An extremely dangerous heat wave is underway for the eastern half of the United States as a potent heat dome reaches its peak, bringing the hottest temperatures of the year so far – the hottest in years for some cities – and putting tens of millions at risk.

Heat alerts have been issued from Colorado to Maine, including more than 150 million Americans. Heat index values in the Midwest will soar into the triple digits this afternoon, w

A heat dome occurs when a large area of high pressure in the upper atmosphere acts as a reservoir, trapping heat and humidity. A heat wave is the persistence of heat, usually three days or more, with unusually hot temperatures.