Sunday, February 23, 2014

Some Icy Thoughts on a Day of Melting

New York City is having a big February thaw! As we start to melt out of all the ice and snow that's built up over the past couple of months, it's a good time to ask: How did some of this stuff freeze in the first place? Like the dripping water from the pipe you'll see in the video below. (Or even more amazingly, Niagara Falls in upstate NY and Canada, seen beneath our video footage in an historic old photo.) Do you know how moving water freezes? Check out the video for clues. See what you think. Then, and only then, read the answer!

Are you ready for the answer? Here it is: 

As you may have guessed, moving water takes a lot longer to freeze than still water. And it doesn't freeze all at once. Instead, it "supercools" when its temperature drops below freezing. That's when water molecules slow down enough to start to stick together into tiny discs. Then these discs clump to each other, and also to any other surfaces they touch—like the brick wall and metal pipe in our video. The icy portion slowly grows and grown, cooling the still frozen water around it. Until that freezes, too.

Did you get the right answer? Or did you have another hypothesis (guess) entirely? Write in and let us know!

Video by Robert Cowan.

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