Recent snow and frosty windshields have provided a preview of winter. On some chilly mornings there has even been a skim of ice on local ponds. I am fascinated by ice formation on the surface of ponds and lakes from a scientific perspective, but also because of an activity which I love to do when the ice is new. It’s only possible when conditions are just right and can’t be done every year. I call the activity “whistle stones.”
It’s common knowledge that ice forms on the top surface of a body of water, but did you ever really wonder why? Most substances become denser when they turn from liquid to a solid form … not water! Ice is less dense than water and that’s why it floats. But there’s a whole process that happens before ice can form.
In the summer when the sun shines on a lake, the upper part of the water heats up. Lower down where the sun does not penetrate, the water remains cooler. As a result, larger bodies of water are stratified with temperature layers called thermoclines.
As the days get shorter and the air temperature drops, the surface water of a lake cools. The colder the water gets, the denser it becomes (up to a point) and it begins to sink. The sinking colder water pushes up warmer water from below. This transfer of water between layers is called lake turnover. It occurs each fall, and again in the spring.
The mixing of water continues until the lake is relatively uniform in temperature, about 39° F. This is the point when water reaches its greatest density. Water is unique in that way; it is still in a liquid form at its densest point. How long it takes for the lake water temperature to become uniform depends greatly on the size of the body of water, both in depth and surface area. The deeper the lake, the later the turnover and the longer it takes to thoroughly mix.
You don’t need a thermometer to determine if turnover is happening. You can notice the shift in water temperature because the clarity of the water also changes. As nutrients and particles of silt and clay are brought up from below, the water gets cloudy. There may also be a detectable odor from rising detritus.
As autumn progresses toward winter and air temperatures consistently dip below 39 degrees, the temperature of the surface water continues to drop, but remains on top, because it is lighter. That water eventually freezes and forms a layer of ice. If water was most dense in its solid form, lakes would freeze from the bottom up and that certainly would change things! No skating, ice fishing or life under the ice.
Once the ice has formed a solid sheet, but remains thin, it’s the perfect time for “whistle stones.” The activity is a bit like skipping stones but instead of bouncing on the surface of water, you slide stones across the ice. It works best with walnut-sized stones, but feel free to experiment with various sizes and shapes.
Stand close to the edge of the water (but not so close that you break through the ice). With a low side arm toss, fling the stone out onto the ice and listen. If the stone has been tossed well and the ice is just right, you’ll hear a mysterious, almost outer-space-like sound as the stone skates across the surface towards the other side.
Unlike when skipping stones, you can’t count the number of times it hops across the water, but you can count how long the sound lasts, or how far the stone glides on the ice.
A word of caution, look around before throwing stones so no one gets hurt, and I would not recommend doing this on a skating pond. Stone collections on the ice can become hazardous obstacles for skaters later in the season!
There is a narrow window of time when Whistle Stones will work. If the ice is too thin or too thick the magical sound doesn’t happen. Hopefully you will get to try this and experience the ethereal resonance which heralds this shift in seasons.