Going out for a walk through the center of a big city at three in the afternoon in the middle of July is a great challenge for many people because they literally begin to roast due to the high temperatures. But in these situations, taking shelter in a dense park promises relief almost immediately, as has been demonstrated in different areas of Valencia such as the Bioparc, where the wind chill drops 13 degrees with respect to the asphalt environment, as its sensors have demonstrated.
A question of perception. When we read “13 degrees minus”, our brain immediately thinks about the air temperature. And this is where it is important to qualify, since a tree is not going to take air at 40 °C and magically convert it into air at 27 °C. However, air temperature is only part of the equation.
What really determines whether we are comfortable or on the verge of heat stroke is the cthermal comfort, measured by indices like PET (Physiological Equivalent Temperature). These indices take into account air temperature, but also humidity, wind speed and, above all, the average radiant temperature.
On the street. Science in this case shows that direct solar radiation on our skin and the heat radiated by hot asphalt trigger our thermal stress. A specific study on shade infrastructure and outdoor thermal comfort demonstrated that urban shading can reduce the PET index between 9.4 and 17.1 ºC. Therefore, the 13-degree drop cited by the Valencia data fits perfectly with what science has been measuring for years. The air temperature does not drop drastically, but the radiant load does plummet.
Why are trees so good? Here science has shown that tree canopies block direct solar radiation, and a field study of tree shading confirms that this interception of sunlight causes large drops in the mean radiant temperature under the canopy, eliminating the “oven effect” of direct sun.
But they also act as a natural air conditioner, since, unlike a metal awning or pergola, which heat up and radiate heat, the trees are alive. This means that they absorb water through the roots and release it in the form of vapor through the leaves. This process consumes thermal energy from the environment, actively cooling the air around them, even if it is a few tenths or a couple of degrees.
We need trees. The debate about whether to put more or fewer trees in a square is no longer a purely aesthetic or landscaping issue. It is directly a matter of public health, since science is quite clear that exposure to extreme heat triggers mortality and morbidity. That is why these natural environments act as a protective shield that is very necessary.
It is no coincidence that institutions such as the Climate Cities Network are publishing technical guides to integrate “climate shelters” into urban planning. And it is no wonder because a climate shelter is nothing more than a space that offers safe thermal conditions for the vulnerable population during episodes of extreme heat. And the data makes it clear, since a well-designed park, with trees with wide, dense crowns, is one of the best possible refuges.
Images | Andy Lee
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