City climate: small parks are better

More effective cooling in tropical areas through parks and windscapes

Park in the city: offers rest and cooling at the same time © SXC
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If city planners rave about large parks in the metropolises, the climatologist Dieter Scherer from the Technical University Berlin is rather skeptical. For, as he has shown, a network of large, but also medium and smaller green spaces acts most effectively as green air conditioning in a big city. If small parks spread across a metropolis, they can cool the warm air of tropical summer nights much better than a few large ones.

So-called tropical nights, during which the temperature does not drop below 20 degrees Celsius, are becoming increasingly common in countries such as Germany thanks to climate change. The big cities are particularly affected because the summer sun heats up the concrete on hot days. While open space stores only five percent of the energy radiated by the sun, densely built-up cities first capture half of the heat in the morning, later sinking to 25 to 30 percent. At night, however, the walls radiate the energy stored during the day and prevent the strong cooling after sunset. "In extreme situations, the nights in the city center are eight degrees warmer than in the surrounding area, " explains Scherer. Large cities therefore form heat islands in the otherwise cooler night.

Parks as air conditioning

As long as the city's parks and greenery have enough water, they look like an air conditioner in the middle of the heat island at night. At best, this coolness radiates three hundred meters away, but normally the houses that are only a hundred yards away still get a cooling wind. Therefore, only a few large parks benefit the immediate residents on tropical nights. On the other hand, many small green areas with at least one hectare in size and thus football pitch extend across the sea of ​​houses, nobody lives far away from the nearest mini-park, and the city heat island cools a little better at night.

Free track for cooling wind

However, the cool park air alone does not cool down when the air can not move: "Buildings block the wind and you do not feel much of the cool night air in the green spaces, " explains Scherer. The wide fresh air corridors that are popular with town planners are indeed very sensible from the perspective of the climatologist, but they do not bring much coolness into the city on such nights, at least in the shallow inland, because the way from the surrounding countryside is simply too far.

Much better there are different levels of development or even many parks with groups of trees and bushes and many meadows. Then the wind encounters obstacles again and again, swirls of air are formed and cold air moves from the height towards the ground. So the air remains in motion and does not accumulate pollutants, as is often the case with stationary cold air islands. display

If climate change raises temperatures in the future, especially in Grosstadt, such climate-optimized urban planning will be very important. But then the city officials should also think of enough water resources. Because the summers should be drier in the future, the climate researchers fear. And then the water could be scarce, with which the city green must be kept damp in times of drought. But if the parks wither, they heat up even more than concrete. Then the desert island of Gro stadt could become the heat island at night.

(TU Berlin, 28.08.2007 - NPO)