Black ice - wikipedia Black ice For other uses, see Black ice ( disambiguation ). Warning sign for icy pavement in Quebec, Canada Black ice, sometimes called clear ice, is a thin coating of glaze ice on a surface, especially on roads. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it. The typically low levels of noticeable ice pellets, snow, or sleet surrounding black ice means that areas of the ice are often practically invisible to drivers or people stepping on it. There is, thus, a risk of skidding and subsequent accident due to the unexpected loss of traction. Contents 1 Definitions 2 Formation 2.1 On roads and pavements 2.2 On bridges 2.2. 1 Problems on I - 35W Mississippi River bridge 2.3 On water 2.4 In mountains 3 See also 4 References 5 External links Definitions The term black ice in the United States is often incorrectly used to describe any type of ice that forms on roadways, even when standing water on roads turns to ice as the temperature falls below freezing. Correctly defined, black ice is formed on relatively dry roads, rendering it invisible to drivers. It occurs when in the textures present in all pavements very slightly below the top of the road surface contain water or moisture, thereby presenting a dry surface to tires until that water or moisture freezes and expands ; drivers then find they are riding above the road surface on a honeycombed invisible sheet of ice. Three other definitions of black ice by the World Meteorological Organization are : A thin ice layer on a fresh or salt water body which appears dark in colour because of its transparency ; A mariner 's term for a dreaded form of icing sometimes sufficiently heavy to capsize a small ship ; Another term for ice on rocks in the mountains known equally as verglas ( glaze ice ). Formation On roads and pavements The American Meteorological Society Glossary of Meteorology includes the definition of black ice as `` a thin sheet of ice, relatively dark in appearance, ( that ) may form when light rain or drizzle falls on a road surface that is at a temperature below 0 ° C ( 32 ° F ). '' Because it represents only a thin accumulation, black ice is highly transparent and thus difficult to see as compared with snow, frozen slush, or thicker ice layers. In addition, it often is interleaved with wet pavement, which is nearly identical in appearance. This makes driving, cycling or walking on affected surfaces extremely dangerous. Deicing with salt ( sodium chloride ) is effective down to temperatures of about − 18 ° C ( 0 ° F ). Other compounds such as magnesium chloride or calcium chloride have been used for very cold temperatures since the freezing - point depression of their solutions is lower. At low temperatures ( below − 18 ° C ( 0 ° F ) ), black ice can form on roadways when the moisture from automobile exhaust condenses on the road surface. Such conditions caused multiple accidents in Minnesota when the temperatures dipped below − 18 ° C ( 0 ° F ) for a prolonged period of time in mid-December 2008. Salt 's ineffectiveness at melting ice at these temperatures compounds the problem. Black ice may form even when the ambient temperature is several degrees above the freezing point of water 0 ° C ( 32 ° F ), if the air warms suddenly after a prolonged cold spell that has left the surface of the roadway well below the freezing point temperature. On December 1, 2013, heavy post-Thanksgiving weekend traffic encountered black ice on the westbound I - 290 expressway in Worcester, Massachusetts. A chain reaction series of crashes resulted, involving three tractor - trailers and over 60 other vehicles. The ice formed suddenly on a long downward slope, surprising drivers coming over the crest of a hill, who could not see crashed vehicles ahead until it was too late to stop on the slick pavement. On bridges Warning sign for bridge on US turnpike Bridges and overpasses can be especially dangerous. Black ice forms first on bridges and overpasses because air can circulate both above and below the surface of the elevated roadway when the ambient temperature drops, causing the bridge pavement temperature to fall more rapidly. In the United States, road warning signs with the advisory `` Bridge May Be Icy '' indicate potentially dangerous roadways above bridge structures. Similar road signs exists throughout Canada, but warnings sometimes appear without words to comply to bilingual requirements. The Canadian sign features a vehicle with skid marks and snow flakes. The same sign 's official and undisclosed description is defined as `` Pavement is slippery when wet ''. Additional signs may be attached with various different wording in Canadian provinces that do not have bilingual requirements : Bridge Ices Slippery When Wet Road Ices Slippery When Frosty Icy Bridge Deck Bridge Ices Before Road Problems on I - 35W Mississippi River bridge See also : I - 35W Mississippi River bridge The I - 35W Mississippi River bridge seen from below in 2006 The I - 35W Mississippi River bridge in Minneapolis, Minnesota, was well known for its black ice before it collapsed in 2007 into the Mississippi River. It had caused several pileups during its 40 - year life. On December 19, 1985, the temperature reached − 34 ° C ( − 29 ° F ). Cars crossing the bridge experienced black ice and there was a massive pile up of crashed vehicles on the bridge on the northbound side. In February and in December 1996, the bridge was identified as the single most treacherous cold - weather spot in the local freeway system, because of the almost frictionless thin layer of black ice that regularly formed when temperatures dropped below freezing. The bridge 's proximity to Saint Anthony Falls contributed significantly to the icing problem and the site was noted for frequent spinouts and collisions. By January 1999, Mn / DOT began testing magnesium chloride solutions and a mixture of magnesium chloride and a corn - processing byproduct to see whether either would reduce the black ice that appeared on the bridge during the winter months. In October 1999, the state embedded temperature - activated nozzles in the bridge deck to spray the bridge with potassium acetate solution to keep the area free of winter black ice. The system came into operation in 2000. While the automated de-icing system prevented further major multi-vehicle collisions, it may have contributed to the collapse of the I - 35W bridge, due to accelerated corrosion of the structure. On water Main article : Congelation ice Black ice on a canal in the Netherlands When the temperature is below freezing and the wind is calm, such as under a high atmospheric pressure at night in the fall, a thin layer of ice will form over open water of a lake. If the depth of the body of water is large enough, its color is black and can be seen through the ice, thus the name black ice. Ice can also be formed by seawater spray and water vapour freezing upon contact with a vessel 's superstructure when temperature is low enough. Ice formed in this manner is known as rime. As the formation progress, the aboveboard weight of the vessel increases and may ultimately cause capsizing. Furthermore, rime ice may impede the correct functioning of important navigational instruments on board, such as radar or radio installations. Different strategies for the removal of such ice are employed : chipping away the ice or even using fire hoses in an attempt to remove the ice. In mountains Main article : Glaze ice Black ice is a great hazard for climbers and scramblers. Cold weather is common at high altitudes, and black ice quickly forms on rock surfaces. Loss of traction is as sudden and unexpected as on a pavement or road, but can be fatal if the rock is in an exposed position with a drop below. An ice - axe and crampons are essential use in such circumstances as they will help to prevent a fall, and a belay rope will help to arrest a fall. See also winter portal Clear ice Congelation ice Freezing rain Freezing - point depression Glaze ice Jumble ice Road traffic accident Sand Sodium chloride References Jump up ^ World Meteorological Organization. `` Black Ice ''. Eumetcal. Archived from the original on December 3, 2013. Retrieved November 28, 2013. Jump up ^ `` AMS Glossary : Black ice ''. Amsglossary.allenpress.com. Retrieved November 28, 2013. Jump up ^ `` Is there really such a thing as black ice ? ''. Retrieved December 22, 2008. Jump up ^ `` Black ice causes treacherous driving conditions in metro ''. KARE 11 TV. Retrieved December 22, 2008. Jump up ^ Abel, David ( December 2, 2013 ). `` On busy travel day, black ice led to massive pileup in Worcester ''. Boston Globe. Retrieved December 3, 2013. Jump up ^ Jeffrey Selingo ( December 13, 2001 ), `` How it works : Black ice '', The New York Times Jump up ^ `` Driver 's Handbook ''. Retrieved December 23, 2016. Jump up ^ Blake, Laurie. ( February 3, 1996 ) Star Tribune February deep freeze. Black ice makes I - 35W bridge treacherous. Section : news ; Page 10A. Jump up ^ von Sternberg, Bob. ( December 27, 1996 ) Star Tribune Minnesota is one big deep freeze. What is the sound of a cold record shattering ? It 's the sound of silence from dead motors, of crumpling metal on icy roads, of resigned grumbling. But take heart -- it will warm up. Section News ; page 1A. Jump up ^ Blake, Laurie. ( January 21, 1999 ) Star Tribune State hopes to speed up north - metro lane project. But it clashes with Met Council over whether addition to interstate should be for car pools. Section news ; Page 2B. Jump up ^ Blake, Laurie. ( October 19, 1999 ) Star Tribune I - 35W bridge getting de-icer system. Unit will target ice before it can form. Section news ; Page 1A. Jump up ^ `` I - 35W & Mississippi River Bridge Anti-Icing Project '' ( PDF ). Archived from the original ( PDF ) on October 5, 2011. Jump up ^ Blake, Laurie. ( January 13, 2000 ) Star Tribune Met Council will survey our citizens ' travel habits. Study will include trip numbers and times, speed of drivers and waits at ramp meters. Section news ; Page 2B. Jump up ^ Blake, Laurie. ( February 3, 2000 ) Star Tribune Richfield may face traffic challenges ; How will I - 494 accommodate Best Buy 's 5,000 commuters ? Section news, page 2B. Jump up ^ `` De-Icing Chemical May Have Corroded 35W Bridge ''. Archived from the original on November 13, 2007. Retrieved September 10, 2007. Jump up ^ National Weather Service. `` Rime ice ''. NOAA. Retrieved November 28, 2013. External links Wikimedia Commons has media related to Black ice. Look up black ice in Wiktionary, the free dictionary. Is there really such a thing as black ice ?, from the Straight Dope staff Ice the solid state of water Crystalline types Ih Ic II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI Formations and phenomena Amorphous solid Anchor Black Clathrate hydrates Cap Cave Circle or disc Clear Crystals Dam ( roof ) Diamond dust Drift Dune Field Firn Fog Frazil Frost Frost heave Frost flower ( sea ice ) Glacier Glaze Hair ice Iceberg ( calving ) Icicle Jacking Jam Nucleus Sheet Shove Spike Stalactites Macroscopic quantum phenomena Névé Needle Rime hard soft Sea Slurry Slush Storm Ice - related activities Bathing Blasting Blocking Climbing Fishing Rafting Sculpture Skating Sports Bandy Boating / yachting Cricket Curling Cycling Figure skating Hockey Racing Tour skating Constructions Bar Bridge Hotel Igloo Palace Pier Pykrete Road Work Cutting Icebox Icehouse Iceman ( occupation ) Pick Trade Other uses Chips Core Cream Cube Pack Ice ages Glaciology Little Ice Age Pleistocene Wikimedia Commons Wiktionary Streets and roadways Types of road Limited - access Freeway / Motorway Dual carriageway / Divided highway / Expressway Elevated highway By country Australia Brazil China Croatia Czech Republic Germany Greece Hong Kong India Ireland Italy Pakistan Portugal Spain United Kingdom United States Main roads Arterial road Collector road County highway Express - collector setup Farm - to - market road Highway Link road Two - lane expressway 2 + 1 road 2 + 2 road Parkway Super two Trunk road Highway systems by country Local roads Alley Backroad Bicycle boulevard Boulevard Country lane Dead end Driveway Frontage road Green lane Main street Primitive road Road Side road Single carriageway Single - track road Street Sunken lane Other terms Channelization Concurrency Detour Hierarchy of roads Private highway Route number Special route Business route Street hierarchy Toll road Road junctions Interchanges ( grade - separated ) Cloverleaf Diamond Free - flow Directional T Diverging diamond Parclo Raindrop Roundabout Single - point urban ( SPUI ) Stack Three - level diamond Trumpet Intersections ( at - grade ) 3 - way junction Bowtie Box junction Continuous flow Hook turn Jughandle Michigan left Offset T - intersection Protected intersection Quadrant roadway Right - in / right - out ( RIRO ) Roundabout Seagull intersection Split intersection Superstreet Texas U-turn Traffic circle Turnaround Surfaces Asphalt concrete Bioasphalt Brick Chipseal Cobblestone Concrete Reinforced concrete Corduroy Crocodile cracking Crushed stone Diamond grinding of pavement Dirt Full depth recycling Glassphalt Gravel Ice Macadam Pavement milling Permeable Plank Rubberized asphalt Sealcoat Sett Stamped asphalt Tarmac Texture Road hazards Aquaplaning Black ice Bleeding Crosswind Dead Man 's Curve Expansion joint Fog Ford Hairpin turn Level crossing Manhole cover Oil spill Oversize load Pothole Road debris Road slipperiness Road train Roadkill Rockfall Rut Speed bump Storm drain Washboarding Washout Whiteout Space and time allocation Barrier transfer machine Bicycle lane Climbing lane Complete streets Contraflow lane Contraflow lane reversal High - occupancy toll lane High - occupancy vehicle lane Lane Living street Managed lane Median / Central reservation Motorcycle lane Passing lane Pedestrian crossing Pedestrian zone Refuge island Reversible lane Road diet Road verge Runaway truck ramp Shared space Sidewalk / Pavement Shoulder Street running railway Traffic calming Traffic directionality Traffic island Traffic lanes Traffic signal preemption Unused highway Wide outside lane Woonerf Demarcation Bollard Botts ' dots Cable barrier Cat 's eye ( road ) Concrete step barrier Constant - slope barrier Curb F - Shape barrier Guard rail Jersey barrier Kassel kerb Noise barrier Raised pavement marker Road surface marking Rumble strip Traffic barrier Traffic cone Structures Bridge Causeway Overpass / Flyover Underpass / Tunnel Glossary of road transport terms Road types by features Retrieved from `` https://en.wikipedia.org/w/index.php?title=Black_ice&oldid=833263142 '' Categories : Snow or ice weather phenomena Water ice Precipitation Weather hazards Road hazards Hidden categories : All articles with dead external links Articles with dead external links from July 2017 Articles with permanently dead external links Pages using div col without cols and colwidth parameters Talk Contents About Wikipedia Wikimedia Commons Беларуская Беларуская ( тарашкевіца ) ‎ Български Čeština Dansk Deutsch Español Esperanto Euskara فارسی Français 한국어 Italiano Latviešu Lietuvių 日本 語 Polski Português Русский Саха тыла Српски / srpski Suomi Svenska தமிழ் Українська اردو Tiếng Việt Žemaitėška 中文 21 more Edit links This page was last edited on 30 March 2018, at 15 : 11. 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Question: at what temperature does water freeze on the roads
Answer: a temperature below 0 °C (32 °F)