A lecture extract on the urban heat island

Good morning, everyone. Today I want to look at a phenomenon that most of you will have felt without ever naming it: the urban heat island. In essence, a city is warmer than the countryside around it, and on a calm, clear night the difference can reach as much as eight degrees.

Why does this happen? Not, as is often assumed, because cities generate so much heat of their own. Engines and air conditioners do contribute, but their share is modest. The larger factor is surface. Stone, brick and asphalt soak up sunlight all day and release it slowly after dark, whereas soil and vegetation stay cooler, partly because plants lose water through their leaves, which draws heat out of the air.

Why does the night matter so much? Because the body needs a cool period in which to recover. When nights stay hot, people sleep badly, and the health effects build up over days rather than hours.

Had planners understood all this a century ago, they might have designed very differently. Instead, we inherited streets lined with dark surfaces and hardly any shade. Rarely do we stop to consider that a single paved car park can raise the temperature of an entire neighbourhood by a degree or more.

Now, a word of caution about the solutions. Planting trees is the obvious answer, and it does work, but only where there is room for roots and a reliable supply of water. A tree struggling in a narrow pavement pit may cool nothing at all. Pale roofs are cheaper and quicker. In a study of twelve districts, they lowered indoor temperatures by roughly three degrees. Yet reflective surfaces can also bounce glare into the street, which is hardly ideal for pedestrians.

What I find most interesting, however, is the question of who bears the burden. Heat is not shared evenly. The hottest streets tend to be those with the fewest parks, and those are often where older residents and low-income families live. So the heat island is not merely an engineering puzzle; it is a question of fairness.

I'm not suggesting that every problem has a technical fix. Some cities have simply opened cool rooms in public libraries during heatwaves, which costs very little and may well save lives. Others have asked residents to paint their own roofs white, with mixed results. For next week, I'd like you to read the two case studies on the reading list and decide which approach you find more convincing, and why.