In 2021, a heat dome parked over British Columbia and 595 people died in the community. Most households in greater Vancouver have no air conditioning. Heat exhaustion and heat stroke did the rest.
A heat dome is extreme heat that builds when the atmosphere traps hot air under a lid. It happens when strong high pressure stays in place for an unusually long time, which stops convection and rain and keeps the heat bottled up over a region. Sea surface temperature anomalies and La Niña can set one off. The upper air patterns barely move, a setup meteorologists call an Omega block.
The term gets stretched. Media often use it for any heat wave, but heat waves are simply long periods of excessive heat and do not always come from a stationary high-pressure system.
How the lid forms
Heat domes tend to arise in still, dry summer conditions. A mass of warm air builds up and high pressure pushes it down. The air compresses, its heat sits in a smaller volume, and the temperature climbs. When the warm air tries to rise, the dome above forces it back down, so it gets hotter again.
Skies stay mostly clear under the dome, so sunlight reaches the ground unimpeded. The 2021 Northwest heat dome followed this pattern. A stagnant high intensified local temperatures, held back cooling breezes from the ocean and discouraged clouds. Rising warm air was pushed back down in a self-feeding cycle. Warmer sea surfaces across the northern Pacific, particularly off Washington, Oregon and British Columbia, create conditions where such domes form.
What it does on the ground
Stagnant air traps smog and pollution. Higher evaporation and drier soil worsen drought, as in California’s Central Valley, where crops and native plants both lose water. A dome can also make a heat wave worse by interacting with other systems such as frontal boundaries.
Trees have been damaged by the intense sunlight, with leaves scorched by heat stress. In 2021, the North American dome heaped thermal stress on intertidal ecosystems and killed marine species. That event also caused widespread power outages and more wildfire activity. Agricultural losses in the Pacific Northwest alone passed $600 million, with fruit crops devastated and widespread livestock deaths. Studies of British Columbians found higher climate change anxiety after the event.
Deaths fall hardest on vulnerable and marginalized people, who are less likely to have air-conditioned homes. In Russia in 2010, a persistent heat dome brought wildfires, crop failures and a jump in mortality, with effects that reached agricultural markets worldwide. The 2025 European heatwave has also been attributed to a heat dome.
Climate link and response
Studies indicate human-caused climate change plays a major role, since domes are more likely in higher atmospheric temperatures. They feed back on the problem as well, because they raise overall temperatures. Research projects an increase in the stationary waves around North America that drive such events. Heat domes are still generally improbable, but the research suggests they may stop being rare.
Mitigation leans on urban planning, public health programs and community involvement. Cities add green space, cool roofs and better ventilation. Agencies run heat health warning systems, monitor data and open cooling centres, and awareness campaigns on heat safety back up the rest.
