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What exactly are heat domes and why are they so long-lasting and miserable?

A high-altitude weather map shows a massive heat dome centered over France on Thursday. (
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Since late June, bouts of extreme heat have scorched both the United States and Europe. To blame are large, stagnant zones of high pressure known as heat domes.

They are known to produce record-breaking high temperatures, and they also can fuel violent thunderstorms, stoke dangerous wildfires and exacerbate drought.

These heat domes baked the western United States and Europe in June and early July, the eastern United States in mid-July and then Europe again last week. Their sweltering conditions helped heat the planet to its warmest June and what may become its warmest July on record.

They form several miles high in the atmosphere and their air sinks down toward the ground, heating up because of compression. Sometimes they produce a dry heat, but when they are near water, they can circulate oppressive humidity as well.

Heat domes often form what are known as blocking patterns in the atmosphere, which halt the west-to-east movement of weather, and their stifling conditions can last for days.

We typically see three different kind of blocks resulting from heat domes in the United States and Europe: omega blocks, rex blocks, or sprawling high-pressure blocks, which can create what’s known as a ring of fire weather pattern. Regardless of their shape, the blocks often provide three to five days of relentless heat until the pattern weakens or is pushed out.

Omega blocks have been the main culprit for the record-breaking heat in Europe this summer. At the end of June, an omega block (see image above) that initially formed over the Atlantic Ocean pushed over central Europe and allowed tropical air to surge over the continent sourced from northern Africa and the Mediterranean. France notched its highest temperature ever recorded of 115 degrees, and the heat wave helped clinch Europe’s hottest June on record.

A similar pattern took residence over the continent again last week, leading to a high of 109 degrees in Paris, shattering its previous highest temperature ever recorded. Germany, Belgium, Netherlands and the United Kingdom all registered their highest temperatures on record.

As the name suggests, the omega block takes on the shape of the Greek letter omega on a weather map. It is anchored by zones of low pressure on its west and east sides, flanking the high-pressure zone or heat dome in the middle.

Whereas the omega block comprises three pressure systems lined up from west to east, the rex block is oriented from north to south, containing just two pressure systems. The two systems are almost stacked, with the high pressure to the north and slightly west of the low-pressure system to its south.

As the air moves around the S-shaped block, it slows down just as a car would decrease speed on a turn. This hinders the air’s movement and leads to the kind of stationary heat dome that California experienced in early June. San Francisco soared to 100 degrees, its earliest instance on record hitting the triple digits.

Rex blocks can create explosive conditions for wildfires due to the combination of their heat, dry air and hot winds rushing down the slopes of mountains.

Sometimes large blocking high pressure systems that form over the southern United States help form a ring of fire weather pattern (although the term may be more commonly associated with volcanoes in the Pacific or the Johnny Cash song).

Such a pattern developed over the central United States in mid-July when a heat dome formed in the South and expanded north and east. Sweltering heat overtook much of the East Coast, fueled by warm humid air flowing north from the Gulf of Mexico. It produced heat indexes exceeding 110 degrees in some areas and suffocating nighttime temperatures that did not drop below 80 degrees. Boston posted its hottest weekend on record.

This expansive heat dome gave rise to the ring of fire pattern, named for the corridor of thunderstorms, some severe, arched along its northern periphery. During the July heat wave in the eastern United States, two fast-moving thunderstorms complexes, known as derechos, formed atop the heat dome, unleashing destructive winds in the Upper Midwest.

As the climate has warmed in recent decades, such heat domes have become more intense. An analysis from meteorologist Ryan Maue showed the strength of summer high-pressure zones in the western United States increasing over time. Similarly, a study conducted by researchers at State College showed increases in the frequency of such warm pockets over much of the Northern Hemisphere.

As the climate continues to warm, expect to hear more and more about these hot high-pressure zones and their punishing impacts.