A new hourly analysis of extreme heat reveals that hot hours are increasing faster than hot days, especially at night and in the tropics. Even days that do not qualify as "hot days" still see an increase of several hot hours, mostly at night, which extends heat stress on the body.

Currently, the average hot day contains about 16 hot hours, increasing to 19 hours in a medium emissions scenario and 21 hours in a high scenario. This means the miserable part of the day gets longer, reducing relief hours.

The trend is strongest in low-income countries. Globally, those born in 2023 would experience about 1.4 to 1.8 times as many hot hours compared to those born in 1981, depending on emissions.

The researchers emphasize that hourly-scale assessments are essential for anticipating health impacts, informing equitable adaptation strategies, and designing early warning systems for round-the-clock heat exposure.