Icicle



An icicle is a spike of ice formed when water dripping or falling from an object freezes.

Formation and dynamics
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 was observed by BBC film crews near Antarctica.

Damage and injuries caused by icicles
Icicles can pose both safety and structural dangers. Icicles that hang from an object may fall and cause injury and/or damage to whoever or whatever is below them. In addition, ice deposits can be heavy. If enough icicles form on an object, the weight of the ice can severely damage the structural integrity of the object and may cause the object to break.

The story of an English youth who was killed by a falling icicle in 1776 has been often recounted.

Armstrong, the scientist from the National Snow and Ice Data Center, told a journalist, "Another twist is very large icicles falling from cliffs along highways hitting cars. Where I used to work doing avalanche hazard assessment and forecasting along U.S. Highway 550 in southwestern Colorado, icicles have fallen and destroyed passing cars."

In 2010, five people were killed and 150 injured by icicles in Saint Petersburg, Russia after a heavy snow that also caused apartment block roofs to collapse, as well as creating water damage to private homes and to the National Library of Russia.

Kathleen F. Jones of the U.S. Army Cold Regions Research and Engineering Laboratory states, "The combination of the ice load and the wind on ice load can break wires, cross arms, and poles and/or other supporting structures. Damage associated with icicles on roofs may be to vehicles parked next to the overhang. Icicles on roofs are also often associated with ice dams that may cause water to infiltrate under the shingles with possible water damage to the house or building and its contents."