Why look darker when the road is wet?
If you get wet you or raining on the ground, bumpy ground to contain the water, it will be flat than when was dry the ground. It is what the figure below has been described it.
Dry ground
Wet ground
In this way, since the ground is bumpy less and get wet, fewer diffuse reflection light, color will also be darker. In addition, the light will be slightly more in the direction of specular reflection.
Summary
Why ground looks darker wet?
To flatten the ground by water entering the bumpy, because the diffused reflection is reduced, the color will be darker.
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2015 身の回りの光と色の現象