How does a bubbler prevent ice on ponds?

Answer 1

This has been explained as "the rising air bubbles keep the water in motion. Still water freezes easier than water that's moving."

But there is much more to it than that.

It's true that water in motion freezes more slowly than still water, for two reasons: The energy of the motion is turned into heat which warms the water just a tiny amount. And water that is moving finds it harder to turn into ice than still water - it has to be just a little colder to make the molecular connections that are necessary in ice. But, in a climate such as I live in - New England - neither of these effects stops the water from freezing, and we have lots of frozen waterfalls to prove it.

The reason why a bubbler prevents ice is that the water at the bottom of the pond or lake is warmer than the water at the surface and the motion of the bubbles upwards carries the water with it, creating a current of warmer water being brought to the surface, which therefore doesn't freeze. (The technical word is that the air entrains the warmer water as it moves upwards.)

The reason why the water at the bottom is warmer is that water has some very unusual properties. Most things expand as they get warmer, and shrink as they get colder. And so does water at temperatures above 4°C (39°F). But below 4°C, water actually expands as it gets colder and eventually turns into ice. (Which, of course, explains the well known but unfortunate effect that water pipes burst open when they freeze.)

This means that water is most dense at 4°C and is less dense at 0°C. As a lake cools, the denser water moves to the bottom and the less dense water, which can be freezing, stays at the surface. That's why lakes always freeze from the top and have a layer of ice on the surface. And then, because ice is a fairly good insulator, lakes that thaw every year don't freeze down for more than one to two feet. The bottom of the lake stays warm at 4°C.

It's interesting to contemplate that if water didn't have this very unusual property of expanding as it cools, lakes would freeze from the bottom, and could become one big chunk of ice. (Whether this would happen in fact is a more complicated story: there is a flow of heat upwards from the earth which continues to warm the bottom of the lake.) And beware: if you have a small shallow pond, which normally freezes at the top and has water underneath, then putting a bubbler in could stir it up enough that it will freeze all the way through. Lakes don't have this problem because there is a huge reservoir of warm water at the bottom, which is barely affected by the little bit the bubbler steals from the bottom.