Eventually, the coldest water that has drifted to the top of the lake in winter conditions freezes and forms a layer of ice. As a result, the colder water floats up near the freezing point and the warmer water sinks down. This process continues until the water at the surface cools below 4 degrees Celsius, causing it to lose density and eventually freeze. This fact could lead to the assumption that ice should form first at the bottom of a lake.
The density of liquid water depends on its temperature, and water is densest at about 40 degrees Celsius.
How does the ice of a lake freeze?
Eventually, the surface of the lake cools to 4 degrees Celsius (39.2 degrees Fahrenheit), the temperature at which the water is most dense. This poses a major challenge for the fish, because if the lake stays frozen for too long, oxygen levels can get so low that the fish die. This top layer of water comes into contact with the cold air, which cools the upper part of the lake even further until it freezes. However, the way lakes, including Iowa’s Great Lakes, are melting is different than you would think.
Heterogeneous nucleation occurs when strong winds blow dust, snow or frozen rain over the surface of a cold lake, and when it hits the water surface, it freezes.
Why do lakes freeze faster than ponds?
Only when the entire pond has reached a temperature of 4 °C can the water at the surface cool below this temperature and still be lighter than the water below. Freezing a pond is much easier than freezing a large lake, but it still doesn’t happen overnight. One of the most basic concepts I always took for granted was water and its miraculous properties. To completely freeze the pond, the environment would have to absorb the same amount of energy needed to lower the temperature by almost 80 degrees Celsius.
The ground under the water is probably not below freezing either, so it will give off heat to the water at the same time.