Tree canopies and shaded canyons

Even though humans build cities, in a way creating our own habitats, we don’t completely understand the interactions of this built environment with nature and how much comfort or discomfort we might experience. Throw in the climate weirding, which brings more frequent extremes of heat, precipitation, as well as quick variations, and you quickly see how this understanding must be much more detailed and the lessons applied. The Science Museum of Virginia and the Portland State University SUPR Lab teamed up with a network of local collaborators, volunteers, and newscasters to measure Richmond’s heat island effect.

We observed a 16°F difference between the warmest and coolest spots in our city at the same time on the same day! […]

In total, we collected over 74,000 temperature readings across the District. Here too, our results showed a ~16°F difference between the warmest and coolest spots in the city during the hottest time of the day (3-4 pm).

Their research confirmed something we know, and added some details to something less well understood. It’s already well established that tree cover can help in greatly reducing cities’ heat island effect, just walk into a park with lots of trees on a hot day and it can almost feel like the place is air conditioned, the same thing happens in neighbourhoods with lots of trees.

The other thing they observed however, is that building height also greatly impacts the heat effect.

[T]he dense, building height-varying urban cores have relatively lower air temperatures than the sprawling, wide-streeted but single-height residential areas that lack extensive tree canopy. Turns out this isn’t a new discovery, but it is one that we could use to advance density as a physical microclimate control.

Basically, when buildings are taller than the street is wide, they form a “shaded canyon,” thus reducing the heat that’s accumulated throughout the day, and of course what dissipates during the rest of the day.

Computer modeling studies that simulate increasing building heights in Adelaide, Australia show that once building heights surpass the width of the street, this creates urban canyons that shade the street surfaces and cool the air.

Those buildings don’t have the “sponge” effect that permeable terrain around trees and other vegetation provide when precipitation comes, but more tree cover and taller buildings on narrower streets do align perfectly with the denser, more human-scale places favoured by the people who can pick where they live in larger cities, as well as many recommendations for more sustainable, less climate-impactful cities.