Thursday, September 18, 2008

Black Ice

I learned today why black ice is so hard to predict: it isn't ice before you drive on it. When the temperature first falls below freezing, water on the road is supercooled - its temperature drops below its freezing point but it stays liquid because nothing provides the seed crystal to start ice forming or provides the energy for the water to change states. As soon as your tire hits the water, it shocks the water into instantly crystallizing into ice and you are sliding across the "wet" road. That's why you can't see black ice when you are driving; watching the thermometer is a better gauge of hazard than watching the road surface.

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