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The heat started in the South and Southwest — baking Phoenix in at least 110-degree Fahrenheit temperatures for 11 straight days — and will migrate into the Midwest, East Coast, and parts of the South this weekend and beyond, triggering warnings of "Excess Heat" from the National Weather Service. The heat index, or what it feels like, will reach over 105 F in many places.

The culprit is a heavy mass of air, called a ridge or "heat dome," that's moving over the Lower 48. "Heat dome" is an apt word because it describes a large mass of air pushing down on Earth's surface, explained Greg Carbin, a National Weather Service meteorologist. Importantly, this region of high atmospheric pressure heats itself up, because the sheer weight of the air sinks down and inevitably compresses, creating heat.

If you add a heat dome to already warm (or hot) summer temperatures, "then you put the ingredients together for a heat wave," Carbin told Mashable.

The heat won't be short-lived. For example, the hot weather is expected to arrive in large parts of the Mid-Atlantic states on Saturday and continue through at least mid-week.

"That's a fairly long run of hot temperatures," saidJeff Weber, a research meteorologist at the University Corporation for Atmospheric Research. "When you get into the three-four-five day heat wave tier is when it becomes dangerous for humans."

That's because when a heat dome sticks around, it amplifies itself even more, exposing people to high temperatures day and night. The nighttime doesn't have a chance to really cool off, so the following day starts anew with warmer temperatures. "As the heat continues it aggregates," explained Weber.

"It’ll be very unpleasant," he added.

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This heat dome will linger because of "stagnancy" in the atmosphere, explained the National Weather Service's Carbin. There's just not much weather flowing across the U.S., and the jet stream — a band of powerful, high atmospheric westerly winds — has shifted north into Canada.

The heat wave will almost certainly break some daily temperature records, as it did in New Orleans this past week. But what will really be notable is the duration of the heat.

"The persistence is the remarkable thing here," Flavio Lehner, a climate scientist at the National Center for Atmospheric Research, told Mashable.

Any heat wave today, however, is warmer than it would be without boosted background temperatures, due to continued human-caused climate change. That's why both all-time and daily high temperature records far outpace low temperature records: 364all-time high temperatures were set in 2019, versus just 70all-time lows. Since the late 1850s, humanity has warmed Earth by a little over 1 degree Celsius (1.7 degrees Fahrenheit), though landmasses have, on average, heated up more, by over 1.5 C.

So today's heat waves will get an added boost, explained Lehner, though just how much depends on the region. (The Arctic, for example, has warmed around three times faster than the rest of the planet, so this polar region will typically get a bigger kick from warming simply because it's heated up significantly more.)

As Earth continues to warm — due to skyrocketing levels of carbon dioxide in the atmosphere — climate scientists expect heat waves to increase in frequency, intensity, and size. Already, U.S. cities are experiencing more heat waves than occurred over half a century ago, in the 1960s. Climate change doesn't make extreme events like storms, drought, and heat waves — it makes them worse.

That doesn't bode well for humans, especially when it comes to heat waves. Among weather events in the U.S., heat waves kill the most people.

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