The Heat Dome

Heat domes are not rare in themselves; ridges of high pressure build over land every summer. What makes one dangerous is persistence. A ridge that moves through in a day or two produces a warm spell. A ridge that gets stuck produces a disaster, because the danger of heat is cumulative. The first hot day is uncomfortable. The third and fourth are when the built environment and the human body stop keeping up, because neither has had a cool night in which to recover.

The overnight low matters more than most people expect. A heat dome that stays clear at night lets some heat radiate away, but the surrounding air is already so warm that temperatures often fail to drop into a range that offers real relief. During the 2021 event, some places recorded overnight lows in the 70s or higher — warm enough that homes without air conditioning never cooled down, and the walls, floors, and furniture that had absorbed heat all day kept releasing it into the night. The house itself became a slow radiator.

The human body has one main strategy for shedding heat in hot conditions: sweat. As sweat evaporates off the skin, it carries heat away, and this works remarkably well — up to a point. That point is set less by raw temperature than by humidity, because evaporation slows as the surrounding air fills with moisture. In very humid heat, sweat pools rather than evaporates, and the cooling system stalls with nowhere left to dump the heat.

When the body can no longer keep its core temperature down, it moves through a sequence that ends badly. First it pushes more blood to the skin to release heat, which strains the heart and drops blood pressure. Then organs begin to suffer as internal temperature climbs past roughly 104°F. Heatstroke — the failure of the body's temperature regulation — can cause confusion, organ damage, and death within hours if the core stays too hot for too long. Much of the harm is not dramatic collapse in the street but a quiet decline indoors over a day or two.

A heat dome sets the same air temperature across a whole region, but the region does not experience it evenly. Cities run hotter than the countryside around them — often by several degrees — because concrete, asphalt, and dark rooftops absorb solar energy through the day and release it slowly at night, while the lack of vegetation removes the cooling that trees and soil provide. This is the urban heat island, and under a dome it turns a hot city into a genuinely dangerous one. The overnight relief that already fails across the region fails hardest in the built-up core.

Within a single city, the gap is sharper still, and it tracks lines that have nothing to do with weather. Neighborhoods with tree cover, parks, and lower building density stay measurably cooler than dense, paved neighborhoods with little shade. Studies mapping urban heat have repeatedly found that the hottest districts are frequently the poorest — often, in American cities, the same areas shaped by decades of disinvestment and redlining, which left them with fewer trees and more heat-absorbing surface. The dome doesn't choose these streets. The city did, long before the ridge arrived.

The dome over the Northwest broke at the start of July 2021. The ridge weakened, the jet stream loosened, cooler air pushed in from the Pacific, and within days the temperatures fell back toward something the region recognized. The sky had never darkened. Nothing had been knocked down except, in one town, by the fire the heat had helped to set. The event ended as quietly as it had arrived, and the counting of the dead went on for weeks afterward, because so many had died alone and indoors.