How a heat dome can push temperatures to dangerous highs

By Jennifer Gray, CNN Meteorologist

(CNN) — Summer is notorious for producing punishing heat waves, often referred to as heat domes.

A heat dome occurs when a ridge of high pressure builds over an area and doesn’t move for up to a week or more.

High pressure results in fair weather with lots of sunshine and very few clouds. It also indicates sinking air, and when air sinks, it warms – causing temperatures to rise.

The “dome” is created because the air can’t escape. Then, temperatures keep warming, often to uncomfortable or even dangerous levels.

Most heat records are set within a heat dome. And the climate crisis is expected to make them more frequent – and even hotter.

Notorious heat events can kill

• Europe, 2003: Among the deadliest heat waves in history, an estimated 30,000 casualties were recorded amid scorching conditions in July and August. Temperatures hit as high as 104 degrees Fahrenheit, with warmth overnight as well. France, which normally has temperatures in the 80s at that time of year, was in the 90s and triple digits for the first three weeks of August.

• India, 2015: More than 2,000 people died over several weeks as temperatures soared to a shocking 118 degrees in some places. It was so hot in Delhi, roads began to melt.

• Chicago, 1995: More than 700 people died in the metro area as a heat dome settled over the Midwest. Temperatures topped 100 degrees but felt closer to 125 because of the heat index. Many died because overnight temperatures stayed warm, so bodies couldn’t recover from daytime heat.

How the climate crisis will worsen heat waves

The climate crisis is expected to increase exposure to dangerous heat index levels by 50% to 100% in much of the tropics and by up to 10 times across much of the globe, according to a 2022 study published in the journal Nature Communications Earth & Environment.

Even a small increase in global average temperature can lead to significant increases in the hottest extremes, which are seen during strong and persistent heat domes.

Places such as Afghanistan, Papua New Guinea and Central America – including Guatemala, Honduras and Nicaragua – have been targeted as “hot spots” for high-risk heat waves, a 2023 study in the journal Nature Communications found. These regions are particularly vulnerable due to their fast-growing populations and limited access to health care and energy supplies, which undermines their resilience to extreme temperatures, according to the report.

In 2023 alone, several jaw-dropping high temperature records fell across the globe:

• In South Texas, Del Rio hit 115 degrees on June 22. This new all-time high temperature broke the record set two days earlier at 113 during an unusually strong heat dome for the month of June.

• Shanghai recorded its highest May temperature in more than 100 years on May 29.

• In Vietnam’s northern district of Tuong Duong, the heat on May 6 reached around 111.6 degrees – the highest temperature ever recorded in the country, weather historian Maximiliano Herrera said. On the same day, Thailand saw the hottest ever temperature recorded in Bangkok: 105.8 degrees.

• Siberia set dozens of records in June as temperatures climbed above 100 degrees during a heat dome that formed especially far north.

Heat waves not only endanger health but also contribute to extreme drought and wildfires. Human-caused climate change has exacerbated the hot and dry conditions that allow wildfires to ignite and grow.

In recent years, some of these fires have exhibited extreme fire behavior, including alarming rotational patterns, creating their own clouds and wafting smoke hundreds of miles away, leading to air quality issues.

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