What is the difference between a heat dome and a heat wave?

It sounds like something out of a horror movie, like “The Blob” or “Godzilla.” But “heat domes” are far from fiction, and the West could face one this week.

The term “heat dome” refers to a ridge of high pressure that persists over a large geographic region, generating high temperatures that persist for days or weeks.

An infamous heat dome occurred in 2021, when triple-digit temperatures smothered the Pacific Northwest for 27 days, contributing to hundreds of deaths and prompting multiple research studies.

Experts say it's not the same as a heat wave, which is conventionally defined as a period of three or more abnormally hot days. But the term has gained prominence in recent years as climate change, El Niño and other variables have raised global temperatures, changed weather patterns and contributed to worsening dangers.

The term “seems to have caught on in the last decade or so,” said John Abatzoglou, a professor of climatology at UC Merced who co-authored a recent study on the 2021 heat dome.

He said one authority on the subject is the American Meteorological Society, which maintains a glossary of such terms. That group added “heat dome” to its index in March 2022, defining it as “an exceptionally hot air mass that develops when high pressure aloft prevents hot air below from rising, thereby trapping the hot air.” as if I were in a dome.”

Abatzoglou expanded the definition further, noting that heat domes are not only large and persistent, but are typically installed above ground and in warm seasons.

“We can have heat waves without heat domes,” he said, particularly along the Southern California coast, where offshore winds can bring warm temperatures south and west of the mountains, while other parts of California remain colder.

“In general, heat domes affect very large geographic areas, synchronizing warm and sometimes very dry conditions, which can create additional challenges that may not materialize with 'local' heat wave events,” he said.

However, not everyone agrees. The National Weather Service does not have an official definition of “heat dome” in its glossary, and meteorologists are less likely to use the term.

“I've heard the term 'heat dome' used in the media, but we don't really use it,” said Dan Harty, a meteorologist with the National Weather Service in Hanford. “We normally refer to a heat wave.”

Harty said the NWS typically defines a heat wave as at least two consecutive days of high heat risk. He said the incoming event is expected to peak in the Central Valley on Thursday with temperatures as high as 108 degrees in Fresno and 107 degrees in Madera, both of which could set daily records.

The agency has issued an excessive heat warning extending from parts of Los Angeles nearly to the Oregon border through Friday, warning residents of dangerously hot conditions and increased potential for heat-related illnesses.

Sara Purdue, an NWS meteorologist in Sacramento, said either term could be used for the incoming event, but “it's not a particularly impressive heat dome.” The heat is expected to dissipate slightly in the region over the weekend, but more heat could follow next week.

“Usually when people refer to a heat dome, it's stationary for whatever reason, so it's not moving quickly out of the area,” he said. “It is being blocked by other pressure areas, to the east and west.”

Even less certain is whether the blocking or ridge formation pattern associated with heat domes is becoming more prevalent. Some scientific literature notes this to be the case, including a 2012 study that found that slower patterns favoring extreme weather are becoming more common in mid-latitude regions.

A more recent study on the 2021 heat dome shows that climate change “is actually favoring the behavior of the jet streams that produce these stagnant high-pressure systems and the extreme heat and drought associated with them,” Michael said. Mann, one of the study's authors and a presidential distinguished professor at the University of Pennsylvania, in an email. (The jet stream is the river of air that moves weather systems eastward around the world.)

Mann noted that heat domes are not only warm on the surface like heat waves, but have a deep layer of hot air and high pressure associated with the sinking dry hot air. He said they are the type of air masses that are often associated with desert conditions and a particular jet stream pattern called the “omega block” because of their shape.

“Our work shows that climate models are not yet doing a good job of capturing this effect, which means that the models are probably underestimating the impact that climate change is having on these persistent summer climate extremes (domes). heat, heat waves, wildfires, as well as extreme flooding),” he said.

And while the term “heat dome” has gained prominence recently, Mann said it dates back almost half a century. However, climate change is causing increased incidents of the underlying conditions associated with such events, he said.

In fact, research has shown that heat waves now occur three times more than in the 1960s. Another study found that heat domes, like the one that covered the Pacific Northwest in 2021, are also 150 times more likely due to climate change.

Climate change is also causing warmer global temperatures and greater temperature extremes, Abatzoglou said, “so when events like this occur, the temperatures along with them are substantially warmer.”

So in some ways, the term heat dome is no different from other weather phenomena that have gained prominence in recent years, including real terms like atmospheric rivers and bomb cyclones, and more colorful terms like fires and hurricanes.

Another recent coinage is the “ridiculously resilient ridge,” a phrase that UCLA climate scientist Daniel Swain used to describe a blocking pattern west of California in the winter months that deflects storms north of the region.

Abatzoglou said heat domes are somewhat similar, except that the ridge is set over land with the jet stream often far to the north, and the ridge pattern becomes “stuck.”

But extreme heat is particularly dangerous — one of the deadliest of all extreme weather events — and often the first heat wave of the year is the most shocking because people have not yet acclimatized, Abatzoglou said. Sometimes the most important task is to accurately communicate that threat to the public.

“I've said these are 'difficult events' and for a lot of people that's jargon and doesn't translate,” he said. “And so we have to move from talking about '500 millibar geopotential heights' that a very small audience appreciates and understands, to something that captures the essence of the event. And this really is a big blockage.”