Supercooled Pond

This morning I had a walk round my back garden and noticed something strange about the pond. It was covered in hundreds of floating needles of ice, all roughly the same length, about 40mm. I put my hand in and fished a few out. The ice must have covered less than 1% of the surface at this point. The pond was in sunshine and although it had clearly been below freezing during the night I guess the air temperature was now approximately two degrees celsius, certainly above freezing at any rate. I assumed that the needles would soon melt and I went back in the house. I think it was about half an hour later when I went out again, and to my great surprise the pond was entirely frozen over! The ice was no more than 1mm thick but it formed a continuous layer. My only explanation is that the surface layer of water had supercooled during the night and something had caused nucleation to occur just before I first went out. I wish I had stayed and watched because presumably the needles were growing when I saw them and I could have watched the freezing process.

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