icicle
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 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.
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.{{cite web |url= http://news.discovery.com/earth/snowmageddon-brings-icicles-of-doom.html |author= Ribas, Jorge |date= 9 February 2010 |title= Snowmageddon Brings Icicles of Doom |publisher= Discovery News |access-date= 19 September 2012 |archive-date= 3 January 2011 |archive-url= https://web.archive.org/web/20110103014513/http://news.discovery.com/earth/snowmageddon-brings-icicles-of-doom.html |url-status= dead }} Impurities in the water can lead to ripples on the surface of the icicles.{{cite web |url= https://www.nytimes.com/2015/03/17/science/why-icicles-look-the-way-they-do.html?ref=science&_r=1 |date= 16 March 2015 |title= Why Icicles Look the Way They Do |publisher= NY Times |access-date= 25 March 2015}}
Icicles elongate by the growth of ice as a tube into the pendant drop. The wall of this ice tube is about {{convert|0.1|mm|in|3|abbr=on}} and the width {{convert|5|mm|in|1|abbr=on}}. 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.{{cite journal|last=Makkonen|first=L.|year=1988|title=A model of icicle growth|journal=Journal of Glaciology|volume=34|issue=116 |pages=64–70|bibcode=1988JGlac..34...64M|doi=10.1017/S0022143000009072|doi-access=free}} The growth rate in length typically varies with time, and can in ideal conditions be more than {{convert|1|cm|in|abbr=on}} 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) sinking from sea ice. These so-called brinicles can kill sea urchins and starfish, which was observed by BBC film crews near Mount Erebus, Antarctica.Ella Davies: [http://bbc.co.uk/nature/15835017 'Brinicle' ice finger of death filmed in Antarctic] {{Webarchive|url=https://web.archive.org/web/20120218134744/http://www.bbc.co.uk/nature/15835017 |date=18 February 2012 }} filmed by Hugh Miller and Doug Anderson, Frozen Planet, BBC Nature, BBC One, broadcast 23 November 2011.{{cite news |last= Praetorius |first= Dean |title= Brinicle, Underwater Icicle, Captured Forming By Time-Lapse Camera |work= The Huffington Post |date= 23 November 2011 |url= http://www.huffingtonpost.com/2011/11/23/brinicle-underwater-icicle_n_1110757.html |access-date= 19 September 2012}}
Dangers
Icicles can pose personal and structural dangers.[http://www.citynews.ca/news/news_31299.aspx CityNews.ca – Dangerous Icicles A Concern As Pieces Fall From Above] {{webarchive|url=https://web.archive.org/web/20090204020214/http://www.citynews.ca/news/news_31299.aspx |date=4 February 2009 }} 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, 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 ice dams, 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.{{cite book |title= Sporting Magazine: or, Monthly Calendar of the Transactions of The Turf, The Chase, and Every Other Diversion Interesting to the Man of Pleasure, Enterprise, and Spirit, Vol. 27 |location= London |publisher= J. Wheble |year= 1806 |page= 95 |url= https://books.google.com/books?id=ibwCAAAAYAAJ&pg=PA95}}{{cite book |last= Billing |first= Joanna |title= The Hidden Places of Devon |location= Aldermaston, England |publisher= Travel Publishing Ltd. |year= 2003 |page= 51}}{{cite book |last= Streever |first= Bill |title= Cold: Adventures in the World's Frozen Places |url= https://archive.org/details/coldadventuresin00stre |url-access= registration |location= New York |publisher= Little, Brown and Company |year= 2009 |page= [https://archive.org/details/coldadventuresin00stre/page/147 147] |isbn= 978-0-316-04291-8 |quote= In 1776, a son of the parish clerk of Bampton in Devon, England, was killed by an icicle that plummeted from the church tower and speared him. His memorial: Bless my eyes / Here he lies / In a sad pickle / Kill'd by an icicle.}}
Large icicles that form on cliffs near highways have been known to fall and damage motor vehicles.
In 2010, five people were killed and 150 injured by icicles in Saint Petersburg, Russia after 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.{{cite news |last= Osborn |first= Andrew |title= Falling icicles kill record numbers in St Petersburg |work= The Daily Telegraph |date= 24 March 2010 |url= https://www.telegraph.co.uk/news/worldnews/europe/russia/7512865/Falling-icicles-kill-record-numbers-in-St-Petersburg.html |quote= Russians risk their lives each year as winter becomes spring causing melting icicles and blocks of ice to fall from roofs, often from a great height, onto defenceless pedestrians below. Regional figures show icicles kill dozens of Russians each year. |access-date= 19 September 2012}}
See also
References
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