Urban heat island effect is measured as the temperature difference between the air within the urban canopy layer and that measured in rural areas. Built urban environments can suppress air movements, obstructing cool flows and exacerbating pollution. However, as Alexander Robinson argues in his research for SWA, “high density cities may be our best sustainable model and so making them comfortable can have some of the largest ecological/sustainable impacts.” (Robinson, Urban Heat Island Countermeasures Summary & White Paper).
Borne of the urban condition, urban heat island effect has even more complex impacts, from precipitating the formation of smog (as temperature rises) to creating scenarios that encourage huge consumptions of energy (e.g. air conditioners during a heat island-induced heat wave). This phenomenon is mainly caused by the absorption of solar radiation by urban surfaces, and subsequent trapping of the heat-energy by the geometries of built environments. Mitigating these impacts is just as complex.
Los Angeles is currently posed for worsening heat island effects. Projected temperature rises from global warming will make Los Angeles hotter regardless of heat island effect. With a growing population, the increased development and mineralization of the urban environment seems inevitable. Surface modification, in the form of rooftop greening, has the potential to mitigate heat islands in Los Angeles. The temperature was moderated at one point in history with surface modification: the agricultural irrigation of orchards drastically dropped summer temperatures. The most water-efficient method of surface modification are trees, which produce deep shade and comfortable microclimates.
Facilitated air movement and surface modification are used as urban heat mitigation techniques. The city of Stuttgart in Germany provides an excellent precedent in designing for air movement to mitigate urban heat island effect. The city’s Urban Climatology department has identified areas critical for nocturnal air flows, called “STEP” zones. Cold-air generators (e.g. agricultural fields, forests, oceans) and flow corridors are identified and protected from development, so cool air can flow into the otherwise poorly ventilated city at night. Corridors must be at least 100m wide and kept free of obstructions (including trees).
Many reports provided in Alexander Robinson’s research for SWA recommend wider streets and stepped back buildings to improve air circulation. Depending on urban geometry, freeways have the potential to act as corridors for cool-air movement. Perhaps freeways can be enlisted in moving cool air from the Pacific Ocean through the urban neighborhoods of Los Angeles?
Alexander Robinson was a designer at SWA from 2007-2009. He is currently an Adjunct Assistant Professor in the Masters of Landscape Architecture Program at the University of Southern California.