Icicles: Water Stopped on the Way Down

Icicles rarely form on the coldest days. They need snow on a roof warmed by sun or by the house beneath it, so that meltwater runs to the edge, and then air cold enough to freeze each drop before it falls. That is why the longest rows tend to hang along sunny eaves and gutters, a few days after a snowfall, in the gap between a bright noon and a hard night.

An icicle grows from the outside. A thin film of water slides down its surface, gives up heat to the air and freezes a layer at a time, while the tip stays open as a small tube of liquid that lengthens drop by drop. Look closely and most icicles carry faint rings around their length, spaced roughly a centimetre apart whether the icicle is short or as long as a forearm. Physicists have traced those ripples to traces of salt and dissolved minerals in the water, and icicles grown from very pure water come out almost smooth.

Against a low winter sun the whole row turns into small lenses, each one bending the light behind it and throwing a narrow bright line along its edge. On a screen, that row works best seen from below or from the side, with the sky dark enough that the clear ice reads as an outline. It is a picture of a few cold hours, and of the water that did not quite make it to the ground.