A third major heat dome in a month is baking large parts of the United States, putting roughly 70 million people across the South, West and Midwest under extreme heat warnings. The system, a high-pressure "lid" that traps hot air and humidity, matters because it is pushing temperatures well above 100°F over a wide area, raising the risk of heat stroke and other serious health effects, particularly for vulnerable groups.

Daytime highs are topping 100°F across the region, with heat index values even higher and near-record temperatures of around 115°F expected close to Phoenix. Extreme heat advisories are concentrated in Arkansas, Tennessee and the Gulf Coast, while western states such as New Mexico, Colorado, Utah, Wyoming and Idaho are also grappling with severe to extreme drought. Health experts warn that heat stroke can cause dizziness, headaches, nausea, confusion and loss of consciousness, with people who have heart disease, high blood pressure or a higher body mass index at greatest risk; they advise staying out of direct sun, drinking water or electrolyte drinks, wearing light loose clothing, and — in a suspected heat stroke emergency — cooling the person with cold water immediately, since doing so within 30 minutes can prevent organ damage and save lives.