The planet faced an unprecedented number of perilous hot and humid days in 2024, as climate change heightened global humidity levels to new extremes.

The worldwide average of humid heat days exceeded the 1991-2020 baseline, reaching 35.6 days last year—a rise of over 9.5 days compared to the previous high recorded in 2023. Climate Report 2024 Status as published by the American Weather Society.

During hot and humid weather, cooling becomes challenging as moist air hinders the body’s ability to cool itself through sweating. This makes such conditions exceptionally hazardous for human health. Kate Willett, who contributed to the report at the UK Met Office, states, “Your body starts to really struggle to offload the heat, so it’s really dangerous.”

Meteorologists track “wet bulb temperature” to assess heat and humidity. This involves utilizing a wet cloth on a thermometer bulb to illustrate the cooling effect of evaporation. High humidity diminishes the evaporation’s cooling effect, causing wet bulb temperatures to approach those of dry air.

As global temperatures rise, the atmosphere can retain more moisture, leading to not only stronger rainfall and storms but also heightened humidity levels. Willett notes that 2024 is “exceptionally” humid and ranks just behind 2023 in moisture content.

Certain regions, including the Middle East, Southeast Asia, and East China, have experienced extreme temperatures of 31°C (88°F) over short periods, with wet bulb temperatures surpassing 84°F multiple times, as indicated in the report. At such levels, prolonged exposure is deemed extremely hazardous and potentially fatal to human health.

Historically, scientists have considered a wet bulb temperature of 35°C as the survival threshold, beyond which individuals cannot endure outdoor conditions for more than a few hours without dire consequences. However, recent research published in 2022 suggests that the actual limit may be significantly lower, around 31°C. “Over 30°C is where your body really struggles,” Willett explains.