter nights. 

​

This analysis is the first to examine how humidity, daytime, and nighttime temperatures are changing within dangerous summer air masses; how their frequency is shifting; and how these trends vary across climate regions, making a significant contribution to the science and helping inform urban heat and health interventions for practitioners everywhere. 

​

The analysis found:  

​

Increases in nighttime temperatures, and decreases in the gap between daytime highs and nighttime lows across much of the globe 

​

• 83% of cities in the study are experiencing sustained, higher nighttime temperatures. 

• Nighttime temperatures are rising fastest in Melbourne, Australia (dry tropical), where they increase by 1°C every 5.36 years, and Dubai, UAE (moist tropical), where they rise by 1°C every 8.81 years. 

During moist tropical weather, Santa Maria, Upington, Seoul, Samarkand, Paris, Kuwait City, Portland, and Abadan are seeing the biggest decrease between daytime and nighttime temperatures. The number of cities seeing decreases per region breaks down as follows: 

​

• Africa: 13 out of 15.  

• Asia: 18 out of 22. 

• Central and South America: 10 out of 11.

• Europe: Seven out of 12. 

• Middle East: 5 out of 5. 

• North America: 14 out of 16. 

• Oceania: Nine out of 11.

​During dry tropical weather, Melbourne, Agadir, Seoul, Mumbai, Cairo, Luxor, Kuwait City, and Santiago are seeing the biggest decrease between daytime and nighttime temperatures. The number of cities seeing decreases per region breaks down as follows:

​

• Africa: 10 out of 14. 

• Asia: 13 out of 22. 

• Central and South America: Seven out of 11. 

• Europe: Four out of six. 

• Middle East: Six out of seven. 

• North Americas: 11 out of 14 

• Oceania: Five out of nine. 

• Some of the regions show weaker differentiation, possibly because dry tropical weather types are rarely present in the cities we evaluated in those regions. 

Increases in the frequency of extreme heat days 

​

• Over the 30-year study period, summertime moist tropical weather patterns have increased close to or over 50 percent in Central and South America, Oceania, and Africa - and have grown by 37 percent globally. 

• Dry tropical weather patterns have grown by 13 percent over the same period, with the largest increase in Australia, which had a 29 percent rise. 
   

Global, regional, and city level data visualizations for the cities in the study can be accessed via this link.   

​

“Before this analysis, we did not know how rapidly nighttime heat has been rising within the most dangerous air masses,” said Larry Kalkstein, climatologist, Chief Heat Science Advisor at Climate Resilience for All, and the study's lead author. “It is critical for us to understand how the heat of summer—that sends people to the emergency room—is shifting, and what we are overlooking when we talk about it.”  

​

Mayor Yvonne Aki-Sawyerr of Freetown, Sierra Leone, leading a city on the frontlines of extreme heat, said, “This study reflects the lived reality of so many cities in the global south. Most of our residents lack access to cooling, and women market traders facing extreme heat all day return to homes that are dangerously hot at night. As we implement our heat action plan, this study sounds the alarm on the hidden threat of nighttime temperatures and pushes us toward solutions that drive awareness and protect people.” 

​

“We want this analysis to mobilize city and health leaders to urgently broaden their view of what is a 24-hour heat crisis. This research uncovers a critical blind spot in our understanding of extreme heat,” said Kathy Baughman McLeod, CEO of Climate Resilience for All. 

​

High nighttime temperatures prevent the human body from cooling down, increasing risks of heat exhaustion, dehydration, and cardiovascular stress. When sleep is disrupted by heat, the body loses its ability to recover from daytime exposure, heightening the danger of illness and death—especially for older adults, women, and those living in poorly ventilated housing. 

​

Heat warning systems are focused on high daytime temperatures and currently minimize the impact of overnight temperatures. The study offers guidance and urges health officials and policymakers to integrate these changing patterns into their work and to ramp up regionally targeted heat warning systems that account for the growing probability of multi-day, high-intensity events that offer little nocturnal relief. 

​​

END​

​

About the Study 

While most meteorological studies look at all weather types (air masses) together to determine whether the earth has warmed or cooled, this groundbreaking global analysis used the Spatial Synoptic Classification to isolate just those weather types that we know are most dangerous to human health.  

​

About Climate Resilience for All 

Climate Resilience for All is a global adaptation NGO dedicated to protecting the health, income, and dignity of women on the frontlines of extreme heat. CRA partners with local organizations, bringing expertise, capacity, and resources to co-design and deliver practical, scalable solutions for a hotter world. 

​​

Media Contacts: 

Geraldine Henrich-Koenis, geraldine@climateresilience.org 

Kelechukwu Iruoma, kelechukwu@climateresilience.org