Ten Things to Know About Your Lake in Winter
Your lake is a very different place when encased in ice for the winter, but it’s still very much a living system with many mysteries to explore. Here are ten things you may not know about the life of a frozen lake.
It all starts with steam. Look out on your lake on a very cold November morning and you may see tendrils of steam rising from the water. The steam forms as warmer, moist air just above the water rises to meet the cold, drier air above. The moisture condenses into tiny droplets to form what’s known as steam fog.
Ice requires cold stillness. Your lake won’t freeze when the days and nights because wave action constantly fractures tiny ice crystals as they form, keeping solid ice from taking hold. In these conditions, the water can actually supercool, remaining as liquid below the freezing point of 32 degrees F. Then along comes a very cold, windless night, and presto! A thin sheet of ice covers your lake.
Your winter lake has layers. Under the ice, lake water has an interesting temperature profile. The warmest, densest water, at about 40 degrees F, lies at the bottom. The coldest, least dense water, at 32 degrees F, lies right under the ice. The ice itself is the least dense of all, which is why it floats.
he lake starts the winter rich in oxygen. Fish and other water creatures need a supply of oxygen to make it through the winter. Fortunately, beneath the new ice, the lake holds more oxygen than at any other time of year. That’s because water can hold much more oxygen when it’s cold than when it’s warm. At 32 degrees, water holds almost twice as much oxygen as it would at 80 degrees.
Water creatures need less oxygen in winter. Fish, frogs, mollusks, crayfish and other critters are cold-blooded. In cold water, their metabolism slows down, and they move about slowly if at all. As a result they use less energy and so less oxygen. They go into winter with the most abundant oxygen they will experience all year, at a time when they need that oxygen the least. It’s a good scenario for survival.
There’s no need to fear ice “booming.” Lake ice makes wondrous, almost musical, sometimes eerie sounds as it expands and contracts with changes in temperature. If you’re on the lake when the ice is booming, or even if you hear a crack sizzle right past you and off into the distance, there’s no need to fear. Booming does not mean the ice is weakening.
Ice is really a collection of crystals. We think of ice as a monolithic, solid substance, akin to a block of glass. In reality, it consists of vertically oriented crystals in the shape of hexagons, tightly packed together. These crystals grow from the surface down as the lake is continuously exposed to cold air.
No one is certain why ice is slippery. Most of us learned in grade school that we can skate on ice because the pressure of the steel blades lowers the melting temperature at the surface and creates a film of water on which we glide. Science has now rejected that explanation in favor of two theories. One is that friction, not pressure, melts the ice. The other is that ice is inherently slippery – that a tiny liquid film remains on the ice surface even at temperatures far below freezing. Scientists disagree on which theory is correct.
Ice is tough – and fragile. There are various ways to assess the strength of ice. One is fracture toughness – how easily a crack spreads through a material. On this measure ice is about one-tenth as tough as window glass. Then there’s tensile strength – how much force a substance can take when stretched from both ends. The tensile strength of ice is about half that of bricks. In flexural strength — resistance to bending under a load,, ice is about the same strength as a pine board across the grain. That’s pretty tough!
Ice melts from the bottom up. As the weather warms, the snow melts off the ice surface. Then the sun’s rays penetrate the ice and warm the water below, while also warming areas of open water near shore. Warm air above the ice contributes to the thaw, but it’s the warming water below that really does the trick.
Winter is a great time to enjoy your lake. Whether you fish, skate, ski, hike, or just observe, lake ice is a resource worth treasuring.