Why are heatwaves worse in cities?

The "urban heat-island effect" refers to heat that is trapped in a city. It has several causes, such as traffic and the layout of a city. The main cause, however, is that paved environments absorb more heat, and when it is released, it warms the surrounding air. In US cities, for example, daytime temperatures are between one and three degrees higher than in rural areas; the difference between nighttime temperatures is even larger.