Chap. 2: Shading & Cooling
Most people live in cities containing very high population densities — densities much higher than the natural ecological world expects. What unique environmental properties do cities possess as a result? Here I first demonstrate how much urban areas have sacrificed vegetation — indeed, that’s why they’re urban — and produced much higher temperatures than surrounding rural areas. In thermal satellite images these so-called urban heat islands (UHIs) poke out of the cooler rural background. Thermally massive urban surfaces, like parking lots and cement buildings, radically change urban environments, making cities somewhere around 5-10C (9-18F) hotter than nearby forests. Interestingly, North American and European cities show distinct urban heat island patterns concerning temperature differentials as a function of a city’s human population. Different building styles, measured through a “skyview factor,” resolve this difference: At street-level, you can view more sky in European cities, and this difference makes them cooler. In one well-documented example, Los Angeles, California’s urban temperature increased about 4C (7F) over the last 120 years. Considering the concomitant population increase, that change corresponds to what one expects from a North American urban heat island.
Urban heat islands can change weather patterns at local, regional, and continental scales, from shifting the time of day that precipitation occurs to inducing localized thunderstorms. Studies from Atlanta, Georgia, for example, even demonstrate that more lightning strikes take place in urbanized areas compared with outlying regions, and lightning strike data suggest a similar pattern for urban areas of Durham County. City atmospheres also influence local climates, just like glass greenhouses and global greenhouse gasses, but particulate emissions complicate the picture by adding a shading effect.
Parking lots, with their extensive impervious surfaces and high thermal mass, represent one location where trees and vegetation can reduce heating and its related issues. However, business owners’ profit motives and safety concerns rarely align with optimal shading, and comparing parking lot trees with their natural shading potential clearly demonstrates suboptimal shading.
The heating of impervious surfaces generally matches and confirms the scale of urban heat island warming, and also matches the amount of heat that evaporates a light rain. I then examine the role urban vegetation plays in cooling cities and treating stormwater, and discuss whether any significant energy reduction and carbon sequestration benefits can be expected. Comparing the cooling potential of a tree shows that a tree just can’t transpire enough water to cool these high thermal mass surfaces. Further, trees experiencing realistic urban scenarios have an even bigger challenge as their transpiration systems shut down due to high heat. Simply painting these cement and asphalt surfaces white, on the other hand, could greatly change how much heat they absorb. Surprisingly, lawns have a higher potential than trees for cooling via transpiration, and interesting approaches combine parking lots with grass.