A heat dome is a persistent, synoptic-scale upper-level ridge of high pressure that traps warm air over a region by suppressing rising motion, clearing skies, and warming the air aloft through subsidence. The AMS Glossary describes the pattern as an exceptionally warm mid-latitude air mass tied to a synoptic-scale high aloft during the warm season.
The physics of the pattern
Two mechanisms concentrate the heat. Air within the ridge sinks, and sinking air is compressed as it moves into higher pressure below, which warms it adiabatically. At the same time, the ridge suppresses clouds and precipitation, so incoming sunlight reaches the surface with almost nothing to block it.
The heating compounds. Days of clear skies dry the soil, which then converts more incoming solar energy into sensible heating rather than evaporation. Downslope flow from nearby terrain can add several more degrees at surface sites east of mountains.
How forecasters read it
Forecasters watch the 500 mb pattern for a ridge that stops progressing eastward. When a strong ridge parks over the same longitude for a week or more, often in a blocking configuration with a downstream trough, that is the setup for a multi-day heat event. NESDIS notes that when a persistent upper-level ridge locks in, sensible weather tends to occur in the same area over and over.
The peak surface temperature depends on how much soil moisture is left to evaporate, how deeply the boundary layer mixes, and whether smoke or high clouds attenuate solar heating. A ridge in the same location can produce a moderate heat event over green landscapes and a record heat event over dried-out ones.
Where it misleads
A heat dome is a synoptic pattern, not a severe weather event on its own. Its dangers, heat stress on people and livestock, drought intensification, wildfire growth, and demand-driven power failures, are downstream consequences that depend on how long the ridge stays and what surface conditions it inherits.
The label is also loose in practice. Some forecast discussions use it for any warm ridge, others reserve it for a blocking pattern that persists for two weeks. Read the pattern itself, not just the phrase.