Recent heat in north America ~ The Washington Post

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Hot air masses are more intense

The blistering temperatures last Friday occurred against a backdrop of more-intense hot-air masses due to climate change.

Multiple analyses have shown that the strength of heat domes, the bulging zones of high pressure that are the source of extremely high temperatures, has trended upward in recent decades.

An analysis conducted by meteorologists at the National Weather Service in State College, Pa., and Pennsylvania State University found an increase in the intensity of heat domes over the entire Northern Hemisphere during the summer months from 1979 to 2010.

The intensity of heat domes is evaluated using a measure known as “geopotential height,” which is the height in the atmosphere at which 500 millibars of pressure occur. The higher this pressure level is, the hotter it is, because hot air is less dense than cold air and fills more space. The most intense heat domes, which are extraordinarily rare, feature geopotential heights exceeding 6,000 meters at their core.

Friday’s heat dome exceeded the 6,000-meter geopotential height threshold in several locations in the Western United States and was nearly that high (5,940 meters or higher, as shown within the red outline in the image below) over a sprawling area from Southern California to southern New England.

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It was this same heat dome that led to the hottest weather ever recorded in Denver and Montreal, where dozens of heat-related deaths have occurred.

Data shows that hot domes this extreme are becoming more common. Last summer, Ryan Maue, a meteorologist for Weather.us,  examined data back to 1958 and found almost all of the heat domes exceeding this 6,000-meter threshold in the Western United States have occurred since 1983 — with the overwhelming majority forming since 1990.

Because of the warming climate, “I’d surmise that the [6,000-meter] threshold — while an arbitrary big round number — is now more easily exceeded,” Maue told the Capital Weather Gang.R

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