What Causes a Heat Wave?

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Try as you might, there are few ways to escape a heat wave. There are only so many fans you can turn on and so much ice cream you can devour before the high temperatures catch up to you.

It's especially difficult because heat waves can cover a wide geographic area, but, then again, heat waves are all relative, so you might be able to find some relief if you travel far enough.

Of course, if the entire world is experiencing a heat wave, well, there's not a lot you can do.

The Science of a Heat Wave

For all the suffering they cause, heat waves are remarkably simple things. Indeed, according to the U.S. National Weather Service, the 10-year average for heat-related deaths between 2005 and 2014 was 124, more than all other weather-related disasters combined.

Apart from what causes them, there's no set definition of a heat wave. Generally, a heat wave occurs when there are multiple days or weeks of higher-than-usual temperatures for an area during a specific time of year. After all, a week of 95-degree Fahrenheit (35 degrees Celsius) July days in Maine would be something to note, but the same temperatures in a week in Texas? Not so much. That being said, heat waves can occur in Texas, too, if the high and low temperatures exceed what's usual for the state during a particular period of time.

Heat waves occur wherever a mid-level high-pressure system develops over an area. This high-pressure system forms a "cap" over the affected area, trapping heat that would otherwise rise into the air to cool off before circulating back to the surface. This reduces the chance for precipitation to form, and the result is just a continual buildup of heat.

An illustration of a high pressure system creating a heat wave
High pressure in the middle layers of the atmosphere acts as a 'cap,' allowing heat to build up at the Earth's surface. U. S. National Weather Service/Wikimedia Commons

These high-pressure systems are slower to change during the summer. This means the heat can linger on and on, and why temperatures may not cool off enough at night to offer a sense of relief.

Moving a high-pressure system relies on strong currents of air that encourage them to move along. The U.S. heat wave of 2012, during which more than 8,000 warm temperature records were broken, stayed put over the middle of the country because winds that normally pull such systems eastward were weak during late June and early July. The 2018 global heat wave is a chain of heat waves happening all over the world, lingering in some areas for months due to a variety of factors, including a weakened jet stream, which is perhaps most at fault.

Can We Blame Hotter, More Frequent Heat Waves on Climate Change?

People carry umbrellas on a hot, sunny day in Tokyo
The 2018 heat wave in Japan has claimed more than 60 lives. The government has declared the heat wave a natural disaster. Martin Bureau/AFP/Getty Images

No, but also yes.

Weather is a complicated, messy thing. Human activity, the orbit of the planets, and ocean currents can all play a part in our weather. So it will probably be a while until we can say we with certainty that climate change is responsible for certain weather phenomena. It's more accurate to say climate change increases the risk of severe weather versus saying that it causes extreme weather.

With the apparent exception of heat waves.

"Heat waves are easy," Michael Wehner, a senior staff scientist at the Lawrence Berkeley National Laboratory, told Slate. "For pretty much everywhere in the world ... climate change has increased the severity of heat waves."

Science pretty much backs up Wehner's position. A study published in Nature Climate Change concluded that climate change caused by human-produced greenhouse gases would overtake natural variability as the main cause of heat waves in the western U.S. by the late 2020s — a mere 10 years away — and by the mid-2030s in the Great Lakes Region.

Here's where the interconnectedness of it all comes into play. Remember the weakened jet stream mentioned in regards to the 2018 heat wave? That stream is propelled by cold air descending southward from the Arctic and warm air rising northward from the equator. The greater the difference in their respective temperatures, the more powerful the jet stream is. When the difference between the two forces of air is minimal, the jet stream moves less. When it moves less, there's less push or pull on pressure systems, like the ones sitting over England and Northern Europe, to move them along.

And why is there less of a difference between the two forces of air? The Arctic is warming twice as fast as the rest of the planet, so there's a steady supply of warmer air already at the Arctic.

So maybe it's time to go buy another pint of ice cream.