#I forgot about Fahrenheit for a moment
what does fahrenheit have to do with anything, i took these figures from an article that gave its figures in celsius (although i now can't find the article again, that's on me not citing my sources cos i was just talking to myself), but here's a scientific article stating the same figures:
The process of urbanisation alters the thermal balance of an area resulting in an urban heat island effect where cities can be several degrees centigrade warmer than the surrounding rural landscape. This increased heat can make cities uncomfortable places and, during heat waves, can pose serious health risks. This study looked at the role that trees and grass can play in reducing regional and local temperatures in urban areas during the summer within the urban landscape of Manchester, UK. In June and July 2009 and 2010, we monitored the surface temperatures of small plots composed of concrete and grass in the presence or absence of tree shading, and measured globe temperatures above each of the surfaces. The same measures were also recorded at mid-day on larger expanses of asphalt and grass in an urban park. Both surface and shade greatly affected surface temperatures. Grass reduced maximum surface temperatures by up to 24 °C, similar to model predictions, while tree shade reduced them by up to 19 °C.
this is about surface/ground temperature rather than air temperature, if those figures seem high to you: the woodland trust notes that
"However, in a study from Manchester, shade from street trees reduced surface temperatures by an average of 12°C and concrete surfaces shaded permanently by a bank of trees were cooled by up to 20°C in the summer (Armson et al. 2013), although these had no effect on air temperatures." [source]
elsewhere on their website they also reference research in cardiff showing that surfaces in direct sunlight are about 20C higher than surfaces in shade
you might think that that means they don't help in a heatwave, if it's not the air temperature that's impacted, but one thing i really noticed in this one was the impact of the bricks of my flat absorbing heat from direct sunlight during the day -- at night, although air temperatures had cooled, the bricks were still radiating heat like an oven. however, the wall that is shaded by a tree did not absorb as much heat, because it did not get direct sunlight, and therefore that part of the flat did not have this heating effect at night
this is one way that trees can help mitigate the heat impacts of concrete and other built-up surfaces! even where they don't directly cool the air in the moment, they can prevent the surface heating that causes further heating after air temperatures drop. also, of course, it is dangerous to walk or rest on hot surfaces, and this disproportionately affects vulnerable people, like children (closer to the ground, using play equipment etc) or rough sleepers. last week, pavements in london reached 57 degrees celsius. in one place it was 65C!! [source]. this is something that trees can mitigate
