A 'heat dome' is driving dangerous heat across the U.S. into the July 4 weekend

A person uses an umbrella to shield themselves from the sun in Central Park during a heat advisory on May 19 in New York City. Adam Gray/AP hide caption toggle caption Adam Gray/AP Extreme heat this week will blanket a majority of American states through the July 4 weekend, according to forecasters. The National Weather Service on Sunday said "dangerous to record setting heat will expand across the eastern two-thirds" of the country. In areas including Ohio, parts of North Carolina and Washington, D.C., the extreme temperatures and humidity will be especially threatening for people with respiratory issues and the elderly. "With the combination of high humidity, heat indices may reach 100-110 Degrees," said the NWS. "Much of the central and eastern U.S. is under a Moderate to Major HeatRisk, which can pose health impacts on those without hydration or cooling." Parts of Iowa, Missouri and Kansas are under extreme heat warnings. A heat dome is driving the heatwave. It occurs when a very hot air mass parks itself over a region and gets trapped under a "lid" above the Earth's surface. But the high temperatures are not the only concern, said NWS forecaster Bryan Putnam. "You get temperatures in the 90s to low 100s, that's obviously pretty hot. But you combine that with the humidity, those heat indices will go well into the 100s and that's the temperature that it's going to feel like," Putnam told NPR on Sunday. …