What’s the deadliest natural disaster? In the United States, it’s heat. Between 1979 and 2003, heat waves killed at least 8,015 Americans, according to the Centers for Disease Control and Prevention. That’s more than hurricanes, lightning, tornadoes, floods and earthquakes combined. And it’s largely an urban problem—the bulk of those deaths occur in cities.

Why are cities so susceptible to heat waves? Well, in part because that’s where most people live (obviously). But there’s another factor, too: Cities tend to run much hotter than nearby rural areas. This is due to what’s known as the urban heat island effect. There are fewer trees and plants in the city to enable evaporation. Buildings and pavements absorb more warmth from the sun. And factories and automobiles give off waste heat. That all adds up. On a hot summer afternoon, a large city can easily run 5°F to 18°F hotter than surrounding rural areas, enough to turn an unpleasant heat wave into a deadly calamity.