Listening Part 4: A lecture on urban heat islands

Lecturer: Good afternoon. In today's lecture I'm going to look at a problem that affects almost every large city in the world, and that's becoming more serious as the climate warms: the urban heat island.

Lecturer: The term describes the way that cities are often noticeably warmer than the countryside around them. On a summer evening, the centre of a large city can be several degrees warmer than the fields just outside it.

Lecturer: Interestingly, the difference is usually greatest not in the middle of the day, but at night. I'll come back to why that happens in a moment.

Lecturer: So, what causes the effect? The first factor is the materials we build with. Asphalt roads and dark roofs absorb a great deal of the sun's energy during the day.

Lecturer: They then release that stored heat slowly after sunset, which is why city nights stay so warm.

Lecturer: The second factor is the shape of the city itself. Tall buildings on either side of a narrow street form what's called an urban canyon. Heat bounces between the walls and has difficulty escaping to the sky.

Lecturer: Third, there's the heat we produce ourselves. Vehicles, factories and, ironically, air-conditioning units all release waste heat into the streets.

Lecturer: And finally, cities simply have less vegetation. Plants cool the air by releasing water through their leaves, a process known as evapotranspiration. Replace a park with a car park, and you lose that natural cooling.

Lecturer: Why does this matter? The most serious consequence is for health. During heatwaves, high night-time temperatures prevent the body from recovering, and elderly people are particularly at risk.

Lecturer: There's also an economic cost. Warmer cities use more electricity for cooling, which puts pressure on the power supply at exactly the moment it's needed most.

Lecturer: So what can be done? One of the simplest solutions is the so-called cool roof. This means painting roofs white or covering them with a reflective material, so that they reflect sunlight instead of absorbing it.

Lecturer: Green roofs go a step further. Covering a roof with plants not only lowers the temperature of the building, but also absorbs rainwater, which helps to prevent flooding.

Lecturer: At street level, the most effective measure is still the humble tree. Trees provide shade, and through evapotranspiration they can lower the temperature of the surrounding air.

Lecturer: However, trees need space and water, and their roots can damage pavements, so city planners have to choose species carefully.

Lecturer: Some cities are also experimenting with lighter-coloured paving, and with fountains and small water features in public squares.

Lecturer: The key lesson, I think, is that no single measure is enough. The cities that are coping best are combining several of these approaches.

Lecturer: In next week's lecture, we'll look at a case study of one of those cities in more detail.