Why do lakes and ponds freeze from the top down rather than from the bottom up?

Answer 1

Water is rather unique among substances. It increases its density with cooling until it reaches 4oC (Above freezing) at that time getting cooler lowers its density. As a consequence the coldest water floats on top of the above freezing point water. The ice (when it forms) is even less dense and floats at the top of the pond on the colder and warmer water. As a consequence the water that freezes is nearest the top.

Even if the water were to be cooled from the bottom, the coldest water would float to the top and when the whole pond became 0°C the ice would float to the top as it formed. In some cases where rocks are below the freezing point (exposed to the air part of the time, ice can form and stick to the rocks under water. There is one unusual case where ice crystals form as needle shaped crystals in the water below the surface. This is called frazil ice. These crystals mat together and block inlet pipes and simiar structures.

In most ponds, however, the water is losing the majority of its heat to the colder air above it, so the ice would form there. And, being less dense than water, the ice would stay on the top as does the coldest water.
All water has a very special property that causes this to happen. (The same thing happens with fresh water in lakes, ponds, rivers, etc.)

Most materials in a given state (solid, liquid, gas) contract as their temperature goes down, and expand when their temperature rises, assuming that pressure does not change. A change in state, such as from liquid to solid, can also be expected to change the volume of a material. And when the volume of a material changes but its mass remains constant, the material's density changes.

In its liquid state, water acts like most other materials in that, as it cools, it contracts, meaning that it becomes more dense and tends to sink. Cooler water will almost always be found at the bottom of a large body of water than at the top. But this contraction stops and actually reverses itself when water's temperature cools beyond a certain point -- around 4 deg. C. From that point on down, water expands as it cools toward its freezing point of 0 deg. C. When water changes its state from liquid to solid (ice) it expands again. Note that these temperatures apply to fresh water, but the same things happen to sea (salt) water at a somewhat lower range of temps.

Thus, when water begins to approach its freezing point, it stops becoming "heavier" (more dense) and starts becoming "lighter" (less dense). Because of this, the very coldest liquid water "floats" on top and when it becomes ice, the even "lighter" ice floats on the coldest layer of water.

Answer 2

The surface of the pond is directly in contact with the freezing air. Once the ice has formed on the surface, the water beneath is partially insulated from the freezing air by the ice, so remains unfrozen. In a very hard and prolonged winter, the whole of a pond, especially if shallow, will freeze solid from top to bottom.
Also, ice is less dense than water, and will float, as seen with icebergs.

Answer 3

the air tepprature is lower than the present water temprature