Frazil ice, as you see here in a recent picture of the Hudson River, forms in turbulent rivers (and the ocean and sometimes lakes) when the air is below about 18 degrees F. The river water itself has to be a fraction of a degree below 32 F (super-cooled).
For winter newcomers to the Upper Hudson, and you who have not read or understood my many attempts to explain this before: That brilliant white stuff that sometimes fills the Hudson from Thurman to The Glen is not snow, and has nothing to do with it. It is “frazil ice” (pronounced like frazzle), which is made of tiny, round, very thin disks of clear ice).
Frazil forms in turbulent rivers (and the ocean and sometimes lakes) when the air is below about 18 degrees F. The river water itself has to be a fraction of a degree below 32 F (super-cooled).
The nucleus of each frazil crystal is an ice crystal, often from the freezing of tiny drops from bursting bubbles. The frazil disks try to put out “dendrites” (like snowflakes) but they keep breaking off in the rough water, creating unending nuclei.
When the tons of frazil floating down the river hang up on gravel islands or get clogged in a curve, a floating cover of loose frazil can form across the river.
The inexorable frazil coming down the river gets pulled under the cover by the current, where it collects and builds “hanging dams”, which eventually block the current.
The river water is slowed down by the blockages. When it moves at less than two miles an hour, it collects on the surface.
The hanging dams cause the water level in the river to rise, loosening and spreading out the frazil cover until the edges are above the sloping cobble shores. (You can hear crackling of the frozen-over pools of water on the edge as they are cracked by the rising water.)
The pressure from the still collecting frazil and the loosening of the former cover cause the hanging dams to give way, lowering the water level quite quickly and leaving huge frazil banks on the shores.
Any questions?? I still have lots, too!
The frigid cold weather was great for frazil-making, of course. The river frazil backed up to just below The Glen. Then the warm rain raised the water level quickly and opened up a channel through the frazil from The Glen almost to Thurman. This cold snap (starting Jan. 27) is again filling up the channel. Maybe the frazil will last till spring this time. (It always used to stay in place all winter.)
If you can find a safe place to park where the frazil is collecting at the upper end of the clogged river, it is fascinating to watch how this kind of ice acts. It is loosely cohesive, like wet sand, and big sections of it hang together, sliding against adjoining sections, instantly raising white ridges on both sections. You can hear the rubbing of the billions of tiny crystals. After a really cold night, there can be great jumbled masses of frazil chunks which have been raised up above the water level because of the shoving and recoagulating that goes on all night, and finally frozen in place. And there will be absolutely smooth-sided masses where one section slid against another, then froze when the water level went down. Caution: Wear a down jacket, snowpants, insulated boots, the whole nine yards if you want to spend any time watching. A really warm coat with hood can work too. Google “Saratoga woods and waterways” on Jan. 25 for some cool pictures of one thing frazil does when it gets squeezed together just a little.
As the chunks and banks of frazil are very porous, like snowbanks, when they get wet from rain or from being in water again, they disintegrate quickly. In the spring it is fun to watch the spectacular white vertical banks calve off like a glacier, leaving the bank still vertical.