Richard Bramwell sent some pictures from January 2011 of an interesting form of ice he found at Ardmore Beach on the south eastern end of Georgian Bay after a night of moderate onshore winds and temperatures around -10˚C. He described them as cobblestone like: rolling around in the water as incoming waves moved under them. I had seen what is probably the same thing a couple times on Lake Champlain at frozen ice edges, far from shore.
After getting Richard's email, I kept watch on several beaches on Lake Champlain until I found them on a very cold (0˚F,-18˚ C), windy (20 mph) day in a small bay just north of the Essex NY ferry dock. The fresh balls are composed entirely of a uniform consistancy slush. It is not as cohesive as water saturated new snow but is more cohesive than fully metamorphosed corn snow slush. Bernard Michel(1) describes slush balls as: "The result of extreme accretion of slush particles. This is produced either by waves along the shores of lakes or in long stretches of very turbulent and wavy flows in rivers."
The day I found the slush balls on Lake Champlain it was snowing lightly out of the ice fog over the open water on the lake but it did not seem like enough to make this much slush and we did not find slush balls at any of the many other bays and beaches we looked at that day. On the other hand there was plenty of snow falling to nucleate supercooled water on the windblown surface. The low flow from the small stream at the end of the bay also did not seem up to the task of filling the bay with frazil. Also, Ardmore Beach does not have a significant nearby stream and the former ice edge where I had seen them frozen into the ice sheet was far from any streams. Slush particles from unfrozen slush balls between polarizing filters did not give any useful clues that helped make a case for a particular source.
While slush balls have little impact on our use of lake ice they are an interesting curiosity. There are several WWW other references2-3 on the web, one of which was published in 1954.
After a 2" snow storm, it has been cold (10 deg F) for the last two days in Burlington. Yesterday we checked a local beach to see if the snow falling into the lake had created to slushballs. There weren't any and it looked like the water was still too warm. Today was another story, there was a band of slushballs about 40 feet wide off the beach. Wind at the time was a few mph and it dropped later in the day. After lunch I came back in my dry suit with lots of clothes underneath. When I waded out to the edge of the slushballs I found evidence for the source and mechanism for making slush as well as got a better understanding of how they evolve. A hypothesis is laid out in the following pictures and captions.
It looks like a bit more onshore wind speed would have caused smaller, rounder slushballs. The key ingredients for slushball formation seem to be:
- Water in the top few inches of the water near the beach is very close to 32 deg (that is the temperature I measured in a bucket full I brought to shore). A wide band of shallow water might help with this.
- Cold temperatures to supercool the surface water off shore to allow fast growth of dendritic needles. It is the opinion of frazil expert Dr Steve Daley that nucleation takes place at far too little supercooling to be anything other than ice to be the nucleating agent. Snow (even a little), ice fog, frozen water spray in windy condition and broken bits of other crystals are all possible candidates.
- Note: While super cooling of a thin surface layer is taking place off shore to grow the dendritc flake ice/needles I do not believe it plays a role in ice ball formation. When the ice balls are bobbing in the water they have the consistency of a very wet snowball where water/ice particle cohesion holds things together. There is not significant hardening that I would expect would be associated sintereing in supercooled water as is seen in rivers with active frazil whick builds up anchor ice under the water surface.
- A moderate onshore wind that will move the needles growing on the surface downwind but not strong enough to deeply mix the surface water.
- Heavier snow will also contribute to the cooling of the water surface and might contribute significantly to the the slush.
Michel Bernard (1) includes spicules (small or broken needle/dendritic ice pieces) in his definition of frazil. The formation mechanism has some similarities but also significant differences from discoid frazil that commonly forms in turbulent rivers. In River Lake Ice Engineering(4) this is described as needle frazil
Slush, its various forms, plays an important role in the world of ice. Snow ice, superimposed on the top of an ice sheet often comprises a fair portion of the thickness of many ice sheets by mid/late winter. Discoid frazil ice can develop large ice deposits on rivers that can have significant adverse economic impact. Needle frazil, in addition to making slushballs, is most likely responsible for the large areas of white slush ice that can be often be found mixed with areas of black ice. Needle frazil also accounts for at least some (maybe most) of the small grain (S2) ice found in ice sheets, especially on larger lakes where the needles and flakes get broken up in the early stages of their formation. Snow falling into water can create slush that freezes into spectacular patterns.
Slightly revised 1/11/14
1) Winter Regime of Rivers and Lakes, Bernard Michel, April 1971, page 3 and 4, Cold Regions Science and Engineering Monograph III-B1a, Cold Regions Research and Engineering Laboratory (CRREL). Note: This is an excellent survey of the practical side of river and lake ice science.
2) Click Here for: A STUDY OF ICE ON AN INLAND LAKE by: James Wilson, James Zumberge and EW Marshall, 1954
3) Click Here from a video taken by Richard Bramwell at Ardmore Beach.
4) River Lake Ice Engineering Edited by G. Ashton, Page 184, Water Resources Publications, LLC, 2010 printing.