Evaluations of Strategies to Cool Urban Environments


Mackey CW, et al., Remotely sensing the cooling effects of city scale efforts to reduce urban heat island, Building and Environment (2011), doi:10.1016/j.buildenv.2011.08.004
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Mackey CW, A Verification that Large Albedo Increases in Chicago Resulted from New Reflective Roofs and an Identification of Building Characteristics Producing Large Albedo Increases. Web published Dec. 19, 2011.

Mackey CW, Grant Proposal: Heat Wave Resilience Beyond Air Conditioning - Incorporating Alternative Cooling Strategies into the United States LIHEAP and WAP. Web published Dec. 29, 2012.


Full Results and Background (1 hr 30 min)
Summary of Results (30 min)

Work With the Yale Heat Budget Group

Project Page on the Yale Heat Budget Website
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As global temperatures continue to rise amidst burgeoning interanational urbanization, the urban heat island effect has become an increasingly prominent issue and has inspired several analyses of techniques to combat it in recent years. However, nearly all of these analyses have either been evaluations of small-scale instances of technique application or they have been attempts to computer model the effects of large scale applications. As a result, a debate has persisted over whether a strategy that increases urban vegetation or reflectivity is more effective with both sides lacking support from analyses of the actual implementation of such methods over entire cities. Within the last 15 years, the city of Chicago has deployed a variety of methods in both the vegetative and reflective categories and has consequently established itself as a suitable testing ground for these strategies' comparison. This study is an attempt to evaluate such efforts by observing Chicago's recent vegetated/reflective surfaces and their temperature in LANDSAT imagery. Results show that Chicago's new reflective surfaces since 1995 produced a noticeable impact on the albedo of the entire city, raising it by about 0.017, while overall increases in vegetation index (NDVI) hover around 0.007 but vary with precipitation. This finding along with the fact that an average of 27,548 pixels in Chicago increased in NDVI but dropped in temperature while 55,357 pixels did so for albedo suggests that the reflective strategies influenced a larger area of the city than the vegetated methods. In addition to this, plots between albedo increase and corresponding LANDSAT surface temperature change over the test period have much steeper slopes (-15.7) and are more strongly correlated to temperature decreases (-0.33) than plots between NDVI increase and temperature change (-8.9, -0.17). This indicates that the albedo increases produced generally lower temperatures than those of NDVI, which is surprising since NDVI exhibits a much steeper slope (-16.20) and stronger correlation to lower temperatures (-0.67) in single scenes of the city. Observation of aerial images confirmed that typical instances of efforts to increase albedo, such as reflective roofs, produced more intense cooling in the LANDSAT temperature change images than common instances of NDVI efforts, such as new green roofs, street trees and green spaces. Accordingly, the reflective strategies were likely much more effective at combating Chicago's heat island from 1995 to the present and may signify a more effective method for similar cities.

Average Reflectivity Change of Chicago Average Surface Temperature Change of Chicago