"Cool surfaces are measured by how much light they reflect (solar reflectance) and how
efficiently they radiate heat (thermal emittance). Solar reflectance is the most important
factor in determining whether a surface is cool. A cool roofing surface is both highly reflective
and highly emissive to minimize the amount of light converted into heat and to maximize
the amount of heat that is radiated away. Every opaque surface reflects some incoming
sunlight and absorbs the rest, turning it into heat. The fraction of sunlight that a surface reflects is called solar reflectance or albedo. White roofs reflect more sunlight than dark roofs, turning less of the sun's energy into heat. Increasing the reflectance of our buildings ... can reduce the temperature of buildings and cities.
Most roofs are dark and reflect no more than 20 percent of incoming sunlight (i.e., these surfaces have a reflectance of 0.2 or less); while a new white roof reflects about 70 to 80 percent of sunlight (i.e., these surfaces have a reflectance of 0.7 to 0.8).
New white roofs are typically 28 to 36 degrees Celsius cooler than dark roofs in afternoon sunshine while aged white roofs are typically 20 to 28 degrees Celsius cooler."
The Albedo Effect
Comparison of a black and a white flat roof with an air temperature of 37 degrees Celsius
Extracted from 'A Practical Guide to Cool Roofs and Cool Pavements', by Global Cool Cities Alliance, 2018