These cities have the worst urban heat, where temperatures can spike 15-20 degrees in a matter of blocks

These so-called urban heat islands are deadly during extreme heat waves.

During heat waves, these so-called urban heat islands are deadly. Extreme heat is an invisible yet dangerous consequence of human-caused climate change, killing more people each year on average than any other weather-related event, according to the National Weather Service. The urban heat island effect amplifies a heat wave's already-oppressive temperatures: areas with a lot of asphalt, buildings and freeways tend to absorb the sun's energy then radiate heat. Areas with green space — parks, rivers, tree-lined streets — absorb less.