The Term Heat Dome

Having not been paying attention, I only recently realized that three years ago the American Meteorological Society added the term Heat Dome to its Glossary of Meteorology. The Glossary entry was edited just last October, and so I guess that explaining the term is still a work in progress. The last sentence of the Glossary entry reads, The term "heat dome" has been popularized by the news media as a way to explain extreme heat and/or drought events across large regions. Yes, I've noticed that. Not just news media, but meteorologists and other scientists often provide Heat Dome as an answer to the question, Why is it so hot? But take away the obfuscation and that answer is like saying It's hot because it's hot. Reasonable follow-up questions would include: Well, why is this heat dome where it is now? Why doesn't it go somewhere else for awhile? Why does it keep coming back here? Are heat domes new? Or have they always been around? Are Heat Domes more frequent now?

Hot air temperatures at the ground need to be accompanied by high pressure aloft, i.e. near the middle of the atmosphere. This is required by hydrostatic stability. (Hot air being less dense than cold air, pressure decreases slowly with height.) The hot temperatures at the ground and the high pressure aloft are two different ways of looking at the same thing. Here is an example from sixty years ago, at 6 p.m. Central Standard Time on July 23, 1965. (This 500-mbar chart was clipped from the NOAA Library online collection of U.S. Daily Weather Maps, link 19650724).

High temperatures on this July 23 included 102 F at Wichita and 99 F at St. Louis. The closed contour around the 500-mbar high is labeled 19500 feet, roughly equivalent to a 5940 meter contour on a contemporary chart.

The high pressure aloft need not be a high center; a broad ridge of high pressure is enough. Below is another example, from a few years after the previous one. This one is at 8 a.m. Eastern Daylight Time on June 6, 1968.

High temperatures on this Thursday, June 6, included 92 F at New York and 88 F at Washington. This was the week of the RFK assassination. By the time of the funeral on Saturday, June 8, the high center at 500 mbar had shifted east into the Atlantic, but from there a broad ridge of high pressure extended northwest over the mid-Atlantic states. High temperatures that Saturday included 88 F at both Philadelphia and Washington. At least one report from among the crowd watching the funeral train pass near Philadelphia described the heat as sweltering.

So do either of these meteorological aspects of my childhood memories qualify as a Heat Dome? As can be seen in the preceeding paragraphs, my preference is for the phrase High Pressure Aloft. Fans of the term Heat Dome apparently restrict that term to certain categories of High Pressure Aloft. Seven years ago in a blog post here I objected to Category Labels being treated as Distinct Things. That was before Heat Dome became a thing. Explanations of Heat Dome as a thing in some cases endow the thing uniquely with a mechanism that sounds something like a wine press, as if subsidence was a consequence rather than a cause of the warm air aloft.

If areas of high pressure aloft associated with hot temperatures at the ground are becoming more frequent, even as areas of flooding become more frequent and devastating, I think the natural explanation would involve the up-moist, down-dry process proselytized by Professor William Gray. In the second example above, on the morning of June 6, 1968, tropical storm Abby was moving slowly near the Atlantic coast of Florida. Abby had briefly become a hurricane, but mostly had remained a tropical storm. Nevertheless heavy rain was falling over Florida on June 6, and on later days north into Georgia and the Carolinas. So there was a lot of up-moist over the southeast United States and adjacent Atlantic and Gulf of Mexico. It's easy to imagine that at least some of the compensating down-dry was over the mid-Atlantic States, where subsidence warming would have counteracted radiational cooling in the vicinity of the high pressure aloft.

Tucson Winter Precipitation, Update 2025

Periodically over the last fifteen years I've posted here updates about Tucson's Winter Rain totals. All of those posts can be viewed together under the heading Winter Rain on the right. Please see those earlier posts for additional discussion about the format and content of the scatterplot below. Here I'll just reiterate my contrarian definition of winter in Tucson as being the five months November through March, instead of the conventional meteorological three month winter of December through February. Either way this past winter in Tucson was very dry. The most significant precipitation events during the five-month winter came in early November and mid-March, and those brief "shoulder month" storms were very wintry, with cold rain in Tucson followed by panoramic views of mountain snow cover. The Tucson Airport recorded a total of only 0.86 inches of precipitation for the five months, with only a quarter inch of that from the middle three months.

The last time a five-month Tucson winter was this dry was fourteen years ago, during the moderately strong La Niña winter of 2010-2011 (unlabeled black square near the lower left corner of the plot). Over the last seven years we have had four La Niña winters (unlabeled 2020-2021 and 2021-2022, along with 2022-2023 and 2024-2025, labeled 23 and 25), one strong El Niño winter (2023-2024), one weak El Niño winter (2018-2019) and one winter that straddled the threshold between neutral and El Niño (2019-2020).

This time last year, as the strong El Niño was winding down, models were predicting continued rapid cooling in the critical 3.4 area of the equatorial Pacific, heading toward a moderate La Niña by winter, something like what happened in 2010. But that large swing did not happen in 2024. Instead, ocean surface temperature anomalies in the 3.4 area stalled in the neutral zone for months. Finally in December 2024 the La Niña threshold was barely crossed. (See the ONI section of the CPC's weekly ENSO update). Now in late March the 3.4 anomaly has already crossed back into the neutral zone. Nevertheless, total rainfall in Tucson for the 2024-2025 winter was near the bottom of the historical range for a weak La Niña (over a period of 76 years). For what it's worth, model predictions into the beginning of next winter are keeping the ONI in the neutral zone, drifting back toward the cold side of that zone.

The next post here will be about heat, and not just in Tucson. But that will wait until summer heat is closer, probably around mid-May.