Daytime high temperatures across the western United States on June 28, 2021 when a heat dome settled over the Pacific Northwest, according to data from NOAA's Real-Time Mesoscale Analysis/URMA.
The summer of 2021 was one for the record books as the now-infamous "heat dome" settled over the Pacific Northwest from late June through early July, resulting in triple-digit temperatures and hundreds of deaths.
In recent years, that term has grown in popularity by the public and news media to describe impactful heat waves, but what exactly is a heat dome?
Portland State climate scientist Paul Loikith and Earth, Environment, and Society doctoral candidate, Siiri Bigalke, saw an opportunity to formally define it from a meteorological perspective — with even more specifics and detail than the recently-added definition to the American Meteorological Society's glossary.
"The term 'heat dome' is being used in a lot of different ways in reference to extreme heat waves, but it often wasn't coming from the meteorology community, and there wasn't a clear concrete definition of what is, or isn't, a heat dome," said Loikith, a professor of geography in PSU's School of Earth, Environment & Society.
In a study published in the journal Weather and Climate Extremes, Loikith and Bigalke along with coauthors from Cornell University, University of California Merced, and Washington State University, introduce their definition, present an 85-year climatology of heat domes and of extreme heat occurring and not occurring as part of a heat dome, and provide some real-life case studies from the 2021 Pacific Northwest heat dome to a multi-day event in 1980 across the Southern Great Plains.
We spoke with them about the features of a heat dome, the difference between a heat wave and a heat dome, and what the public should know about them.
How does this paper define a heat dome from a meteorological perspective?
Loikith: There are two criteria that need to be met. One is that you have to have an extreme heat object, so you have to have very high temperatures for the location, and the other is that you have to have a circulation feature — an area of high pressure aloft in the upper part of the troposphere. When those two things come together, that weather feature is considered a heat dome.
You can have extreme heat objects without the heat dome that could be associated with some other atmospheric process, and you can have these areas of upper-level high pressure without an extreme heat object, but when the two come together, that's kind of a special type of meteorological phenomenon associated with exceptionally extreme heat.
Talk through the progression of the 2021 heat dome and how it fits into this definition.
Bigalke: It officially started on June 26. That's when the circulation feature resembled what we're using as a heat dome definition with the associated yet extreme heat at the surface, and it persisted until July 2.
Loikith: On June 25, there was an extreme heat object, but it didn't yet meet the criteria of a heat dome, so we call that a precursor. It was the development of the extreme heat but it wasn't closely associated with an upper-level high pressure yet.
Then maybe the more interesting aspect of this is the two days after — July 3 and 4. A very spatially large extreme heat object continued to travel across central and eastern Canada, but it was no longer associated with this dome of high pressure; in fact, it wasn't even associated with high pressure at all. So we call that a heat dome remnant in the paper.
There could be a couple of reasons for remnants, including that the atmosphere just takes time to cool off. There could also be interesting things going on between the land surface and the atmosphere, where really high temperatures can dry out soil, and if the soil gets drier, you have more heating of the lower atmosphere, which can prolong heat waves.
This was a case where we saw both heat dome precursor and remnant part of the same multi-day extreme heat event.
We often hear "heat wave" and "heat dome" used interchangeably. What is the actual difference, and why does misusing the term matter?
Bigalke: A heat wave is a subjective term because it depends on the local climate. A heat wave near the high Arctic is a lot different than a heat wave in Texas. Usually, we're talking about the temperature at the surface. A heat dome is specifically talking about the atmospheric circulation that's going to be causing the extreme heat changes.
We're trying to understand the meteorology of what's driving surface events, so it's important to have some kind of more robust or physically constrained criteria.
Loikith: The term heat dome shouldn't necessarily be used as a way to describe the magnitude of the heat — it's a type of meteorological feature. I think we've all seen the term used that way like 'It's going to be hot, but it's not a heat dome,' so it's not just temperature based, it's also atmospherically based if we're going to be precise about its use.
Why is it important for the public to know?
Loikith: When you think about a hurricane, does it really matter if the wind and rain and damage is coming from a hurricane or something else? In the abstract, maybe not, but for communication purposes to the public, when they know a hurricane is coming, they know that it's a type of storm that they may have experienced and thought about before. The public looks at these catchy defining characteristics to equate to the severity of something they're going to experience. You can get flooding without a powerful atmospheric river, but if the news says a powerful atmospheric river is coming, you know you're going to get flooding. Similarly, you can get extreme heat without a heat dome, but if you have a heat dome, you know it's going to be very extreme.
Where are heat domes most common in North America?
Bigalke: There are other atmospheric circulation processes that do create extreme heat, but roughly 70% across North America are generated or associated at some point with a heat dome, and that's especially true for the Western United States, the Southeast, and a little bit less so once you go into interior Canada and the high Arctic.
Are heat domes becoming more common in the Pacific Northwest, and if so, why?
Loikith: The temperature is increasing, and as you go forward in time, more days are exceeding that threshold of an extreme heat object. The probability that you end up with a high pressure system that's associated with an extreme heat object is just increasing, but those high pressure systems are not making up a greater proportion of extreme heat objects over time, so it really depends on how you look at it.
We're getting more heat waves because it's getting hotter, and because of that we also pick up more heat domes, but the weather phenomenon isn't necessarily changing — it's just the background temperature.
This conversation has been edited for clarity and length.