Heat wave or heat dome? Weather experts don’t agree on the definitions

MONTREAL — There’s no doubt the weather across much of Eastern Canada this week is very, very hot. But is the region experiencing a heat wave or a heat dome, and what is the difference between the two?

It depends on whom you ask.

While the term heat dome isn’t new, it gained prominence during the record-shattering June 2021 heat event over the Pacific Northwest. The B.C. provincial coroner’s service linked more than 600 deaths to the extreme heat, with 93 per cent of them occurring between June 25 and July 1.

The term heat dome — like heat wave — doesn’t appear in Environment Canada’s glossary. Several weather experts contacted by The Canadian Press gave slightly different meanings and disagreed on whether the term heat dome is useful to inform the public about climate change and the dangers of extreme weather.

For Jalena Bennett, a spokesperson for the Weather Forecast Research Team at the University of British Columbia, heat domes and heat waves have separate meanings. Heat waves, Bennett said, are periods of hot weather that can be caused by multiple factors, while heat domes are more specific.

“Simply put, heat domes will always be a heat wave, yet not every heat wave is a heat dome,” she wrote in an email.

A heat dome, Bennett said, occurs specifically “when high pressure exists in a large region for a very long period of time, with both surface high pressure and high pressure higher up in the atmosphere remaining stationary.” The high pressure, she wrote, is associated with little to no clouds, allowing the sun to heat the surface for an extended time.

“Since the system isn’t moving, the air within will essentially be trapped until the high pressure lessens or moves on,” she wrote.

Bennett said members of the UBC weather research team, like many meteorologists, prefer to use the term heat wave to describe extended periods of high temperatures — whether or not the phenomenon occurring is technically a heat dome. The two terms, she said, are sometimes falsely conflated and can generate confusion.

“There needs to be careful analysis of a heat event before it can be classified as a ‘heat dome,’ even if it is widely described as such,” she said.

David Phillips, a senior climatologist at Environment Canada, also defines a heat dome as a stationary high-pressure area. But for him, the main differences between a heat wave and heat dome are that the latter covers a bigger area, doesn’t cool down at night and is deadlier.

“It’s like the air is trapped in there and doesn’t move,” he said in a phone interview.

Unlike other meteorologists, Phillips said he uses the term heat dome because it is “descriptive” and easy for the public to understand — even though he recognizes that some of his colleagues may not agree. Weather terms, he added, differ by region and have shifted over time.

Jean-Philippe Bégin, an Environment Canada meteorologist from Quebec, said he prefers not to use the term heat dome, describing it as a “useless” description to inform the public.

Talking to people about heat domes — which differ with heat waves in the temperatures in the upper atmosphere — “it’s making things complicated for nothing. So let’s just call it a heat wave. People understand that.”

When asked about the difference between a heat wave and a heat dome, B.C.-based Environment Canada meteorologist Armel Castellan said “there isn’t one.”

“It doesn’t matter what we call it,” he said. While there can be some small nuances, he said, “It’s kind of the same mechanism.”

What’s important, he said, is “whether or not we can incite behavioural change, both at the individual level and the societal level,” he said.

Caroline Metz, the managing director of Climate Resilience and Health at the University of Waterloo’s Intact Centre on Climate Adaptation, said the extreme weather events of recent years have driven the popularity of a number of new meteorological terms, from atmospheric river to heat dome, in an effort to explain the new realities to the public.

“I think it stems from the fact that our planet is warming and it’s changing the atmosphere and ocean conditions,” Metz said. “These terms are trying to describe what’s happening with our weather, which is being influenced by climate change.”

She said a heat wave is conventionally described as an extended period of higher-than-average temperatures, but the specifics depend on the region and time of year. Similar to the other experts, she described a heat dome as a “stagnant stationary hot air mass” created by a high-pressure system that traps a layer of hot air below it like a lid.

Metz said that in Canada, heat domes may be “unofficially” associated with heat events that are particularly deadly, due to the connection with the 2021 extreme hot weather in B.C., but she said the rest of the world may use the word heat wave for a similarly deadly event.

What’s not in doubt, she said, is that heat events are among the greatest hazards to human health, responsible for “more illness and deaths than most other extreme weather hazards combined.”

“At the end of the day, the reality is that things are getting hotter, faster and we need to adapt,” she said. “Acting now, to build protection from extreme heat will safeguard health and save lives.”

This report by The Canadian Press was first published June 18, 2024.

Morgan Lowrie, The Canadian Press