U.S. cities are "heat islands" boiling under deadly extreme temperatures. It's only expected to get worse.

This summer's deadly heat waves have left people throughout the U.S. and Europe desperate for relief. It's urban centers that have felt the intense temperatures the most — and where experts say it's only expected to get worse. 

That's because of something called the urban heat island effect. 

"Simply put, it means that urban centers are hotter than the surrounding suburban areas," Liv Yoon, a postdoctoral research scholar at Columbia University's Climate School, told CBS News. She explained that the effect is like a "dome overarching the city" that's created by carbon emissions, air getting trapped amid tall buildings, and a lack of open space and greenery, among other things.