The most severe and persistent heat waves are often associated with an atmospheric pattern called a heat dome–a persistent high-pressure system over a large area created by sinking air. The high pressure also serves as a lid keeping hot air on the surface from rising and dissipating and can persist for days or even weeks. The longer a dome lingers, the more heat will build up, creating sweltering conditions. It’s getting worse thanks to climate change. Check the link for explanation.