28 December 2017:
This morning it’s 3o F in Pittsburgh with a forecast high of 15o F. Yesterday was just as cold. It’s 20 degrees below normal here.
Less than a week ago, on 22 December, the low was 40o with a high of 57o F. It was 19 degrees above normal. To accomplish this temperature swing it rained half an inch on December 23 and froze solid on Christmas Eve night. We had black ice on Christmas Day.
Black ice isn’t really black. It just looks that way because it’s such a thin, smooth coating of clear ice that the dark pavement shows through.
Technically speaking “black ice” forms when pavement looks dry but its porous surface contains water. When that water freezes it’s invisible. So the photo at the top isn’t really “black ice” (it was probably laid down by freezing rain) but who can find a photo of something invisible?
Pittsburgh has black ice but we don’t have blue ice.
Blue ice is very old ice from the lowest layers of a glacier. It’s blue because the weight of the glacier above compressed all the air bubbles out of it. The lack of air makes the ice look blue like sea water.
I learned about blue ice in the description of this photo taken in Iceland.
So… black ice occurs where temperatures move above and below freezing (Pittsburgh). Blue ice occurs where it’s so cold that the glaciers are old (Iceland).
Pittsburgh or Iceland? Where would you rather be on this cold day?
p.s. Be careful out there! If you fall on the ice you’ll be black and blue.
(photos from Wikimedia Commons. Click on the images to see the originals.)