last update: 17 June 2017
The Nature of Lake Ice
The Nature of Lake Ice
First off, the book “Peterborough and the Kawarthas” by Peter Adams and Colin Taylor (Trent University) addresses the subject of the lakes in winter very well (Chapter 6). The following section on the nature of ice and snow cover draws from this reference. Peter Adams has graciously allowed the use of his fig 6.7 below.
The ice and snow cover consists of three components: Black Ice, White Ice, and Snow.
The Black Ice is the first ice to form and from then on, forms from the bottom at the ice-water interface. It appears black because it is translucent, and light passes through without being reflected back.
Snow, of course, is the natural frozen precipitation that forms the top cover.
White Ice is the sandwich layer. As the snow accumulates, the weight forces the snow/ice cover down, and water from the lake comes up through the holes and cracks, and floods the surface and mixes with the snow. This then freezes and forms “white ice”, which is opaque, and reflects all light. If any of the snow melts and refreezes, it is also part of this white ice component.
As the snow/white ice cover continues to grow, it insulates the black ice, thus slowing or ceasing its formation.
So there is a pattern of growth and decay of these various components.
The following figure from Adam’s and Taylor’s book captures this very well. (used with permission from Peter Adams)
Candling of ice is a state that the ice might go through as it thaws. It is fairly dangerous, because even though the ice is “thick”, it has no strength. If you have an interest in ice, have a look at the Swedish site: Lake Ice This is by far the best site I have found regarding ice science, ice safety, and practical information.
There is a difference between “Break Through” ice thickness, and “Getting Back On” thickness. if you are on skis or gliding on 2 skates, and break through, the ice likely isn’t strong enough to let you crawl back onto it. Another big concern is that the ice is not uniform, in thickness or in strength..
So, on Chandos, for my money, you had better see no open water, and a thickness of 5 inches before venturing out.
At times during the late winter, one can encounter slush beneath the snow. This can be quite alarming, but is usually not an indicator of thin ice. Typically it is formed because a burden of snow weighs the lake ice down, thus forcing water up through any cracks or crevices as the ice layer tries to sink. Because the snow is an insulator, this water forms a layer of slush at the snow-ice interface. And when the ambient temperature does not provide enough cooling (usually late February or March), there is then not enough heat extraction to allow it to transform to ice. (The heat of fusion is 80 cal/gm, which means that a considerable amount of energy is involved in changing a gm of water to a gm of ice at 0 degC)
Melting snow, or rain, can also contribute to the water content of the slush layer. But simply stated, it will remain slush until conditions are such that sufficient heat can be extracted to allow it to freeze. If it is able to freeze, it usually freezes from the top of the slush layer down, and creates what some call “snow” ice, which, if not frozen all through, can cause a sandwich of slush between the snow (white )ice and the lake (black) ice.
See the following articles for more information:
fall and spring turnover
The Fall Turnover and “Ice In”
Rounding up a wayward dock, Dec 29, 2014
Two key things to know are that water is densest at 4 degC, and that during the cooling of the lake in the fall this causes a mixing to occur wherein oxygen rich surface water is transported to the depths of the lake thereby replenishing the oxygen that is continually consumed by respiration and decomposition.
The variables that affect “ice in” are a bit different that those that affect “ice out”. In order to freeze, the lake must cool down, and thus heat must be extracted until the water column is at 4 degC (where water is the densest). Before then, as top water is cooled, its density increases and thus it falls to where the water below it is also at 4 deg C. As it falls, warmer water rises, and it then undergoes cooling. Once the entire column is at 4 degC, then ambient cooling can begin lowering the temperature of the top water. This “almost frozen” water is lighter than the 4 degC water so it does not fall and continues to give up heat until eventually sufficient heat is extracted that it undergoes a phase change to ice at 0 degC. Again, ice is lighter than water, and so it stays at the surface.
The lake water below the ice continues to freeze (“black ice”) so long as heat can be extracted below the already formed ice. Ice thickness is also added to by the freezing of snow and water above the ice (“white ice”). The speed and depth of ice formation will be determined by the ambient air conditions and the amount of snow cover (an insulator). Another factor, though I’m not sure how big it is, is that Chandos is fed by springs, and this heat input likely retards the ice from coming in.
Thus the major variables for determining when the lake will freeze are the amount of heat in the lake that needs to be extracted; the rate at which it can be extracted; and when the onset of winter actually occurs. The amount of heat in the lake can be expressed by the average temperature. Thus a warmer summer usually means that the ice will come in later.
“Ice Out” and the Spring Turnover
Spring is on its way!
Once the ice is out, the lake has a slight temperature gradient, going from 0 deg at the surface to 4 degC at the bottom . As the surface of the water warms to 4 degC, (where water has maximum density), the surface waters fall, and a turnover similar to that in the fall will occur. This mixing due to vertical density differences is weaker than that of the fall turnover. Mixing is further aided by natural heat exchange due to temperature differences and also due to wind action. Particularly if there is a good fetch, the wind will push the water towards the far shore, which will create an upwelling condition at the near shore, thus creating a circulating current. (As an aside, along lake Erie, which runs west to east, storm and wind surges and seiches can create up to a 12 foot difference in water elevation from one end to the other! This can rapidly bring cold oxygen depleted water up from the depths, possibly resulting in fish kills.)
After a while, and as warming continues, stratification of the lake sets up, and mixing between the epilimnion and hypolimnion layers ceases.