What Is a Heat Dome? The Science Behind Summer's Deadliest Weather Pattern

A heat dome does not look like much on a weather map — just a broad area of high pressure sitting over a region for days on end. But the physics underneath that pressure system can push temperatures 30 degrees above average, shut off rain entirely, and turn a stretch of summer into a life-threatening emergency. Heat is already the deadliest weather hazard in the United States, responsible for more fatalities each year than tornadoes, hurricanes, or floods. Heat domes are how it gets that bad.

How a Heat Dome Forms

The atmosphere is always in motion, with air masses rising, cooling, and sinking in large loops. A heat dome forms when a ridge of high pressure establishes itself in the upper atmosphere, roughly 10,000 to 25,000 feet above the surface, and then stalls. Instead of moving through in a day or two as weather systems normally do, this mass of high pressure parks itself over a region for days or even weeks.

Under high pressure, air sinks. As that air descends toward the surface, it compresses, and compression generates heat. The effect is the opposite of what happens when air is released from a pressurized container — instead of cooling as it expands, the sinking air warms as it is squeezed. By the time it reaches the surface, it arrives hotter than before, adding to whatever heat the sun is already generating.