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It’s not just you: L.A.’s heat waves are getting more humid—and more dangerous

These muggier weather patterns create more intense heat stress than their hotter but drier counterparts according to a new NASA study.

Michael Juliano
Written by
Michael Juliano

Los Angeles may be known for its relatively pleasant temperatures throughout the year, but as any Angeleno knows, just how pleasant it is largely depends on where you live: coastal areas stay nice and cool, the rest of the basin gets a little bit warmer and the valleys get downright hot (though still cool at night).

That’s all started to change a bit, though, as L.A.’s heat waves get increasingly more humid (and longer and more frequent) due to climate change and increased moisture from the warming ocean surface. In fact, according to a new study out of NASA’s Jet Propulsion Laboratory, both coastal and inland areas of L.A. are prone to increased nighttime stress on the human body during humid heat waves. Moreover, these muggier weather patterns actually create more heat stress than their hotter but drier counterparts.

Taken all together, NASA says Southern California could experience more weather patterns that are typically associated with Texas or Florida (you remember the hurriquake, right?).

This all becomes increasingly problematic when paired with L.A.’s natural and manmade heat-trapping characteristics. And—if we can go on a bit of a tangent here—maybe the most fascinating component of this study to us was a map that NASA highlighted about the disparity in the surface temperature across Southern California. This was actually first released amid a heat wave in August 2020, but it’s worth another look with this new study in mind.

Courtesy NASA/JPL-CaltechA map of surface temperatures in L.A. County on August 14, 2020 observed by NASA’s ECOSTRESS.

The map measures the temperature of the land—as opposed to the air—thanks to data from NASA’s ECOsystem Spaceborne Thermal Radiometer Experiment (or ECOSTRESS) aboard the International Space Station. The resulting image confirms what you already know: The Valley is very hot during a heat wave, with land surface temperatures in the San Fernando Valley reaching 128.3 degrees during a humid August heat wave in 2020.

But you can also see the effects of L.A.’s urban heat islands; exposed spans of concrete and asphalt (roads and parking lots) generate considerably more heat than areas with more vegetation. It’s why Pasadena’s leafy, ritzy neighborhoods appear notably cooler than its more urban core. Or why Disneyland is clearly cooler than the rest of Anaheim, except for the sprawling Toy Story parking area, which is considerably hotter. You can very clearly see the hot grid of streets that criss-crosses the Valley (that cooler patch is the thanks to the golf courses in the Sepulveda Basin, though you can see the hot spot from the adjacent field of dry brush).

With all that in mind, NASA scientists looked at this humid August 2020 heat wave across L.A.’s microclimates and found that both inland and valley areas had elevated levels of heat stress at night. In the valleys specifically, you can normally count on relatively cooler and drier nights on even the most scorching days—but not so when there’s increased humidity. In San Bernardino and Riverside specifically, there was up to 10.8 degrees of added heat stress at night compared to non-heat wave evening conditions.

The study compared the August heat wave to one just a month later in September. Both were in the 90th percentile for August or September temperatures, but the August event was notably humid while the September one was extremely dry. Though the air temperature was as much as 18 degrees cooler during the humid August heat wave, the very high wet bulb temperature (which essentially means that it’s more difficult for our bodies to cool down by sweating) meant that the heat stress was more intense than the hotter but drier September one.

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