Albedo Modification

The urban surface is generally less reflective than the unbuilt surroundings. This difference in albedo is a contributor to the urban heat island effect, but also points to a significant opportunity to mitigate heat islands. Surfaces that are candidates for albedo modification are rooftops (flat roofed commercial buildings, in particular), and paved roadway and parking lot surfaces. In MIST albedo modification is assumed to be uniformly applied across the entire city. Albedo fractional increase is limited in MIST to the range of -0.5 to +0.5, where negative values reflect a reduction in albedo. Users should refer to the resource links at the left for more information on potential for albedo modification.

Cool roofing materials have two properties that make them significantly cooler than traditional materials during peak summer weather: (1) high solar reflectance (albedo); and (2) high thermal emittance. High solar reflectance means that these materials reflect away more of the sun’s energy than dark-colored materials characteristic of traditional roofs. The coolest materials also have high thermal emittance to help them radiate away heat that is absorbed. Surface temperatures of cool roof materials generally stay below 120°F (50°C) in even the hottest, sunniest summer weather, while traditional roof materials can heat up to 190°F (90°C). Cooler roof surfaces help reduce the heat island effect because they store and transfer less heat to the air both during the day and overnight. On a clear, dry summer day, up to 80% of the solar heat reflected by cool roofs passes back through the atmosphere and out into space, and does not heat the air in the city.

Unlike cool roofing, there currently is no official definition of “cool paving” materials. In addition, it is important to note that non-heat island factors such as performance, safety, durability, and minimal traffic delays, are critical factors to consider when selecting pavements. Also, while with cool roofing the choices are typically between similar products (just with different colors), with pavements the choice is among fundamentally different products with different performance characteristics. Cool paving products provide heat island mitigation benefits by either reflecting more incoming solar radiation than traditional materials or enhancing permeability.

Advanced users seeking more detailed information on this and other topics related to the scientific and modeling underpinnings of the MIST software tool should read the detailed model description document that can be downloaded from the MIST website.