heat image of shade cooling by urban trees

Woods are cool

Trees help provide local environmental cooling in two ways: by providing shade, and through a process known as evapotranspiration. Locally, trees provide most of their cooling effect by shading. How warm we feel actually depends less on local air temperature, and more on how much electromagnetic radiation we emit to, and absorb from, our surroundings. A tree’s canopy acts like a parasol, blocking out up to 90% of the sun’s radiation, and increasing the amount of heat that we lose to our surroundings by cooling the ground beneath us.

In total the shade provided by trees can reduce our physiologically equivalent temperature (that is, how warm we feel our surroundings to be) by between 7 and 15°C, depending on our latitude. We really notice that on the walk to and from camp – as soon as we get into the shade of the trees everyone heaves a big sigh of relief, and even on the hottest days we are quite comfortable under the canopy.

Trees can also cool down buildings – especially when planted to the east or west – as their shade prevents solar radiation (sunshine) from penetrating windows, or heating up external walls.  This is the same priciple that tells us to keep our curtains closed on hot days when the sun is shining directly on them. Experimental investigations and modelling studiesin the USA have shown that shade from trees can reduce the air conditioning costs of detached houses by 20% to 30%.

Air conditioning is, of course much more common in some places than in others: for example, while three out of four Australian households have an air conditioner, but very few households in the UK are equipped with it, as (so far) our summers have not warranted it. At times like the current heatwave that  leaves the population here much more vulnerable to urban heat. During the 2003 European heatwave, there were 70,000 more deaths recorded, compared with equivalent cool periods. Shade from trees could cool down the (un-airconditioned) terraced houses and flats that most of us live in, if we planted enough of them.

Beating the heat

Trees can also be used to tackle the phenomenon urban planners call the urban heat island. During periods of calm, sunny weather, the air temperature of cities can be raisedabove that of the surrounding countryside by up to 7°C, especially at night. In cities, the hard, dark asphalt and brick surfaces absorb almost all the incoming short-wave radiation from the sun, heating upto between 40°C and 60°C, and storing energy which is then released into the air during the still of night, when it can be trapped in the narrow street canyons.

Urban trees counter this process by intercepting the solar radiation before it reaches the ground, and using the energy for evapotranspiration. Evapotranspiration is the process that occurs when the sun’s rays hit the trees’ canopy, causing water to evaporate from the leaves. This cools them down – just as sweating cools our skin – thereby reducing the amount of energy left to warm the air.

Calculating the effects of evapotranspiration by measuring how much water a tree loses showss show that tree canopies can divert over 60% of the incoming radiation to evapotranspiration. Even a small (4m high) Callery pear tree – a commonly planted species in Northern Europe – can provide around 6kW of cooling: the equivalent of two small air-conditioning units.

But trees only provide this cooling effect when they are growing well. By measuring water loss from individual leaves, researchers showed that sparser, slower-growing plum and crab apple trees provided only a quarter of the cooling effect of faster growing trees. What’s more, the effectiveness of trees can be greatly reduced if the growing conditions are poor. Research found that the transpiration of Callery pears could be reduced by a factor of five, if the roots were growing through compacted or poorly aerated soil.

The human inhabitants of urban spaces usually ignore the trees that are growing around them – at most we will tut in disapproval at fallen leaves or branches on the pavement. It’s not easy to appreciate the role that those trees play every day in making our environment easier and more pleasant to live in. But if we can appreciate that, and try to make more room in our cities for trees, and plants more generally, the minor effort and inconvenience we incur will be repaid many times over in a cleaner, cooler and more comfortable climate for us.


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