Transcripts
Part 4: You will hear part of a lecture about urban heat islands.
LECTURER:
In this lecture, we’ll look at a phenomenon that many of you have experienced without necessarily naming it. Cities often stay warmer than the areas around them. Scientists call this the urban heat island effect, and it matters because it can turn a warm summer into a health risk for large numbers of people.
Now, you might assume this is mainly a daytime problem, after all, that’s when the sun is strongest. But what I mean by an island of heat is a difference that often becomes most noticeable after sunset. If you compare a dense city centre with nearby open land, the city commonly cools more slowly. In practical terms, you might see a gap of two or three degrees between an inner district and the countryside, especially on calm, cloudless nights.
So why does the city hold on to warmth? Let’s break it down. The first reason is surface materials. Natural ground can release heat efficiently, particularly when it contains moisture. Urban surfaces are different. They are often dark, dense, and designed for durability rather than cooling. Roads are the classic example. Just to be clear, it isn’t that every road behaves the same way, but tarmac tends to absorb a great deal of solar energy during the day and then release it slowly later, keeping nearby air warmer into the evening.
A second factor is the shape of the built environment. In areas with tall buildings packed closely together, wind movement can be reduced. Warm air gets trapped, and streets behave like channels that hold heat rather than letting it disperse. Let’s take a quick example. If you walk from a tree-lined street into a narrow road between high blocks, you can feel the temperature change within a minute, even though you’ve only moved a short distance.
Third, there’s greenery, or the lack of it. Plants cool the air in a way that concrete cannot. They move water from soil and leaves into the atmosphere, and that process removes heat from the surface. The scientific term is evaporation. If a neighbourhood has fewer trees and less exposed soil, it loses that cooling route and becomes more prone to higher temperatures.
We should also mention heat produced by daily activity. Cars, buses, machinery, and cooling units release warm air directly. Now, it would be a mistake to say this is the main cause of the heat island effect, but during busy periods it can intensify the warmth already stored in buildings and roads.
What are the consequences? On a pleasant summer day, the heat island effect may only mean a stuffy evening. The more serious impacts appear during heat waves, when the baseline temperature is already high. At that point, extra warmth in cities increases strain on the body, particularly for older people, infants, and anyone with heart or breathing conditions. Nights are crucial here. If temperatures remain high after dark, the body has less time to recover.
Energy use is another key issue. When evenings stay warm, people run fans or air-conditioning for longer. If many households do this at once, demand rises sharply and can put pressure on the electricity grid.
Urban warmth can also worsen air quality. Higher temperatures can cause polluted air to hang around for longer, and this increases urban smog, the hazy mixture that reduces visibility and can irritate the lungs, especially for people with asthma. In other words, heat doesn’t just feel unpleasant. It can change what you breathe.
So, what can cities do? The encouraging point is that solutions do not require a single dramatic invention. Instead, they involve combining several practical changes. One option is to increase greenery where it will cool people’s everyday routes, along streets, near schools, and around housing. Another option is to rethink rooftops. Some councils support rooftop gardens, not simply for appearance but because they can reduce indoor temperatures and cool the air immediately above buildings.
Surface colour also matters. Dark roofs absorb heat. Lighter surfaces reflect more sunlight. Some cities have trialled reflective coatings using a durable pale paint designed to withstand weather. These coatings can lower roof temperatures and reduce indoor heat build-up, cutting the need for mechanical cooling.
Transport policy can contribute as well. Vehicles release heat directly, in addition to emissions. If a city reduces car dependence and encourages walking, cycling, and public transport, there is less engine heat released into the street environment. It’s not a quick fix, but it’s part of a broader strategy.
Finally, we need to identify where interventions will have the biggest effect. Heat is not evenly spread across a city. Some streets are shaded. Others are exposed and paved. Increasingly, planners combine local readings from fixed sensors with mapping software to show which blocks store the most heat. That allows cities to target the hottest zones and prioritise changes where they will have the greatest benefit.
To summarise, urban heat islands are driven by heat-absorbing materials, the shape of streets, reduced vegetation, and human activity. They increase risk during hot periods, raise energy demand, and can worsen air pollution. But with targeted greening, reflective surfaces, smarter travel choices, and careful monitoring, cities can reduce long-lasting urban warmth and make summer conditions safer.
