Introduction

Early in 2011, Mackey et al. (2011) used data from LANDSAT 5 to identify significant albedo increases around 0.016 happening over the City of Chicago between 1995 and 2009 that were well correlated to drops in remotely-sensed temperature. These drops were so large in some sites (around 3-5 oC) that they suggested the city's microclimate might be cooling from this overall albedo increase. Such a finding is particularly important since Chicago, over the last century, has been experiencing a worsening of its urban heat island effect, up to the point that it was largely blamed for over 500 deaths in a heat wave occurring in 1995, which boasted the largest death toll of any heat event in American history. (Changnon et al., 1996). In addition to the threat from urban heat island, several recent climate change forecasts have projected that the future climate of Chicago over this next century will approach that of the present American South (Kaufman, 2011). In these forecasts, the number of days in a year with temperatures above 32oC will rise to over 72 and will inevitably have detrimental impacts on citizen heath and energy use with air conditioning. Faced with these two sources of increasing temperature, it is clear that cooling strategies will be an essential part of the city’s policies in the coming decades.

While Mackey et al. were able to show that the albedo had been increasing over the city and that the remotely-sensed temperature had been dropping, they were only able to hypothesize what the cause behind this increase could be using “qualitative visual observations.” The specific hypothesis that they reasoned was that the increase had resulted from a mandate in 2003 that roofs meet specific requirements of reflectivity in order to receive a permit for renovation or construction. Specifically, this code required that roofs with slopes greater than 5% have a minimum reflectivity of 0.25 and roofs with slopes less than this have a minimum 3-year reflectivity of 0.5 or an initial reflectivity of 0.7. (CEEPPU, 2008).

Recently, the City of Chicago has made publicly available a shape file with aerial image traces of every building footprint in the city. (The City of Chicago’s Official Website, 2010) Using this file, it is possible to quantitatively verify Mackey et al.’s hypothesis by observing the changes in albedo in relation to the building footprints since it can be assumed that any large albedo increase occurring in a building footprint must be the result of a change in roofing material. Additionally, with the building characteristics listed in the shape file, it is possible to identify the building characteristics producing the greatest increases in reflectivity, which will help identify future sites for a reflective roof campaign.

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Contents:

Introduction