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Question 803

At times a very thin invisible layer of ice forms on road surfaces. The road surface appears dry and does not have the telltale reflections of ice. Many people refer to this as "black ice." How is this ice formed? What are the crystal properties that make it invisible? - BK
Black ice is a layer of ice that is almost free of internal defects or air bubbles and that does not have a smooth surface. The absence of internal defects or air bubbles is what makes it transparent rather than white. Snow and crushed ice appear white because they contain countless tiny surfaces. Whenever light changes speed, as it does in going from ice to air or air to ice, some of that light reflects. Since snow and crushed ice contain many ice/air interfaces, they reflect light extensively and appear white. In contrast, black ice contains no internal ice/air interfaces and doesn't reflect any light from inside. Any light that makes it into the black ice goes all the way to the roadway. If the roadway reflects any of this light, it again passes unscathed through the black ice. The only evidence that the black ice exists at all comes from its surface, but here again the ice offers little that you can see. Since true black ice is microscopically rough, the small amount of light that reflects as it enters the ice from the air is reflected randomly in all directions. So little of that reflected light travels in any one direction that you can barely see it at all. Overall, black ice reflects so little light that you see only the roadway itself. While I am not sure, I think that it forms when moisture in the air condenses to dew on the roadway and then freezes into ice. Whatever process forms it must leave it almost without holes and therefore invisible.

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