Urban Forests: Natural Air Conditioners

What are urban heat islands and how do urban trees perform air conditioning services? Find out more in this blog post by Ana Simčič, a forestry engineer from Slovenia.

What do you prefer in a summer afternoon – a walk in a crowded city center or a walk in a forest? I’m sure everyone gets relieved when after a hot walk a tree shadow appears in front of them, waiting to cool them down and let them breathe a bit fresher summer breeze. Dense canopies certainly provide a much needed relief in hot summer hours.

Every city that is surrounded by forests or comprises forested fragments can be grateful for the effects they bring, especially in the summer time. Forests mitigate and moderate heat by absorbing less heat than neighborhood concrete areas and buildings. Trees lower surface and air temperatures by providing shade and through evaporation of water from the soil and leaves.

The extra heating load that comes with replacing natural vegetation with buildings, streets and sidewalks, has been recognized many decades ago. Cities are often warmer than surrounding rural areas – a phenomenon known as the “urban heat island effect”. It is caused by the absorption and storage of the sun’s thermal energy in urban infrastructure (steel, concrete or asphalt). A heat island forms over an urban area and is like a large bubble with higher temperatures than the rural surroundings. The phenomenon is common in large metropolitan cities, but it’s also noticeable in smaller cities like Ljubljana. The temperature differences between forested areas and urban environments are most pronounced during heat waves. When temperatures in urban concrete areas are near dangerous to impair human health, neighborhood green areas provide conditions that are more human friendly.

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Urban heat island effect, caused by solar energy.

Recreation in summertime is much more healthier in forested areas, where air is less polluted. A dense canopy of healthy trees can reduce the effects of air pollution associated with increasing urban temperatures. Air pollution is a serious public health threat linked to asthma, migraines, respiratory and heart diseases and most of them get especially pronounced during summer heat waves.

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A picnic in the shadow of the trees.

Trees also save energy by shading our homes and paved surfaces. Mature trees can significantly reduce summer air conditioning use by shading the sides of our homes that are exposed to overheating. In autumn, deciduous trees lose their leaves and allow us to solar heat our homes and reduce winter energy use.

Heat tires everyone – people, animals and also plants – that’s why a walk in the forest is always a good decision. In the forest, the air is more humid, temperatures are lower and the canopies protect us from harmful UV rays and sunburns. If we don’t forget a bottle of cool water, there is a pleasant walk in front of us despite the high temperatures outside of the woods.

This blog post is authored by Ana Simčič and is part of the #EFUF2016 blog competition.

Featured photos by WeatherQuestions and FHWA.

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