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I have a very basic question that I'm struggling to find a clear answer to as I do research about cool pavements.

The literature says cool pavements should have high albedo, high thermal emissivity. The high albedo makes sense. However, my understanding of emissivity is that this property describes how stored heat is released into the environment. Wouldn't a high thermal emissivity basically mean that heat is captured by the pavement, then reflected back into the street? Seems like that would increase rather than decrease the ambient temperature. (On the other hand, storing that heat increases the temperature of the pavement material we're standing on, which also doesn't sound good). What am I getting wrong?

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Wouldn't a high thermal emissivity basically mean that heat is captured by the pavement, then reflected back into the street?

The good part of the energy captured would be released on the streets right away but less than if the pavement was white, another part would be emitted later in the day

There's a trade-off to be made between having a clear pavement capable of reflecting incident light (which increases the amount of energy received by a passerby walking on the street) and a material that would absorb this energy. Cities located on the Mediterranean coast (in Greece, Morocco) seem to have solved the issue by opting for a light gray pavement for the ground and walls up to human height (thus a rather absorbent pavement to limit the radiation received by passersby) and by painting other surfaces white to store a minimum amount of energy to make evenings more bearable.

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  • $\begingroup$ Hi Hugo, thanks for the response. I'm not sure that 100% answers my question, however (or at least I'm still not clear). So, to paraphrase, 'some part of the energy would be released right away, and some later in the day.' What is preferable? Releasing right away (high emissivity), or later in the day (low emissivity)? It's not clear to me which is preferable from the perspective of a pedestrian or somebody actually using the street. Thanks! $\endgroup$
    – user131627
    Commented May 9 at 10:58
  • $\begingroup$ @user131627 It depends on what you want. If you are designing a public area used only in the day you may prefer to have a rather dark pavement in order not to reflect too much energy on the pedestrian (but it leads to some inconveniences, i would not walk a dog on such dark pavement ). If you want to be able to have a fresh area for the evening you may prefer a lighter pavement color. Hope this helps ! And sorry, english is not my first language. $\endgroup$ Commented May 10 at 11:20

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