Icicle ripples

The ridges that embellish the surfaces of most icicles are similar regardless of variations in air temperature, humidity, icicle thickness, or growth rate. An icicle grows when thin sheets of water flow down the ice shaft. A portion of the flowing water freezes and the rest drips from the icicle tip. The ice that’s left behind forms ringlike ridges stacked along the icicle’s length. Those ridges always measure about 1 cm from peak to peak, although their heights can vary from one icicle to another. Researchers at Hokkaido University in Japan have developed a model that explains the surprisingly universal structure of icicles: The preferred periodicity arises from a competition between two effects. The Laplace instability, which entails the diffusion of heat from the water sheath into the colder air, amplifies the ripples by promoting ice buildup in convex regions. Flow in the thin water layer, however, tends to even...