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The Heat Dome: Why It’s Been So Hot on the East Coast

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Everything you need to know about the record-breaking heat

Over the past week, the East Coast has been hammered with temperatures well over 90 degrees for days on end. In New York City, residents fled to city pools and nearby beaches to try to beat the heat, while in Washington D.C., locals braved 100-degree temperatures for the first time since 2012. Excessive heat warnings also hit Boston and Philadelphia, where the Democratic National Convention is in full swing.

Hot weather in July is common. But this week’s record temperatures are thanks to something called a "heat dome." In the summer months, the jet stream moves north over the U.S.-Canada border and prevents cooler air from pushing southward. Often a "dome" of high pressure traps hot air over much of the country, resulting in temperatures at least 5-10 degrees above average. As the hot air sinks under the dome, it gets warmer and warmer and more difficult to dislodge, often preventing thunderstorms from producing rainfall. Whenever the U.S. experiences a summer heat wave (three or more days of at least 90-degree weather) across multiple regions, heat domes are usually the culprit.

On July 24, at least 26 states had heat warnings or advisories in effect, stretching from California to New York. A day later, Washington was the only state in the lower 48 that did not see temperatures in at least the 90s. As is typical, for much of the past week the dome was centered over the Midwest. But just because it’s located over the middle of the country doesn’t mean its effects are limited to that area. The persistence of the high pressure ridge created by the dome means that a heat dome affects weather throughout the country—and the longer it persists, the higher the temperatures.

In the East Coast’s biggest cities, this pattern has brought oppressive temperatures and humidity. Urban residents already face warmer temperatures than their rural counterparts thanks to the urban heat island effect—i.e., the asphalt, concrete, and buildings of cities retain heat throughout the day, resulting in temperatures 2-10 degrees warmer than nearby rural areas. Throw in the oppressive temperatures caused by a heat dome, and it can make cities feel downright inhabitable. The recent heat dome has also pushed the Northeast into a prolonged dry period. According to the U.S. drought monitor, much of the region is now categorized as abnormally dry, in a moderate drought, or in a severe drought.

Relief should come this weekend, when temperatures return into the 80s. But the 10-day forecast hints that while the heat dome might break down over the next few days, we haven’t seen the last of it this summer.