Urban planners are searching for ways to cut down on the solar heat that is absorbed from asphalt. It appears that a well-managed shade plan for trees is the most efficient way to go.

About a quarter of a century ago, in the past quarter century, the City of Chicago discovered, in the most negative way, the urban heat island (UHI) result. It was during that summer in 1995 when Chicago's Windy City had three days of temperatures of triple degrees and top at 105 degrees Fahrenheit. The reason for this was that, unlike areas of deserts and rural cities, the city didn't get much cooler at night. Around 739 people died from heat-related causes.

The importance of trees in urban areas

The idea behind cutting UHI is to restore the natural world: increasing the number of trees as well as other vegetation, in essence. Also, Chicago has been working on this goal by implementing tax policies that pay building owners who install rooftops with greenery (i.e., gardens over buildings). The city is also embarking on a tree-planting initiative, which aims to expand the canopy of trees from 17 percent to 20%.

One study conducted in the City of Montreal discovered significant correlations between the amount of trees in the city and summer temperatures, both during daytime as well as at night. Tree cover could reduce air temperature at the tree level (4ºC at 20 meters height from the ground), as well as at a higher level (2ºC at 60 meters from the ground).

Trees between concrete and asphalt aren't simple.

It is essential to choose the right species that will provide the best environment that promotes good root development, putting the trees in areas that cause the most damage to sidewalks, giving consideration to the signage of retailers, and tackling the problem of pavements that are compacted are all a part of the task. Another aspect to take into consideration is climate change and anticipating the kinds of plants that could live for a long time to come, or drought, extreme stormwater, and invasive insects are threatening the landscape.

Fortunately, there are cultivars of trees being developed to perform better in urban streets as well as with natural root systems, which are less likely to cause heave on sidewalks and pavements. Underground structures designed to limit the compaction of soil and funnel rainwater towards the tree's roots are becoming more sophisticated. Landscape designers have grasped how to plant trees where they can work well; however, they must also be able to relate to the existing structure and signage for retail in an approach that benefits instead of detracting from the community and local business.

Could White asphalt be an option?

Another relatively easy and quick modification to the pavement is to reflect the trend of white roofs in which buildings have changed from white membranes to black for flat roofs. This can result in significant savings in cooling costs and the heat that is absorbed by the atmosphere. If possible white asphalt coatings that increase the light reflectivity (i.e., that is the effect of albedo) by 5% when using fresh black asphalt to 50% when coated with white coating bring the temperatures down considerably (from 123ºF to 90ºF on the day that is sunny). This naturally reduces temperatures around the edges of the highway.

Although it is a desirable option, white-coated pavements are uncommon at the moment. The tests conducted in the Los Angeles area undertaken since 2015 have yielded mixed results. The treatment has lowered the temperatures on the pavement; however, the reflected solar energy heated the air over it. The heat is dissipated more easily through the night; however, it can be somewhat unpleasant for pedestrians walking on sidewalks that are near these streets. Research is ongoing to determine the real benefits of the sun if there are any.