I have a friend in the United States who has been involved in phytoplankton research for a while and spent a lot of time aboard ships sampling off Maine. About a week ago she recommended that I visit the sailing vessel Tara, which is currently docked in London having just completed a two and a half year, 70,000 mile voyage called Tara Oceans, investigating marine ecosystems and biodiversity.
Tara at St Katherine Docks
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.