Scientists say we need a real definition for ‘heat wave’

man rests during a heat wave

Extreme heat can lead to a variety of health risks, including dehydration, hyperthermia, and even death—especially during sustained periods of high temperatures.

However, a uniform definition of a heat wave hasn’t exist, making it difficult for public health agencies to know when to activate heat alerts, cooling centers, and other protective measures. But now, scientists have developed a uniform definition of a heat wave that may help public health agencies be prepared.

“According to climate models, temperatures in Florida are predicted to increase over the next 100 years, yet there can be confusion regarding what constitutes a heat wave,” says Emily Leary, assistant research professor in the biostatistics and research design unit at the University of Missouri School of Medicine.

A uniform definition

“As temperatures rise, it’s important to have a uniform definition that best allows public health agencies to prepare for heat waves, whether that means issuing more frequent heat advisories or opening more cooling stations. Using Florida as our model—a state known for its heat—we set out to develop a data-driven definition of a heat wave that can be used for public health preparation. This formula can be adapted and applied to other parts of the country as well.”

The US National Weather Service currently initiates heat alert procedures when the heat index—the perceived temperature in relation to humidity—is expected to exceed 105 to 110 degrees Fahrenheit, depending on the area. However, the United Nations’ Intergovernmental Panel on Climate Change defines a heat wave as five or more consecutive days with maximum temperatures approximately 9 degrees Fahrenheit higher than normal. These definitions can become confusing when different sources use differing methods to define climatology norms.

Further, the definitions may not be suited for certain regions, such as Florida, because the area may have consistently high temperatures and fewer true seasons, which do not account for extreme temperatures or resident acclimation. Previous research also has shown that using local or region-specific meteorological thresholds better reflect a temperature extreme for a certain area.

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The new definition, reported in the journal PLOS ONE, factors in relative and absolute heat index thresholds for a given region and time. The temperature must exceed the 80 percent relative heat index threshold, meaning the heat index must be higher than 80 percent of the region’s temperatures for a given period. In addition, a region also should have at least three non-consecutive days with a heat index above an absolute regional heat index threshold, a predetermined temperature based on regional climates.

For example, in Pensacola, Florida, a heat index higher than 100.6 degrees Fahrenheit for three days means that the area has the potential to experience a heat wave. A heat index higher than 110 degrees Fahrenheit for three days would be considered a heat wave.

“This formula better explains when a heat wave is occurring because it accounts for missing weather data and better captures what extreme heat means for a region,” Leary says. “Because this formula uses National Weather Service regions, there also is an existing infrastructure to communicate alerts.”

The Centers for Disease Control and Prevention supported the work.

Source: University of Missouri

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