A city is warmer than the countryside around it, often by several degrees at night. Concrete and asphalt absorb heat through the day and release it slowly after dark, while open country cools quickly. Because there is little vegetation to lose water, the cooling that evaporation provides is largely absent. The difference matters most during a heatwave, when rural temperatures fall enough for people to recover overnight and urban ones do not, which is why heat deaths cluster in cities.