August 29, 2018

Warnings of Extreme Heat Aren't a Matter of Wimpiness—It's All About What You're Used To

The true great American pastime is putting down people from other parts of the country. Those southerners eat so unhealthily. Northerners are so rude. People in California are so snobby. These rivalries thrive when it comes to the weather. Summertime heat brings out the worst of our mockery. “Why do those northerners need a heat advisory for temperatures cooler than our normal high?” For once, the answer to that question doesn't boil down to “because they're wimps.” There really is a difference between the north and south when it comes to extreme heat or cold.

The National Weather Service issues dozens of watches, advisories, and warnings to alert us to hazardous weather and help us make decisions to keep ourselves safe. The alerts range from the mundane—something like a fog advisory—to the most urgent tornado warnings.

Some of these alerts are the same all around the country and some vary from county to county. Alerts like tornado warnings and severe thunderstorm warnings are static. A severe thunderstorm warning is issued for a thunderstorm capable of producing hail the size of quarters or larger and/or 60 MPH wind gusts. It's the same whether you're in Seattle, Washington, or Mobile, Alabama.

Then there are relative alerts. A winter weather advisory is a relative alert. It only takes a dusting of snow in Pensacola, Florida, to trigger a winter weather advisory, but it takes 4 inches of snow in 12 hours to meet the criteria for a winter weather advisory in Cleveland, Ohio.

Extreme cold and extreme heat are relative because it's all about what you're used to. We're all acclimated to different temperatures. It's easier for someone in North Carolina to suffer through three months of heat and humidity than it is for someone in Atlantic Canada to deal with it for a few days. 'Stifling' is the default setting somewhere like Tampa for much of the year. If places like Louisiana or Texas followed the same heat advisory guidelines as Michigan or Vermont, they'd be under a heat advisory all day and all night for months on end. It would be meaningless.

A heat advisory is issued when an expected period of hot temperatures—either air temperature or heat index—could pose a risk to those who are sick, elderly, or working outside for an extended period of time. An excessive heat warning is issued when dangerously hot temperatures are in the forecast that could quickly cause heat-related illnesses to set in for even healthy individuals.

The heat index is the temperature it feels like to your body when you take into account the amount of moisture in the air. Your body cools off less effectively when there's more moisture in the air, making it feel much hotter—and allowing heat illness to set in more quickly—when it's more humid.

I did my very best to map out the heat index required for a heat advisory for areas east of the Rocky Mountains. The criteria for a heat advisory is different across the country. It's pretty hard to map out the exact criteria because it's subjective for some offices and the temperatures needed can change quickly across short distancse.

Many NWS offices—especially in the south and east—have their criteria helpfully laid out in various places online (see here, here, and here). Some of the heat index requirements on the above map are inferred based on the text of past heat advisories and the requirements of the surrounding offices. My piecemeal map works as a good illustration for these purposes.

It only takes a heat index of 95°F or higher for a heat advisory to be issued in much of New England. The heat index (or air temperature) required for a heat advisory slowly rises the farther south you go. The criteria reaches 108°F along the Gulf Coast and 110°F in desert areas of the southern Plains. (The 110°F requirement in parts of South Carolina and Georgia is due to these areas routinely seeing some of the hottest and muggiest days along the East Coast.)

The heat index routinely climbs above 100°F during the summer months in Miami. Using New England's criteria, a heat advisory would be issued for Miami almost every day for months on end. But residents of the city are acclimated to the heat. Even vulnerable populations—such as the elderly and outdoor workers—mostly know how to handle the heat in a way that doesn't make them sick.

On the other hand, wind chills dipping into the lower 30s is all it takes for a wind chill advisory in Miami. It takes a wind chill of -15°F and -24°F for at least three hours for NWS Boston to issue a wind chill advisory. It's all a matter of what you're used to.


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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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