This story was originally published by Grist and is reproduced here as part of the Climate Desk collaboration.

From Texas clear to Georgia, from the Gulf Coast on up to the Canadian border, a mass of dangerous heat has started spreading like an atmospheric plague. In the days and perhaps even weeks ahead, a high-pressure system, known as a heat dome, will drive temperatures over 100 degrees Fahrenheit in some places, impacting some 160 million Americans. Extra-high humidity will make that weather even more perilous — while the thermometer may read 100, it might actually feel more like 110.

So what exactly is a heat dome, and why does it last so long? And what gives with all the extra moisture?

A heat dome is a self-reinforcing machine of misery. It’s a system of high-pressure air, which sinks from a few thousand feet up and compresses as it gets closer to the ground. When molecules in the air have less space, they bump into each other and........

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