Reported February 2007

BACKGROUND: Researchers at the University of Arizona found the same mathematical formula used to describe the shape of stalactites that form in caves also describes the shape of icicles. This is surprising because the physical processes that form icicles are very different from those that form stalactites. Both have a unique underlying shape, resembling a kind of elongated carrot. This sheds light into the physics of how drips of icy water can swell into long, skinny spikes (icicles).

HOW THEY FORM: Stalactites are formations that hang from the ceilings of caves, formed when water erodes limestone and taking the calcium carbonate. As the water drips inside the cave and evaporates, it leaves behind the calcium, which forms a stalactite. The continued diffusion of carbon dioxide gas fuels the growth of a stalactite. In contrast, heat diffusion and a rising air column are keys to an icicle's growth. Icicles form when melting snow begins dripping down from a surface such as the edge of a roof. There must be a constant layer of water flowing over the icicle in order for it to grow. The growth is caused by the diffusion of heat away fro the icicle by a thin fluid layer of water, and the resulting updraft of air traveling over the surface. That updraft occurs because the icicle is generally warmer than its surrounding environment, and thus convective heating causes the surrounding air to rise. As the rising air removes heat from the liquid layer, some of the water freezes, and the icicle grows thicker and elongates.

PUT TO THE TEST: To compare the predicted shape to real icicles, the researchers compared pictures of actual icicles with their predicted shape. They found that it doesn't matter how big or small the actual icicles were, they could all fit the shape generated by the mathematical equation. The next step is to solve the problem of how ripples are formed on the surfaces of both stalactites and icicles.

ICE, ICE, BABY: Ice is the frozen form of liquid water. The same substance will behave differently at various temperatures and pressures. Water (H₂O) is the most familiar example. It can be a solid (ice), a liquid (water), or a gas (steam), but it is still made up of molecules of H₂O, so its chemical composition remains unchanged. At sea level, water freezes at 32 degrees Fahrenheit (0 degrees Celsius) and boils at 212 degrees Fahrenheit (100 degrees Celsius), but this behavior changes at different altitudes because the atmospheric pressure changes. In fact, get the pressure low enough and water will boil at room temperature. The critical temperature/pressure point at which H₂O changes from one form to another is called a phase transition.

The American Meteorological Society and the American Geophysical Union contributed to the information contained in the TV portion of this report.

If you would like more information, please contact:

Martin Short, Ph.D.
Physicist
University of Arizona
(520) 621-6786
mshort@physics.arizona.edu

American Meteorological Society
45 Beacon Street
Boston, MA 02108-3693
(617) 227-2425

American Geophysical Union
Washington, DC 20009-1277
(800) 966-2481

FACTOID...

In some planets and in most moons, ice is the major constituent. Pluto is 80 percent ice; Jupiter's moons Ganymede and Callisto and Saturn's Titan contain 40 percent ice.