It might seem like a futuristic idea, but it’s beginning to look like driverless cars are here to stay. Imagine having your driverless car take you downtown to shop. There’ll be no need to look for a parking spot. You can simply step out of the car and tell it to drive somewhere and wait for you to call it back. It could go home and wait in your garage, go to a designated parking area outside the urban core, or better yet, it could be a shared car and you could let it transport someone else around while you’re shopping. If cars no longer need to be parked near a destination it could mean the beginning of the end for those expanses of hot asphalt in the centres of our cities. There would be no need to park in the urban core. If this land was converted to almost any other use it would reduce the intensity of the urban heat island. But if the land was converted to well-watered, shady green space, urban heat islands might be converted to urban cool islands. An energy budget analysis helps to explain the reason for this transformation. Let’s first compare the effect of solar radiation falling on an asphalt parking lot and an expanse of grass. Solar radiation is either reflected or absorbed by a surface depending on its colour. Black asphalt absorbs a greater proportion of the solar radiation that it receives than does green grass. The energy absorbed by the surface is then converted into various streams of energy. If the surface is wet or transpiring, like the grass, then a lot of the energy will go into evaporation. But if the surface is dry like the asphalt then this stream is not available. All the energy absorbed by the asphalt goes into heating the asphalt and then either heats the air next to it through convection, or is emitted as terrestrial radiation. Both of these streams of energy help to create the urban heat island – warmer air and more terrestrial radiation. Meanwhile, the grass, having started with less energy, and then having lost some to evaporation, has much less energy available to heat itself or the air next to it, or to emit as terrestrial radiation. Cooler surfaces translate into less of an urban heat island.
Now let’s add trees to both landscapes and consider the effect. Instead of the solar radiation falling directly on the surfaces much of it would be intercepted by the leaves and branches of the trees. The radiation that makes its way through the canopy would then be transformed in the same way as the radiation in the previous example. The only difference would be that there would be a lot less energy to start, and the effect on the urban climate would consequently be much less. Even parking lots will stay reasonably cool if they are shaded by healthy trees.
Here’s a thermal image of an asphalt surface beside a park with grass in the foreground and trees in the background. Using the scale along the right-hand side of the image you can ‘see’ the hot asphalt (pink and white), the cool grass (yellow), and the very cool shade of the trees (blue). Some of the grass in this image is quite dry, so is not as cool as the well-watered grass and appears as a blotchy red/yellow pattern.
Using this information there are two things you can do to help reduce the urban heat island in your city.
Lesson One: Plant trees that shade asphalt surfaces to reduce the effects of urban heat island intensification.
Lesson Two: Prepare for a future of driverless cars by starting to think about transforming parking lots into well-watered, shady green spaces.
Who knows… maybe we can unpave parking lots and put up a paradise.
Read more about energy budgets in Microclimatic Landscape Design by Robert D. Brown and Terry J. Gillespie, chapter 3, pages 45-63.