Major cities across the United States are preparing for extreme temperatures caused by a heat dome that hit the U.S. Plains on Friday and is expected to expand to much of the rest of the country over the coming days.

Maps show where extreme humidity and 90- or 100-degree temperatures will affect around 255 million people across the United States.

Weather forecasters have warned of extreme and dangerous temperatures as a major heat wave sweeps across the central and eastern U.S. states.