Heat Dome has become a buzzword of the 21st Century. You'll mainly hear it in conjunction with talk of an extended and potentially record-breaking heatwave.
It's an informal name given to a massive "dome" of high pressure that can stretch over 1,000 miles in diameter — about the distance from St. Louis to New York City. High pressure brings sinking air, and as air sinks, it dries out and becomes even warmer.
That means the weather inside these domes will typically be clear with nary much moisture available for cloud development, allowing the sun to cook the ground while free from obstructions. Combine the sun’s heating with the sinking air, and you get a double-whammy of warming that can many times bring near-record or record heat.
If a dome forms over a region that had recently had heavy rains or saturated soils, it can also trigger oppressive humidity levels as that water evaporates yet becomes caught in the nearly stagnant pattern.
As these domes strengthen, they become a formidable barrier on the atmospheric highway, steering the jet stream far around to its north. The result is a giant dome of hot air that is anchored in place, outmuscling any attempts by weather disturbances that come to nudge it along.
Thus, another calling card of heat domes is not just their intensity but their duration. A heat dome can bring several days of oppressive heat, with the only hope for improvement coming if the heat dome finally begins to weaken — and then be shoved along out of the region -- like if you finally summon enough friends to push that disabled SUV off the highway.
Some of the worst heat domes, though, are so large and entrenched it’s more like trying to push a disabled semi-truck off the road.