WATER behaves strangely when the times are cold. Most materials expand when heated and shrink as they cool without further complications. But water shrinks in volume as it is cooled only until the temperature reaches 4 C and then it begins to expand again.
This anomalous behaviour can be irritatingly inconvenient. Expansion with the change to ice is the reason, for example, why pipes sometime burst in wintertime and why the cylinder-block of a water-cooled motor-car engine may develop cracks if the anti-freeze has been forgotten.
It also means that unlike almost any other substance, water in this solid form, weighing less per unit volume, floats on the surface of its corresponding liquid. This has advantages if you happen to be fond of skating but had a well-remembered downside for the captain of the Titanic.
But there are consequences even more germane for life on planet Earth. As the temperature of the air falls on a frosty night it cools the water at the top of any roadside pond. This coldness makes the surface water heavier per given volume than the warmer layers beneath and this cold water sinks leaving the coldest water always at the bottom.
But if this state of affairs were to continue until the temperature of the pond fell below zero, the pond would freeze from the bottom up and many important things about our lives would be quite different.
Our climate, for one thing, would not be the same as it is now and many creatures which live under water could not exist which would affect the whole food chain and the pattern of life upon the planet.
But this is not the way things happen. When the top-most layer of water becomes colder than 4 C, it no longer sinks since it now becomes lighter rather than heavier as its temperature decreases further.
As the temperature approaches freezing point, the coldest water lies on top and ice forms first at the surface of the pond.
A layer of ice already on a pond serves as an efficient insulator to delay the process of further freezing. The thicker the layer of ice, the more the freezing underneath is slowed down.
As a first approximation, the thickness of ice increases with the square root of time; if the first millimetre of ice forms in one hour it will take four hours of the thickness to reach 2 mm and nine hours before the ice is 3 mm thick - assuming, of course, that the air temperature above remains at the same sub-zero level.