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Narrator: Part 4. You will hear part of a lecture on urban heat islands.

Lecturer: Good afternoon. Today I'm going to talk about a phenomenon that affects almost every large city in the world: the urban heat island. The term was first used several decades ago, but the effect itself had been noticed long before that, when early weather observers found that their city thermometers consistently gave higher readings than those placed in nearby farms and villages. In short, the urban heat island describes the way in which a city is noticeably warmer than the surrounding countryside. The difference can be several degrees. Now, you might expect the gap to be widest in the afternoon, when the sun is at its strongest. In fact, the opposite is true. During the day, both the city and the rural areas heat up. It's at night that the difference really shows, because the countryside cools quickly while the city slowly releases the heat it has stored.

Lecturer: So what causes it? Researchers generally identify four main factors, and I'll take them in turn. The first is the materials we build with. Brick, concrete and stone all hold heat, but the worst offender is the dark surface that covers most of our roads and many flat roofs – I mean asphalt. It absorbs a great deal of the sun's energy during the day and, rather like a storage heater, keeps it for hours afterwards.

Lecturer: The second factor is the lack of vegetation. In rural areas, a great deal of the sun's energy is used not to heat the ground but to turn water into vapour. As water evaporates from leaves and from damp soil, it draws heat out of the surrounding air. Evaporation, in other words, works as a kind of natural air conditioner. In a city covered in concrete, rainwater runs straight into the drains, there's very little moisture left at the surface, and so that natural cooling is largely lost.

Lecturer: Third, there's the shape of the city itself. Planners sometimes talk about 'urban canyons' – narrow streets lined by tall buildings. These are very good at trapping heat. The sun's energy bounces from one wall to another instead of escaping to the sky, and, just as importantly, tall buildings standing close together block the wind, so the warm air simply sits there instead of being carried away.

Lecturer: And finally, cities produce a lot of heat of their own. In the past the main sources were vehicles and factories, and they still contribute. But increasingly, a large share comes from air conditioning, which, ironically, cools the inside of buildings by pumping heat out into the streets – making the outdoor problem worse and, of course, increasing the demand for more air conditioning.

Lecturer: Why does all this matter? Well, higher temperatures increase the demand for energy, and they also speed up the chemical reactions that create certain air pollutants, such as ground-level ozone. But the most serious effect is on health. During heatwaves, the risk of illness rises sharply. Young children are often mentioned in this context, and they do need protection, but the research shows that the group at greatest risk is elderly people, who are less able to regulate their body temperature and who often live alone.

Lecturer: Fortunately, there's a good deal that cities can do. One of the simplest measures is the so-called cool roof. Instead of absorbing the sun's energy, a roof painted in a light colour can reflect much of it back into the sky. The cost is low, and studies have found that the temperature inside the top floor of a building can fall noticeably as a result.

Lecturer: Trees are another powerful tool. Most people think of them simply in terms of shade, and shade is certainly valuable. But trees also cool the air directly, by releasing moisture through their leaves – the same process I described earlier. Parks, green roofs and even small street gardens can all help in the same way, though a single large tree generally does more than a lawn of the same area.

Lecturer: However, a city can't do everything at once, so it needs to decide where to act first. For that, planners need detailed maps of temperature across the whole city. Ground-level measurements are expensive to collect, so these are now usually produced using data from satellites. And what they reveal is often uncomfortable: the hottest neighbourhoods tend to be the poorest ones, with the fewest trees and the most closely packed housing. In the second half of today's session, we'll look at how two cities have used this information to decide their priorities.

Narrator: Part 4. You will hear a lecture on biomimicry in architecture.

Lecturer: Good morning, everyone. In today's lecture I'm going to introduce an approach to design that has attracted a great deal of attention over the last thirty years or so: biomimicry. The word comes from the Greek for 'life' and 'imitation', and in simple terms it means designing buildings, materials and systems by learning from solutions that have evolved in nature. Now, architects have always borrowed shapes from nature – think of columns carved to look like trees, or decorations based on leaves and shells. But that isn't biomimicry. Biomimicry is about imitating the function of natural forms, not just their appearance. The question a biomimetic designer asks is not 'What does this plant look like?' but 'How does this plant solve the problem I'm facing?'

Lecturer: Let me give you a classic example. In hot parts of Africa, certain species of termite build enormous mounds of soil, sometimes several metres high. Inside, the temperature stays remarkably stable, even when the air outside swings between very hot days and cold nights. For a long time it was thought that the termites simply opened and closed holes, but we now know that the key is a network of tunnels running through the mound. As warm air rises and escapes through the top, cooler air is drawn in lower down, so the mound ventilates itself without any machinery.

Lecturer: In the 1990s, an architect designing a large office and shopping complex in Zimbabwe used this idea. The building has no conventional air-conditioning system. Instead, fans draw in cool air at night, and that coolness is stored in the building's heavy concrete structure, which then slowly absorbs heat during the day. Warm air is released through chimneys on the roof. According to the figures usually quoted, the building uses only around a tenth of the energy of a conventionally cooled building of the same size.

Lecturer: A second example comes from the plant world. The leaves of the lotus, a water plant, always seem to be clean, even though it grows in muddy water. Under a microscope, you can see why. The surface of the leaf is covered in tiny bumps, and these are coated with wax. Water can't spread out on this surface; instead it forms round droplets, which roll off and pick up dirt as they go. This has inspired a range of self-cleaning paints and coatings, which are now used to keep the outside walls of buildings clean without the need for chemicals or expensive maintenance.

Lecturer: My third example is a beetle that lives in one of the driest deserts in the world, on the south-west coast of Africa. People often assume it survives by drinking dew, or the occasional shower of rain, but rain almost never falls there. What the beetle does is collect water from the fog that blows in from the sea in the early morning. Its back has a pattern of bumps that attract water and channels that repel it, so droplets form and run down towards its mouth. Engineers have used the same principle to design nets that collect drinking water from fog in dry coastal regions.

Lecturer: So far, so impressive. But I want to end with some words of caution, because biomimicry is not without its critics. One criticism is that the term has become fashionable, and that some projects use nature mainly as a marketing tool – a building is described as 'inspired by a beehive' or 'like a forest' when in practice it works in exactly the same way as any other building.

Lecturer: A second, more technical problem is scale. Many of the most elegant natural solutions work at a very small scale – the bumps on a lotus leaf are measured in millionths of a metre – and what works for a leaf or an insect doesn't always work for a building of thirty storeys. Physics changes as things get bigger.

Lecturer: In my view, the most successful projects are those where the collaboration starts early. Too often, architects design a building and then invite engineers to make it work. With biomimicry, I'd argue, architects need to be working with biologists from the very beginning, because it's biologists who understand how and why a natural system functions.

Lecturer: Which brings me to your assignment. Rather than starting with a building, I'd like each of you to choose one organism – a plant, an animal, even a type of bacteria – and research how it solves a particular problem, such as keeping cool, staying dry or saving energy. Only then should you think about how that solution might be applied to architecture. We'll discuss your choices next week.