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What is the Heat Dome and How Does it Affect California?

Everything you need to know about the summer’s biggest weather phenomenon

Much of the country has experienced record-setting temperatures this summer, thanks in large part to a weather phenomenon known as the heat dome. And while San Francisco and Los Angeles lucked out with mostly average temperatures in early July, much of California right now is in the midst of what the National Weather Service is calling triple digit heat. That means increased fire danger at a time when two massive wildfires have burned more than 60,000 acres in California in the past seven days. Let's break down what exactly the heat dome is and how it’s affecting weather in the state.

Hot weather in July is common. But heat domes tend to produce temperatures at least 5 to 10 degrees above average and occur during the summer months, when the jet stream moves north over the U.S. and Canada border. With the jet stream so far north, cooler air is prevented from pushing southward, and often a "dome" of high pressure traps hot air over much of the country. 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 many 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. For much of the past week, the dome was centered over the Midwest, but it’s been growing and broadening its reach. Now, in California, the weather service warns the heat dome will create low humidity levels and extreme temperatures (110 degrees near Sacramento, for example) at least through the weekend.

That’s especially bad news for a state that has struggled with almost five years of intense drought resulting in precarious fire conditions. On Tuesday, acting California governor Tom Torlakson issued emergency proclamations for two fires burning in Los Angeles and Monterey counties. Temperatures in Southern California hit 101 degrees in the past two days, fueling the Sand Fire in Santa Clarita to overtake 57 square miles—bigger than the city of San Francisco. The Soberanes fire has burned more than 23,500 acres between Carmel and Big Sur and is now bigger than the size of Manhattan.

Smoke from both fires are plaguing urban areas. According to the Los Angeles Times, the Sand fire has triggered air-quality advisories as far away as Reno, while the smoke from the Soberanes fire can be seen in Silicon Valley and even Lake Tahoe, 300 miles east. Even as firefighters work to contain both blazes, high wind warnings and the excessive heat caused by the dome will contribute to dangerous conditions through the end of the month.

Heat domes aren’t rare. But for California, the resulting heat wave is coming at an exceptionally bad time.