Shade Trees Are Cool

by Sam Cox

In the dog days of summer, nothing beats a shady spot under a big tree for a cool, relaxing picnic. Not only is the air temperature 2 to 9°F cooler in the shade, but your clothes, skin and food can be 20 to 45°F cooler than if they were in the sun (EPA 2019).  Anyone who has ever tried to sit on a metal bench in the sun knows this. Lighter leaf color, higher foliage density, thick leaves and rougher leaf texture all enhance the cooling effect (Lin & Lin 2010). You can see this for yourself by enjoying the deep, dark, cooling shade under a Norway maple, which has thick leaves and a very dense canopy, and comparing that to the less-cooling dappled shade under a honeylocust, which has thinner leaves and a sparse canopy.

Strategic placement of shade trees around a home can reap huge energy benefits. For example, a single shade tree planted on the southeast corner of a house can provide up to 5°F hourly cooling to the east façade of the building, and 2 trees planted in the same spot can provide up to 14.5°F hourly cooling (Zhao et al 2018). The EPA recommends planting trees on the west side of houses for maximum cooling effect. Shade from properly-placed trees can reduce air conditioning costs of a house by 20-30% (Ennos 2015).

In addition to blocking sunlight, the trees cool the air through evaporative cooling, the same process swamp coolers use to cool many residents’ homes in summer. Every leaf has tiny pores that lose water vapor to the air as the tree exchanges oxygen for carbon dioxide. As water vaporizes, it pulls heat energy from the air, providing a cooling effect, just like sweat from your body. In fact, by measuring the water loss from a single 13-foot pear tree, scientists in Europe calculated its cooling effect was equivalent to around 6kW of cooling, the equivalent of two small air-conditioning units (Ennos 2015). All plants cool the air around them, which is why a green soccer field is so much cooler than an adjacent parking lot.

Trees combine the cooling effects of evapotranspiration and shading to create excellent additions to a landscape if you want to beat the heat in the summer. By planting deciduous trees that drop their leaves in the fall, sunlight is allowed to shine through tree canopies on to houses and sidewalks to warm those areas when warmth is needed, and cool those same areas in the summer when cool temperatures are desired.