On a hot afternoon last August, a group of high-school students walked through their neighborhood carrying thermometers. What they found surprised them. The temperature in a wide car park covered in dark asphalt was several degrees higher than in a small park just two streets away, where tall trees shaded the grass. "It felt like walking from an oven into an air-conditioned room," said Tyler, 16, one of the students. Their measurements became the starting point of a project that has since planted more than two hundred trees in the area.
The students were observing a well-known effect called the urban heat island. Cities are often noticeably warmer than the countryside around them, and the difference is usually greatest at night. There are several reasons for this. Dark materials such as asphalt absorb the sun's energy during the day and slowly release it as heat after sunset. Tall buildings trap warm air and block cooling winds. In addition, cars, air conditioners and factories all give off heat of their own.
Trees are one of the simplest ways to fight the problem. Their leaves block sunlight before it reaches roads and walls, so the surfaces underneath stay cooler. Trees also release water vapour through their leaves, a process that cools the air around them, rather like sweat cools the human body. As a result, a street lined with trees can feel far more comfortable on a summer day than a bare one, and the buildings along it may need less energy for air conditioning.
Heat is not only a matter of comfort. During heat waves, high temperatures can be dangerous, especially for elderly people, small children and people with health problems. This is one reason why many cities around the world have begun planting trees, painting roofs white to reflect sunlight, and creating more green spaces.
The students' project has not been easy. Young trees need regular watering for their first few years, and some residents worried that roots would damage pavements or that falling leaves would block drains. The students met with residents to answer their concerns and organized volunteer teams to water the trees in summer. "A tree we plant today will only give real shade in ten years," admitted their teacher, Ms. Hadad. "But my students say that is exactly the point. They are planting for the people who will live here after them."