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Slippery When Wet
Bar Harbor
alexx_kay
What was it about last night's rainfall in the Boston area that made footing so unusually treacherous? I nearly fell on a number of occasions. Was there some unusual chemical compound coming down with the rain, that was extra-slippy? Or was it because of the way the temperature was hovering right around the freezing mark? I didn't observe any actual *ice*, mind you -- just patches of wet sidewalk that were unexpectedly slippery. Or is the answer more obscure than either of those?
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I suspect the sidewalks hadn't warmed up with the air temp, and there was a VERY thin layer of ice. I had that same issue coming home last night as well. ("Now, if could just be sure of the right trajectory, I can skate down the hill to my front door, right? Right?")

My usual finding for that set of conditions: it mists gently as the temperature went below freezing, giving time for a nice layer of ice to fill the rough pockets in the concrete. As the air temperature warms up, a thin layer of water forms over the smoothed concrete, leading to very slippery conditions. It doesn't really look like ice but if you look very closely you can see that the apparently wet concrete is actually filled in a bit.

That sounds to me like a pretty good definition of so-called "black ice".

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