Here's my annual frazil ice column, my apologies. But some of you are new to the area, and for oldtimers on whom I have inflicted this topic many times before, I have a wonderful film of active frazil you can google. "Youtube Yosemite frazil ice" should do it. If you don't have high speed internet, have your grandkid show it to you. The explanation by the naturalists is not totally adequate but it is wonderful to see someone else excited about this fascinating natural phenomenon. They don't mention fairies touching their wands to the water as a possible cause, so I think that is a fairy tale invented by a certain local character!
The white stuff that fills the Hudson in winter between The Glen and Warrensburg, and sometimes for 20 more miles, has nothing to do with snow. When it finally occurred to me that it is only when the sun is shining brightly that it floats down the river, I slapped my forehead and got to work researching the mystery.
When the temperature is below about 20 F., starting as early as late November and continuing till spring, a slush forms which, when the tops of the clumps drain, turns very white. Floating snow is gray, soggy looking and ugly.
The river water has to be slightly below 32 F. (supercooled) for frazil to form, which means cold air and wind need to make up for the heat that is released when ice forms, to keep the water supercooled and producing more frazil. (Physics is hard for us non-scientists to understand, but take my word for it!) Microscopic ice particles, which I think come mostly from bubbles bursting above and falling into the turbulent water, grow big enough to float, then collect in loose clumps which eventually bump each other into "pans" with raised white edges, a foot or so across and sometimes that thick. When these collect in a cove they can freeze together making a jigsaw puzzle effect. Google "youtube lull of frazil ice" for another short video. (I think that "lull" is a typo!)