Notes on Ice-Out at Chandos
Historically, for the last 40 years or so, we have declared “Ice-Out” based on The Main Lake appearing to be ice free.
It is apparent, especially this year, that it is not a perfect proxy for the entire lake.
As you know Chandos has 3 major bays (Gilmour, South, West) along with what is termed “The Main Lake”.
So the question is, when declaring Chandos to be Ice free, should it be the date that there is no ice anywhere? The short answer is that for reasons of historical consistency we must continue with our observations on the Main Lake, but will be more precise by referring to it as “The Chandos (Main) Lake Ice-Out Date”.
A wee primer on ice formation
Ice begins forming from the bottom, and this first layer is called “black Ice”. It is highly transparent and conductive and continues to form as heat is extracted from the water at the ice-water interface via conduction. The ice that forms on the top of the Black Ice is from a mixture of snow and water and is called “White Ice”. So, between the air and the water is a layer of snow, a layer of White Ice, and a layer of Black Ice.
As the air temperature drops, heat continues to be extracted through the snow and white ice via conduction from the water and thereby more black ice forms.
As snow falls it generally acts as a thermally insulating layer limiting the extraction of heat from the water and thus slowing the rate of formation of black Ice. However, if the wind blows a lot of this snow from the middle of the lake away, usually to the edges, then black ice will form more quickly in the middle and more white ice will accumulate at the edges.
In the spring, when there is a lot of solar radiation, the White Ice acts as a reflector and limits the amount of solar energy entering the ice and water.
Eventually though sufficient solar radiation reaches the ice-water interface that the Black Ice starts to melt from below. Of course, it can also melt from the warm air above, so there are two mechanisms at work.
With all conditions identical, (wind, snow, rain, currents) then in theory the deepest and largest water masses will thaw last, and in our case this means that the Main Lake should be the last to become ice-free. The reason for this is that as the water at the interface warms up, it falls and is replaced by colder water just below. (this is because the density of water is highest at 4 deg C, so as soon as the 0 degC water at the interface becomes, say 2 degC, then the 1 degC water just below it rises to the interface thereby reducing the rate of ice melt. This process continues, and the length of time depends on the depth of the water. If the ice hasn’t melted already then this process continues until the entire water column is 4 deg C. After that, the ice melts very quickly because the warmest water stays at the interface. eg, the 5 degC water becomes 6 degC, etc)
To add greater clarity, in the winter the lightest water is just below the ice, at 0 degC, and the most dense water, at 4 degC is at the bottom. As the water at the interface warms up its density increases and so it falls to be replaced by less dense water, which is cooler. This process continues until the entire column is at 4 degC. Now the density of water above 4 deg C decreases with temperature, and so any water warmer than 4 deg C will just continue to stay near the surface and heat up even more, quickening any thawing that has not yet happened, See the graph below.
So with all conditions being equal, once the Ice is off the main lake, it is usually off everywhere.
Ice-Out date Decision
I expect if one were to plot yearly ice out date for each of the bays and compare their statistics to the main lake, that they would be much noisier, in that the swings would be larger and any trend would be more difficult to detect. This is because they are more sensitive to variations in the affecting variables.
The fact remains, however, that our long-term data has really been based on what is happening in the Main Lake. This year, Cathy Burgess declared the Ice-Out date on The Main Lake to be April 19. (and such was also reported by Clark B.)
In our attempts to extract a trend from the Ice-Out dates, it is important to remain consistent year over year in how that date is measured. So, April 19, 2025 is what our historical graph and data will reflect. However, as mentioned in the introduction, to add a bit more preciseness to the observations we are modifying the title to “Ice-Out Chandos (Main) Lake”.
Over the 40 years of data we have accumulated, on a trend line basis, the ice is now going out on the Main Lake about 4.8 days earlier. Note also that the ice is coming in about 11.6 days later, so that we have about 16.5 fewer days of Ice-Cover.
Chandos Lake Science 2 