Why Cities Are Hotter Than the Countryside

A The discovery was made by an amateur. Luke Howard, a London chemist better known for giving clouds their names, spent the early years of the nineteenth century comparing thermometer readings taken in the city with readings taken in the fields beyond it. The results, published from 1818 onwards, showed that London was consistently warmer than its surroundings, and that the gap was widest not at noon but in the hours after sunset, when the countryside cooled quickly and the streets did not. Howard had no explanation for it. More than a century later the effect acquired a name, the urban heat island, and it has since been measured in every city where anyone has looked, from tropical capitals to towns inside the Arctic Circle. It is strongest on calm, cloudless nights and can almost vanish in wind and rain.

B Three groups of causes are at work. The first is the surfaces themselves. Fresh asphalt reflects only a small percentage of the sunlight that lands on it, so nearly all of that energy is absorbed, and brick, concrete and stone store it in bulk and release it slowly for hours after dark. The second is shape. A narrow street is a canyon whose walls face each other, and the strip of open sky visible from the pavement is small; heat radiated upward by one wall is intercepted by another instead of escaping, a relationship the climatologist Tim Oke set out in detail in the 1970s. The third is what the city itself produces: engines, factories, cooking and, increasingly, air conditioning, which cools interiors by pumping heat into the street, so that the hotter the night the more heat is added to it.

C Measuring the effect is harder than it sounds. Satellites see the temperature of surfaces, and a black roof at midday may be forty degrees hotter than the air above it, which makes satellite maps dramatic but misleading if they are read as maps of what a person feels. Air temperature has to be recorded by shielded sensors about two metres above the ground, and permanent weather stations are too widely spaced to show how one street differs from the next. The gap has been filled by sensors mounted on cars and bicycles driven along fixed routes at the same hour, often by volunteers recruited for a single hot afternoon. Campaigns of this kind, run in dozens of cities, routinely find differences of five to eight degrees between neighbourhoods a few kilometres apart.

D Those differences are not distributed at random. Tree cover in most cities tracks income closely, and the districts with the least shade are usually the ones with the least money, the oldest housing and the fewest air conditioners. The health consequences follow the same pattern. What kills during a heatwave is less the afternoon peak than the failure of the night to cool: the body recovers from heat load while asleep, and when the overnight minimum stays high for several days in a row, deaths among the elderly, the ill and outdoor workers rise sharply. Heat now causes more deaths in many countries than floods and storms combined, and it does so quietly, in apartments rather than on the news.

E The remedies are well known and each has a catch. Coating roofs in reflective paint is cheap and immediately effective for the building beneath, though it does nothing for the pavement. Street trees are the strongest single measure, cooling twice over, by shading surfaces before they heat up and by releasing water vapour from their leaves, but they need watering through exactly the droughts that make them most valuable, and in a very narrow canyon their canopy can trap exhaust fumes at head height. Green roofs cool a little and, usefully, hold back a large share of the rain that would otherwise pour into the drains during a storm. Reflective pavements are the most awkward case: they lower air temperature, but the sunlight they bounce upward strikes pedestrians, who may feel hotter on a cooler street. In my view the evidence still favours trees over every alternative for cost per degree, provided a city is willing to wait fifteen years for the canopy to grow.

F Cities have begun to organise. Several have appointed heat officers with budgets and authority across departments, an idea that spread from Athens and Miami. Ahmedabad in India, after a heatwave that overwhelmed its hospitals, built the first heat action plan in South Asia, combining forecasts, text alerts, training for clinic staff and cool rooms in public buildings, and the death rate in later heatwaves fell. Building codes are slowly following, requiring shading, ventilation and lighter roof colours. None of this addresses the cause of the warming that is making each summer worse, and it would be a serious error to treat cool surfaces as a substitute for reducing emissions. It is, however, the part of the problem a mayor can actually solve, and it buys time for the people least able to buy it for themselves.