2024 saw a record-breaking number of dangerously hot and humid days

The planet experienced a record number of dangerously hot, humid days in 2024 as climate change increases global humidity to unprecedented levels.

The global average number of high humid heat days over land exceeded the 1991-2020 average by 35.6 days last year, 9.5 days more than the previous record set in 2023, according to the State of the Climate report 2024 published by the American Meteorological Society.

In hot, humid conditions, people struggle to cool down, as the moist air smothers the evaporative cooling effect of sweating. This makes such weather extremely hazardous to human health, says Kate Willett at the UK Met Office, who worked on the report. “Your body starts to really struggle to offload heat, and so it can be really dangerous,” she says.

Meteorologists measure heat and humidity using a “wet bulb temperature”. Traditionally this is done by sliding a wet cloth over the bulb of a thermometer to account for the cooling effect of evaporating water, so it records a lower temperature than one without this set up, which is known as a dry bulb, or dry air, thermometer. When humidity levels in the atmosphere are higher, the cooling effect from evaporation is more limited, bringing the wet bulb temperature closer to the dry air one.

As the planet warms, the atmosphere is able to hold more moisture, producing not only more intense rainfall and storms, but also higher levels of humidity. Willetts says 2024 was an “exceptionally” humid year, and second only to 2023 for the intensity of humid heat experienced.

Some parts of the world, such as the Middle East, South-East Asia and eastern China, saw wet bulb temperatures exceed 29°C (84°F) multiple times, and even hit 31°C (88°F) for short periods, the report shows. At this level, prolonged time outside is considered extremely dangerous – even deadly – to human health.

Traditionally, scientists have considered a wet bulb temperature of 35°C as the limit for human survivability, where people can’t cope outside for more than a few hours before perishing. But research published in 2022 suggests the actual threshold may be much lower, at around 31°C. “30°C and above is where your body is really struggling,” says Willett.