What Are Icicles Made Of?
Icicles form through a natural scientific process that requires certain weather conditions. First, you need a source of frozen precipitation, such as snow or ice. If you get a blanket of snow or ice on your roof, there’s a decent chance you could see icicles form soon.
Next, you need a combination of below-freezing temperatures and some sunshine. The sunshine is necessary to melt some of the snow or ice to form small water drips. As the water begins to drip off the edge of a surface, such as a roof, the below-freezing air temperature will cause the drops to freeze again, creating the base of a new icicle.
If the process ended there, icicles might look like small, icy warts along your roofline. As you already know, though, icicles eventually take on a long, thin, cone-like shape. Why does that happen?
As the Sun continues to melt snow or ice on the roof, water continues to drip down the slowly-growing icicle. The thin sheet of water slides down the outside of the icicle and gets colder as it reaches the bottom. At the bottom of the icicle, it refreezes and adds to the length of the icicle.
This occurs because the below-freezing air that surrounds the icicle acts like an insulating blanket. Because heat energy rises, this insulating layer of air is thicker at the top, so the top of the icicle stays warmer. Dripping water thus continues to flow toward the bottom of the icicle, where the insulating layer is thinner and allows the drops of water to freeze again.
This thawing and refreezing cycle repeats as long as weather conditions allow, leading to the familiar icicle shape that features a wider base with a long cone that becomes progressively thinner at the bottom. When the water flow that created the icicle stops, the icicle can still change shape.
When temperatures are below freezing, some ice can change directly into water vapor, creating a smoother surface along the outside of the icicle.
Recently, some scientists have theorized that an icicle’s shape can be described by the same mathematical equation that applies to a similarly-shaped natural feature of many caves: the stalactite. This idea has puzzled many scientists, since the physics of icicle formation is completely different than that of stalactites.
Other scientists believe that icicle formation may be much more complicated than a simple mathematical equation can describe. They believe many other factors, including wind, water impurities, etc., may affect icicle shape.
Why bother studying this at all? Some scientists hope to understand icicle formation better, so that improvements can be made to the design of things such as airplane wings and power lines that can be negatively affected by icicles.
Icicles can form during bright, sunny, but subfreezing weather, when ice or snow melted by sunlight or some other heat source (such as a poorly insulated building), refreezes as it drips off under exposed conditions. Over time continued water runoff will cause the icicle to grow.
Another set of conditions is during ice storms, when rain falling in air slightly below freezing slowly accumulates as numerous small icicles hanging from twigs, leaves, wires, etc. Thirdly, icicles can form where ever water seeps out of or drips off vertical surfaces such as road cuts or cliffs. Under some conditions these can slowly form the “frozen waterfalls” favored by ice climbers.
Richard Armstrong, a senior research scientist at the National Snow and Ice Data Center in Boulder, Colorado, has explained why icicles come in various shapes and sizes. Icicles form on surfaces which might have a smooth and straight, or irregular shape, which in turn influences the shape of an icicle.
Another influence is melting water, which might flow toward the icicle in a straight line or which might flow from several directions. Impurities in the water can lead to ripples on the surface of the icicles.
Icicle physics have been resolved by Lasse Makkonen at the Technical Research Centre of Finland. Icicles elongate by the growth of ice as a tube into the pendant drop. The wall of this ice tube is about 0.1 mm and the width 5 mm. As a result of this growth process, the interior of a growing icicle is liquid water.
The growth of an icicle both in length and in width can be calculated and is a complicated function of air temperature, wind speed, and the water flux into the icicle. The growth rate in length typically varies with time, and can in ideal conditions be more than 1 cm per minute.
Given the right conditions, icicles may also form in caves (in which case they are also known as ice stalactites). They can also form around salty water sinking from sea ice. These so-called brinicles can actually kill sea urchins and starfish, which were observed by BBC film crews near Antarctica.