The same alert can mean 100 degrees in one city and 105 in the next, because heat-index thresholds are set locally, not nationally.

A central AC running flat out at 79 degrees on a 99-degree afternoon is usually doing exactly what it was engineered to do. The symptom that actually signals a fault is different, and easy to miss.

A heat dome is the stalled high-pressure lid behind record-breaking heat waves. Here is how it forms, why humidity makes it deadly, and how long it lasts.

With 67 million people's power on the line, PJM leaned on paid demand cuts and data-center curtailments to keep the lights on through the heat.

More than 165 million Americans face major or extreme heat risk this week as a record-threatening heat dome settles over the East Coast and Midwest through the July Fourth weekend — and complicates World Cup matches.