Increasing levels of humidity are here to make heat waves even worse

Enlarge / A tourist refreshes at a vapor barrier in Budapest, Hungary, on July 16, 2023. (credit: Arpad Kurucz/Anadolu Agency via Getty Images)

Because you’re a smooth-skinned mammal, no weather feels quite as oppressive as a humid heat wave. The more water vapor in the air, the less efficiently your sweat can evaporate and carry excess heat away from your skin. That’s why 90° Fahrenheit in humid Miami can feel as bad as 110° in arid Phoenix.

Climate change has supercharged this summer’s exceptionally brutal heat all around the world—heat waves are generally getting more frequent, more intense, and longer. But they are also getting more humid in some regions, which helps extend high temperatures through daytime peaks and into the night. Such relentless, sticky heat is not just uncomfortable, but sometimes deadly, especially for folks with health conditions like cardiovascular disease.

One of the more counterintuitive effects of climate change is that a warmer atmosphere can hold more water vapor than a colder one. A lot of it, in fact: Each 1.8° Fahrenheit bump of warming adds 7 percent more moisture to the air. Overall, atmospheric water vapor is increasing by 1 to 2 percent per decade. That additional wetness is why we’re already seeing supersize downpours, like the flooding that devastated Vermont earlier this month.