An icicle is a spike of
ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
formed when water falling from an object
freezes
Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid o ...
.
Formation and dynamics
![Icicles on a street lamp](https://upload.wikimedia.org/wikipedia/commons/8/83/Icicles_on_a_street_lamp.jpg)
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
An ice storm, also known as a glaze event or a silver storm is a type of winter storm characterized by freezing rain. The U.S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least of ice on ex ...
, when rain falling in air slightly below freezing slowly accumulates as numerous small icicles hanging from twigs, leaves, wires, etc. Thirdly, icicles can form wherever 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
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opa ...
.
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.
Icicles elongate by the growth of ice as a tube into the pendant drop. The wall of this ice tube is about and the width . 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 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 within salty water (
brine
Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...
) sinking from sea ice. These so-called ''
brinicle
A brinicle (brine icicle, also known as ice stalactite) is a downward-growing hollow tube of ice enclosing a plume of descending brine that is formed beneath developing sea ice.
As seawater freezes in the polar ocean, salt brine concentrates are ...
s'' can kill
sea urchin
Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s and
starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...
, which was observed by
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...
, Antarctica.
Icicles can pose personal and structural dangers. Icicles that hang from an object may fall and cause injury and/or damage to whomever or whatever is below them. In addition, ice deposits can be heavy. If enough icicles form on an object, such as a wire, beam, or pole, the weight of the ice can severely damage the structural integrity of the object and may cause the object to break. This can also happen with roofs, where failure can damage nearby parked vehicles or the contents and occupants of the structure. Icicles on roofs can also be associated with
, which can cause water damage as the water penetrates below the shingles.
The story of an English youth killed by a falling icicle in 1776 has often been recounted.
Large icicles that form on cliffs near highways have been known to fall and damage motor vehicles.
after heavy snow that also caused apartment block roofs to collapse, as well as creating water damage to private homes and to the
.