Resource: Urban vegetation makes heat bearable

Even more than water, green spaces make us feel comfortable in a city on a hot summer’s day. So a good amount of well-distributed urban vegetation essential when we are trying to cope with the city’s heat. This has been proven by research carried out by Wiebke Klemm.

The PhD candidate in landscape architecture studied the contribution of urban vegetation to what is known as thermal comfort on hot summer days in the city. Thermal comfort is the term used to describe how we experience the heat, how (un)pleasant the interaction of temperature, wind, air humidity and radiation feels.

When it is hot, we seek shade. And there’s no more comfortable shade than that of a good tree. But oddly enough, the relationship between thermal comfort and urban vegetation has been little studied, says Klemm. Meteorologists measure and calculate everything about heat in the city, but their work says nothing about how a person experiences the heat and the role of urban vegetation in that experience. Klemm inquired into this in Arnhem, Rotterdam and Utrecht. The conclusion is clear: urban vegetation ensures that we feel ‘thermally’ comfortable. In our perception, water or the shade provided by buildings can’t compete with the cooling effect of a tree.

In Utrecht, as well as the thermal comfort in the centre, 13 parks and a rural site outside the city were tested. This involved two delivery bicycles loaded up with measuring equipment being ridden around in circles. Detailed information was produced, about the temperature, the radiant temperature and what is known as the physiological equivalent temperature, a biometeorological measure of thermal comfort. The measurement data confirmed that green spaces are a city’s islands of coolness.

In parks the air is cooler (one degree Celsius) than in the centre, the radiant temperature is lower (2.5 degrees) and the physiological equivalent temperature is also lower (1.9 degrees). The coolness varied from park to park by as much as two degrees. According to Klemm this is due chiefly to their layout. Ten percent more tree coverage, for example, knocks a good three degrees off the radiant temperature. Another factor is whether there is vegetation upwind.

The results prove, believes Klemm, the importance of urban vegetation. Owing to climate change, the heat in cities will increase. She believes this will make thermal comfort increasingly important in the design of outdoor areas. ‘Let’s retain and maintain existing vegetation and, wherever possible, provide more and better urban vegetation, so that on hot days people have a choice of thermally comfortable places.’