Why Cities Stay Warm at Night

[A] A city is reliably warmer than the countryside around it, and the difference is largest not at midday but a few hours after sunset. The reason lies in what cities are made of. [B] Stone, brick and asphalt absorb heat slowly during the day and release it slowly at night, so an urban surface is still giving back the afternoon's warmth long after a field has cooled. The effect is measurable in almost every city that has been studied, whatever its climate.

Two further factors deepen the effect. [C] Rain that falls on a field is taken up by soil and plants and later evaporates, and evaporation costs energy, which is drawn from the surroundings and cools them. Rain that falls on a street runs into a drain and is gone, so that cooling never happens. [D] And the geometry of a street traps what heat there is: radiation leaving a wall is as likely to strike the building opposite as to escape upwards. The consequence is that the difference between city and country is not a single number but a nightly cycle, one that grows through the evening and then shrinks again towards dawn.