On a hot summer afternoon in downtown Salt Lake City, I found myself scuttling from one patch of shade to another.
No one would argue that we find relief from the urban heat in the circles of shadow beneath our streetside trees.
In one study of urban trees, identical test surfaces placed beneath tree crowns were as much as 20 to 40 degrees cooler than those placed in full sun.
Why does tree shade feel so good?
The obvious answer is that tree leaves block sunlight. By obstructing that incoming solar energy, our skin — and the pavement around us — doesn't heat up as much.
Of course, buildings and awnings cast shade, too. But trees have an additional way to cool their surroundings: transpiration.
Trees draw water from the soil and release it as vapor through their leaves. Evaporating water uses heat — just as evaporating sweat cools our skin. Across thousands of leaves, that turns a tree crown into a living evaporative cooler.
So a tree does two things at once: its crown intercepts sunlight while its leaves release water that helps dissipate the heat.
Not all trees create the same kind of shade. A maple tree, with layers of broad leaves throughout its crown, blocks much more sunlight than the lacy canopy of a honey locust tree, whose small leaflets create shifting patches of sun and shade.
When I step from an open sun-baked sidewalk to a place beneath a tree, I'm not just stepping into its shadow. I'm entering a patch of summer that the tree has made cooler.
