Much of the Midwest and the Northeast is broiling — or set to broil — in extreme summer heat this week. Meteorologists are talking about heat waves and heat domes. Both terms mean it’s really hot. And people will hear them a lot more as the world heats up. What’s the difference? Ken Kunkel, a research professor of atmospheric sciences at North Carolina State University, says it’s helpful to think of a heat dome as what’s happening in the atmosphere. And a heat wave is how it affects people on the ground. As both hit a wave and a dome hit the US, the National Weather Service says that the most extreme heat is expected in the Ohio Valley and the Northeast.