What is the heat dome causing record-breaking temperatures in the US?

Death Valley in California could record temperatures up to 54°C

A sign warns of high high temperatures in Phoenix, Arizona. AP
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The US South-West is broiling under record-breaking temperatures as a heat dome threatens to set historic highs through next week.

More than 111 million people across the US were under extreme heat advisories and warnings, the National Weather Service (NWS) said. The agency also reported that 27 million people across the lower 48 US states are forecast to experience a heat index above 43°C next week.

The city of Phoenix has been particularly affected by the latest heat dome. In the Arizona city temperatures have reach 43°C or higher for the past 13 days. The longest stretch Phoenix has endured similar temperatures was 18 days in 1974.

Phoenix set a new “daily record low” with a low temperature of 34°C on Wednesday morning.

Other cities across the US South-West could also face record-breaking heat.

California's Death Valley, which holds the record for the highest temperature yet recorded on Earth, could face temperatures of 54°C this weekend. The US national park recorded similar temperatures in 2020 and 2021. A 65-year-old man died last week after his car broke down in Death Valley, officials said.

Las Vegas's all-time record of 47°C could also be matched this coming weekend.

Other cities in the south-western US including Reno, Fresno and Salt Lake City may also experience temperatures nearing record highs.

What is a heat dome?

A heat dome occurs when high-pressure atmospheric conditions trap heat over an area. The atmospheric conditions are combined with high winds caused by La Nina, according to the National Oceanic and Atmospheric Administration.

This high pressure also causes air to sink. As the air sinks it warms, leading to higher temperatures. And since the air cannot escape it creates a dome, with temperatures getting hotter and hotter.

The area trapped under the dome can stretch across several countries and linger for weeks.

Most records are set during heat domes, which can result in dangerously high temperatures.

More than 2,000 people in India were killed in 2015 during a period of intense heat that lasted several weeks. One of the deadliest heatwaves yet experienced was in 2003, when searing conditions resulted in approximately 30,000 casualties across Europe between July and August.

Climate change is a driver in the increasing frequency of heatwaves. A 2023 study found that Afghanistan and parts of Central America are both areas susceptible to high-risk heatwaves.

Updated: July 13, 2023, 3:07 PM