Practice Test 22: Urban Heat Islands and the Science of Habit

A On a still summer evening, the centre of a large city can be several degrees warmer than the countryside around it. This difference is known as the urban heat island effect, and it is now measured in cities on every continent. The extra heat is not a matter of comfort alone: it raises demand for air conditioning, worsens air pollution and increases the risk of illness among older people and those with heart conditions.

B The main cause is the materials from which cities are built. Dark roofs, asphalt and concrete absorb sunlight during the day and release it slowly at night, so the city does not cool down as the countryside does. Cities also have fewer plants. Vegetation cools the air because water evaporates from leaves, a process that uses up heat, and trees shade the ground below. Tall buildings add to the problem by trapping heat between them and blocking breezes.

C Solutions fall into two groups. The first changes surfaces: light coloured or reflective cool roofs send more sunlight back into the sky, and pavements can be made from materials that stay cooler. The second adds greenery, from street trees to roof gardens. Studies suggest that trees work best when planted in continuous belts that provide shade for pedestrians, rather than as isolated specimens. Each approach has limits. Cool roofs help the buildings beneath them but do little for people at street level, while trees need water and care, which is costly in dry regions.

D Planners increasingly combine the two, and they add a third element: the layout of the city itself. Leaving corridors for wind to pass through the city, and keeping open spaces such as parks and rivers, can lower temperatures across whole districts. The benefits are greatest for poorer neighbourhoods, which tend to have fewer trees and more paved surfaces, and so suffer the highest temperatures.

A Much of what we do each day is not decided afresh. Psychologists estimate that a large share of daily behaviour, from brushing teeth to the route taken to work, is repeated automatically in the same setting. This is efficient: it frees attention for new problems. But it also means that changing a habit is harder than simply deciding to change.

B Researchers describe a habit as a loop with three parts. A cue, such as a time of day, a place or a feeling, triggers a routine, the behaviour itself, which is followed by a reward that the brain registers as worth repeating. Over time the link between cue and routine strengthens until the behaviour happens with little thought. Because the cue is so important, people who move to a new home or start a new job often find old habits easier to break, since many of the old cues have disappeared.

C How long does a new habit take to form? A popular claim is twenty one days, but there is little scientific support for it. A well known study that followed volunteers as they tried to adopt a simple daily behaviour found an average of about two months, with a wide range from person to person. Simple actions, such as drinking a glass of water after breakfast, became automatic sooner than demanding ones such as fifty sit ups.

D Missing a single day did not appear to damage the process in that study, which is an encouraging finding for anyone who has given up after one lapse. Experts suggest practical steps: attach the new behaviour to an existing cue, keep it small at first, and make the reward immediate. They also warn against relying on willpower alone, because willpower tends to be lowest at the end of a tiring day, exactly when many habits are enacted.