Heat wave or heat dome? Yes, there's a difference

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 be facing one this week.

The term “heat dome” refers to a ridge of high pressure that persists over a large geographical region, delivering high temperatures that linger for days or weeks on end.

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

Experts say it’s not the same as a heat wave, which is conventionally defined as a spell 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 warmed global temperatures, shifted weather patterns and contributed to worsening hazards.

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

He said one authority on the issue 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 warm air below from rising, thus trapping the warm air as if it were in a dome.”

Abatzoglou expanded on the definition even further, noting that heat domes aren’t just large and persistent, but typically set up over land and in warm seasons.

“We can have heat waves without heat domes,” he said, particularly in coastal Southern California, where offshore winds can bring hot temperatures south and west of the mountains while other parts of California stay cooler.

“Generally heat domes instead affect very large geographic areas — synchronizing hot and sometimes very dry conditions — which can lead to additional challenges that may not materialize with ‘local’ heat wave events,” he said.

Not everyone agrees, however. The National Weather Service has no official definition for “heat dome” in its glossary, and meteorologists there are less likely to use the term.

“I’ve heard in the media the term ‘heat dome,’ but we really don’t use that,” said Dan Harty, a meteorologist with the National Weather Service in Hanford. “Heat wave is typically what we refer to.”

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 hot 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 that stretches from portions of Los Angeles almost to the Oregon border through Friday, advising residents of dangerously hot conditions and increased potential for heat-related illnesses.

Sara Purdue, a meteorologist with the NWS 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 in the region slightly by the weekend, but could be followed by more heat 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,” she said. “It’s being blocked by other other areas of pressure, east and west.”

Even less certain is whether the blocking, or ridging, pattern associated with heat domes are becoming more frequent. Some scientific literature points to this being the case, including a 2012 study that found 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 jet stream behavior that produces these stagnant high pressure systems and the extreme heat and drought associated with them,” said Michael 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 across the globe).

Mann noted that heat domes aren’t just warm at the surface like heat waves, but have a deep layer of warm air and high pressure associated with sinking dry hot air. He said they’re the sort of air masses often associated with desert conditions — and with a particular jet stream pattern called an “omega block” because of its shape.

“Our work shows that climate models aren’t doing a good job yet in capturing this effect, meaning that the models are probably underestimating the impact that climate change is having on these persistent summer weather extremes (heat domes, heat waves, wildfires, as well as extreme flooding events),” he said.

And while the term “heat dome” has recently risen in prominence, Mann said it goes back nearly half a century. However, climate change is leading to increased incidents of the underlying conditions associated with such events, he said.

In fact, research has shown heat waves now occur three times as often as they did in the 1960s. Heat domes, like the one that smothered the Pacific Northwest in 2021, are also 150 times more likely due to climate change, another study found.

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

In some ways, then, the term heat dome is not unlike other meteorological phenomena that have gained prominence in recent years, including real terms such as atmospheric rivers and bomb cyclones, and more colorful terms such as firenadoes and hurriquakes.

Another recent coinage is the “ridiculously resilient ridge,” a phrase from UCLA climate scientist Daniel Swain used to describe a blocking pattern to the 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 sets up over land with the jet stream often well to the north, and the ridge pattern gets “stuck.”

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

“I’ve said these are ‘ridging events,’ and to many people that’s jargon and it doesn’t translate,” he said. “And so we have to go from talking about ‘500 millibar geopotential heights’ that a really small audience appreciates and understands, to something that captures the essence of the event. And this really is a large blocking.”