I was reminded earlier today of Cathal Bui Mac Giollagunna. My office in Darmstadt, Germany, where the temperature these mornings hovers around -5C, looks out on an ornamental lake.
It is frozen over, and there sliding on the ice was a large bird, pecking at the surface, presumably in search of liquid water. Perhaps in time this poor bird will resemble Cathal Bui's yellow bittern, in the freezing stiffness of a death brought on, by the poet's heart-felt diagnosis, through simple lack of drink:
A bhonnan bhui, 'se mo lean do lui
Is do cnamha sinte tar eis do ghrinn;
Is ni easpa bidh ach diobhail di
A d'fhag in do lui thu ar chul do chinn.
Water is a strange substance. Most materials expand when heated and shrink as they are cooled, without further complications. But water, as we noted a day or two ago in Weather Eye, shrinks in volume until the temperature reaches 4C and then it begins to expand again. It is at its densest and heaviest at 4C.
This has important consequences for my ornamental office lake. As the temperature of the air falls on a frosty night, it cools the water at the top. This coldness makes the surface water heavier than the warmer layers beneath, and the cold water sinks. In this way the coldest water is always at the bottom of the lake.
If this state of affairs were to continue until the water temperature fell below zero, the lake would freeze 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 the goldfish and other creatures in my lake could not exist.
But things do not happen just like this. When the top layer of the lake becomes colder than 4C, it no longer sinks. Further cooling makes it lighter rather than heavier, so at low temperatures the coldest water rises to the top, and freezing starts adjacent to the surface.
Moreover, when a thin layer of ice has formed on top, it serves as an efficient insulator to retard the dissipation of any further heat, and so the process of freezing underneath slows 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 for 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.