Winter has arrived, albeit late

Although late, winter has arrived! We finally have some snow AND some consistent freezing temperatures.

Many of the little wetland pools in my forest are at least partially frozen, with thicker ice extending from the edges and thinning out toward the middle. This is called border ice. Solid ice usually, but not always, forms along the shore of a pond or stream first. The water is typically shallow along the shore and so the water temperature drops faster and freezes quickly on a cold night. In the case of rivers and streams, the water moves slower along the shore and so ice forms more easily along the edges. If conditions stay cold, the border ice will grow gradually toward the center until the entire pond freezes over.

Each lake, river, pond and stream has its own nuances. For example, because the water is moving more quickly than in surrounding areas you will find thinner ice where a river narrows or where channels form between islands. Likewise, ice thins where streams or springs enter a pond or lake. The marsh near my house usually has thinner ice near the shoreline where it is shallow and sunny.

We are lucky that water has the unique thermal properties that it does. If water was like most substances on Earth, its density would be greatest when frozen and it would sink — ice would form at the bottom of water bodies first.

However, unlike most substances, water is less dense when frozen and therefore ice floats. The peak density of water is around 4 degrees Celsius and it decreases, or gets lighter, at temperatures both above and below this. If water didn't do this it is unclear whether life on Earth could exist; all bodies of water would freeze from the bottom up and potentially freeze solid, causing real problems for most aquatic species.

Consider this: because the maximum density of water is at 4 degrees Celsius, the layer of water that forms at the bottom of deep bodies of water — the hypolimnion — can remain at around 4 C year-round. The heavy, cold water just sits there. In the winter, lakes stratify, meaning the layer of cold liquid 4 C water remains on the bottom, warmer water in the middle, ice on top. The ice on top acts as an insulator, protecting the warmer water from freezing air temperatures, further ensuring that the lake or pond doesn't freeze solid.

It is in these cold but unfrozen depths that life such as algae, insect larvae and fish can safely overwinter. These aquatic organisms are all cold-blooded, their body temperatures following external temperature changes; as the lake cools their metabolism slows and they become sluggish. Obviously this whole freeze-thaw system works; when spring comes and northern lakes, ponds and rivers shed their icy covers, aquatic organisms shrug off their winter torpor and come back to life.