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.
Water vapor is actually a greenhouse gas, like carbon dioxide or methane, responsible for about half of the planet-warming effect. (It's supposed to be up there, whereas humans have been pumping in way too much extra carbon.) More warming evaporates more water, which causes more warming—a climatic feedback loop.
In landlocked areas, heat waves evaporate water from plants and soils. But humidity gets especially oppressive near the ocean, where water is more readily available. “Coastal regions in general are seeing more humid conditions as ocean temperatures warm,” says Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography, who studies humidity and heat waves. “Air sitting over a water body tends to be close to saturated. It has a lot of moisture in it—close to 100 percent relative humidity.”
Sea surface temperatures have been steadily climbing globally, as the oceans absorb something like 90 percent of the excess heat that humans are adding to the atmosphere. But since March, global sea surface temperatures have been skyrocketing above the norm. The North Atlantic, in particular, remains super hot, loading Europe’s air with extra humidity.
The waters around Florida are also logging truly astonishing sea surface temperatures: On July 24, a buoy recorded a temperature of 101° Fahrenheit. “You have incredibly warm Gulf water that warms the atmosphere, which can then absorb more moisture. So it's kind of a feedback loop,” says Kent State University biometeorologist Scott Sheridan. “In a lot of the areas around the Mediterranean, where there's been really bad heat, and then in Florida and the Gulf Coast, those have been the really big driving factors for why the humidity is so high.”
Accordingly, in Miami the heat index—a measurement that combines temperature and relative humidity—has been above 100 for over 40 days in a row, smashing the previous record of 32 days in 2020.
Meanwhile in California, Gershunov’s research has confirmed that heat waves are getting stickier. “It's not just more frequent, more intense, and longer-lasting heat waves, like is the case all over the world with the warming climate,” says Gershunov. “Here, the heat waves are also changing flavor. They're becoming more expressed disproportionately in nighttime temperatures. It turns out it's because of humidity, and that's related to the warming of the ocean.”
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