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Ice Storms

Page history last edited by JoAnn Park 15 years, 3 months ago

 

 Ice Storms

 

 

 

This website was designed by the 2008 James Scholars in ATMS 120 at the University of Illinois at Urbana-Champaign.  It was made to provide interesting information about the weather associated with ice storms and the destruction they cause.  Here you will find a vodcast and a case study about the devasting ice storm of 1998.

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Authors:  Katherine Crane

              Chinonyelum Nwosu

              JoAnn Park

              Howie Leibach

Email:  crane1@illinois.edu

           nwosu1@illinois.edu

           park119@illinois.edu

           hleibac2@illinois.edu

 

                                                                    

 

Table of Contents

 


 

 

Overview

 

 

rack! Snap! Boom! That is the sound of a tree breaking and hitting the ground. Why? This tree has been weighed down by the massive amounts  of ice that covered its branches from a severe ice storm. Ice storms are brutal storms that wield supercooled rain (rain that has been cooled between 27ºF and 32ºF, which freezes on contact with a surface) and snow as weapons.[i]Eastern Canada and New England are the two most popular areas for freezin rain, and collect on average of twenty to forty hours of freezing rain per year.  There is almost no freezing rain to the south along the Gulf of Mexico and in the western Great Plains.

 

 

 

ce storms typically occur during the months of December and January, when temperatures are low enough to support freezing rain. Freezing

precipitation can occur at any time of the day, but it is two times more likely to occur early in the morning before sunrise than later in the afternoon. At pre-dawn, ground surface is at its coldest and the surface-based temperature inversions are most common. An ice storm warning is issued by the National Weather Service when freezing rain produces at the least ¼ of an inch accumulation of ice.[ii] The Midwestern and Eastern regions of the United States see the most frequent occurance of freezing rain events as shown in Figure 2. [iii]

 

 

 

reas that are most often hit by these storms are central North Carolina, West Virginia, Pennsylvania, New York, the Columbia River Valley in southern Washington, and northern Oregon. [iv]  One of the most severe ice storms is the Oklahoma Ice storm of January 29-31, 2002. 

 

 

ne very recent severe ice storm hit Oklahoma in January 29-31, 2002. This large winter storm brought four different types of precipitation: rain, freezing rain, sleet, and snow to Oklahoma hitting it hard by taking down power lines, trees, and telephone lines, as well as causing massive power outages across the state leaving 250,000 people in the dark and the cold.[v]These storms produce an extraordinary amount of damage, which cost hundreds of millions, even billions of dollars to clean up and repair damaged structures, One of the most expensive, single ice storms in recent history was the ice storm that occurred February 9-13, 1994, which required $3 billion for cleanup costs across the Southeastern U.S. because of all the damage that the ice caused by bringing down power lines and trees, as well as additional damage caused by ice.[vi]

 

lthough the Oklahoma ice storm of 2002 and the February 1994 ice storm were massively destructive, they are not the worst ice storms to hit the United States, this title belongs to the 1998 ice storm that hit the northeast U.S. and southeast Canada; the so-called “Great Ice Storm of 1998”. The storm started January 5, 1998 and continued through January 10, 1998 impacting both Canada and the United States.[vii] The “Great Ice Storm of 1998” caused 1.4 billion dollars in damage and killed sixteen (16) people in the United States.[viii] This storm dropped close to ten (10) inches of sleet in some areas of Maine, between a one eighth 1/8 and 3 inches of ice in various areas and more than two feet of snow in others.[ix] Millions were affected by this storm, between the states of New York, New Hampshire, Maine, and Vermont where 1,073,000 lost electricity, some people even going without power for as long as 21 days.[x]  Ice storms like these can produce damage on a variety of levels ranging from downed phonelines and treacherous roads, to leaving millions powerless and entire forests destroyed. Regardless of their strength, ice storms are a formidable part of winter storms that frequently leave behind a path of destruction.


 

 

 

 

 

 

Description

 

 

 

Ice storms, produced by freezing rain, are one of the most dangerous weather occurrences in the world. 

 

Figure 1:  Trees weighed down by the ice

Courtesy of Danzfamily.com

 

Freezing rain accounts for a quarter of all winter related events in the U.S. and half of these develop into ice storms.  Ice storms are classified by their structural damage or by the approximate accumulation of 0.25 inches of ice or more [1].  Minor glazes of ice is hazardous for traffic and pedestrians, but the real severity of ice storms are seen in full effect by the extensive power outages, halted air and ground transportation, and considerable property damage. 

 

Figure 2:  A street in Elora after an ice storm - Frozen utility lines are pulled over by weight of ice

Courtesy of John R. Connon

 

      Since the 1950s, there has been a $16 billion loss of property due to ice storms [xi].  And although infrequent in the South, this part of the country experience the greatest number of insured property losses exceeding $1 million [xi].

 

          Two essential elements are required to generate freezing rain:  a deep warm layer (above freezing) over a shallow cold layer at the surface that has temperatures below freezing [xii].  Whether the precipitation is rain or snow, as it passes through the warm layer, it is converted into rain.  Then as the rain passes through the cold shallow layer just above the surface, its internal temperature drops below the freezing point of water (32°F 0°C), however, the raindrop remains a liquid; a phenomenon called supercooling [xii]. When this supercooled liquid water drop hits a surface that has a temperature below freezing, it immediately freezes upon contact.  This process can be visualized in Figure 3.   

 


Figure 3:  Supercooling

Courtesy of www.noaa.gov

 

 

     There are many weather patterns associated with freezing rain east of the Rocky Mountains.  Two of the most common weather patterns, those responsible for 2/3 of freezing precipitation, are focused around the arctic front and warm front in the extra-tropical cyclone [xi].  Other weather patterns include cold-air damming and cold-air trapping, which are most frequent along the Appalachian Mountains.  In nearly all freezing rain weather patterns, the zone of freezing precipitation is narrow, usually less than 160 km (100 miles) across and is formed as warm moist air overruns cold air at the surface in the vicinity of these fronts [xiii]. If the front is immobile (like the stationary front depicted in Figure 4), it allows the zone of freezing rain to remain at the same location for a long time, allowing enough accumulation of glaze to produce a devastating ice storm. 

 

Figure 4:  Stationary Front

Courtesy of www.atmos.uiuc.edu

 

 

Two such examples include the 2002 January ice storm in Oklahoma and the “Great Ice Storm of 1998” in the northeast U.S. and southeast Canada.

 

Figure 5:  Great Ice Storm of 1998

Courtesy fo John Ferguson

 

 

Formation

Freezing precipitation can be classified into two different categories, freezing rain and freezing drizzle.  Each occurrence involves different processes in which they form, and each presents different hazards.  A common misconception is that when the temperature is less than 32⁰ F or 0⁰ C frozen precipitation will fall.  This is not always true because supercooled, liquid water with an internal temperature less than 32⁰F (0⁰C), water plays a large role in the formation of freezing rain.  For a molecule of pure liquid water to change phase and become ice, the temperature actually needs to be about -40⁰ C.  Because our atmosphere is filled with many microscopic particles including dust, organic particles, and many other impurities, liquid water is able to freeze at much warmer temperatures because these impurities aid in the phase change process.  Ice formation in clouds happens most effectively at temperatures between -15⁰C (5⁰F) and -5⁰C (23⁰ F).  The small microscopic particles, which provide attachment sites for water molecules to lock into their lattice ice structure, are referred to as ice nuclei.  Ice crystals can form somewhat effectively between the temperatures -5⁰C (23⁰F) and 0⁰C (32⁰ F) with the help of these ice nuclei, but ice nuclei are rare.  Therefore, supercooled water is a common feature in our atmosphere.[xiv]

[xv] 

 

Ice on a supercooled cloud courtesy of Penn State College of Earth and Mineral Sciences 

Freezing rain is formed through the melting process which usually occurs beneath the clouds.  To understand this process, it is most easy to think of the atmosphere in three layers when describing how freezing rain is formed (See figure I).  Snow is present in the uppermost layer.  The snow then travels down to a layer where the temperature is greater than 32⁰F (0⁰C) and melts into raindrops.  As the raindrops fall into the next layer where the temperatures are less than 0⁰C, they supercool.  When the raindrops reach the surface, they freeze on contact with objects forming a glaze.  When starting from the surface and moving up, we can see that the temperature is increasing with height for a period of time, and then decreases again.  When temperature increases with height it is called an inversion.[xiv]

 

 

 

Figure 1

Temperature profile for freezing rain formation courtesy of Keith C. Heidorn “The Weather Doctor”

[xvi]

 

 

 

 

The supercooled warm rain process, which usually occurs in shallow clouds (1 to 3 km), is yet another way for freezing precipitation to form.  Shallow clouds that have temperatures between -10⁰C to -15⁰C are composed entirely of supercooled liquid water drops.  These small liquid droplets fall from the cloud resulting in what is called freezing drizzle.  In this process there is no melting layer where the snowflakes melt but rather the very small supercooled water droplets fall as a misty rain that freezes on contact with cold surfaces.  Some of the droplets grow to the size of rain droplets form because cloud drops collide and coalesce together.  Freezing drizzle drops have diameters from about 0.2 mm to about 0.5 mm while freezing rain drops have diameters larger than 0.5 mm.  Freezing drizzle produced by clouds containing supercooled droplets pose a danger for aircrafts because of aircraft icing. Freezing drizzle also creates an accumulation of a thin glaze on roads which results in hazardous road conditions.

Want to see how dangerous icy roads can be? Watch this video of cars on icy streets!

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The weather patterns that commonly accompany freezing precipitation are centered on the air flow over arctic fronts and, warm fronts in extra- tropical cyclones, as well as cold air damming along the Appalachian Mountains.  Arctic fronts account for about a third of all freezing precipitation events east of the Rocky Mountains, while warm fronts in cyclones account for about one third of events in the U.S., and cold-air damming causes about 15 percent of events east of the Rockies.  The zone of freezing precipitation associated with each of these weather patterns is usually less than 100 miles across.  As this narrow band moves with the moving weather pattern it often results in the transition either from freezing precipitation to ice pellets and snow, or freezing precipitation to rain over the course of a few hours. This is because as these fronts move with the extra-tropical cyclone, the position and the depth of the 3-layers mentioned above change causing a shift in the precipitation type and location.[xiv]

Arctic Fronts

Arctic fronts flow from Canada and move southeastward across the U.S.  As the front, which is followed by a high pressure system, approaches the Gulf of Mexico warm air rises over the cold air dome, forming a shallow cloud layer that creates freezing drizzle.  In southern states, deep clouds that produce freezing rain can form from warm, unstable air overrunning the arctic air mass.  This overrunning process is best seen in figure 1.1 below. 

Figure 1.1

Low pressure system and temperature separation.  Courtesy of Keith C. Heidorn “The Weather Doctor”

[xvii]

 

 

Warm Front of a Cyclone

A low-pressure system associated with an extra-tropical cyclone typically has a warm front that extends out to the east of a low-pressure center.  This warm front is the product of the counterclockwise circulating winds that pull warm, moist air northward from the Gulf of Mexico, which in turn collides with colder air in the northern U.S. and Canada.[xviii] 

Warm fronts develop best in the winter and the freezing precipitation associated with them occurs in a narrow band ahead of (to the north of) the warm front.[xix] As the warm, moist air overruns the warm front, the vertical structure of the air in the lower parts of the atmosphere mimic the three layer structure discussed above (see figure I).  Clouds and precipitation form over these layers, and the winds along the front force air upward where it will condense into clouds and form precipitation.  During the freezing rain event, strong winds may accompany the winter storm due to the contrast of this low-pressure system and a high-pressure system to the north.  This results in greater potential for damage because the added weight due to the ice combined with the increased wind speeds adds to the stress on trees and power lines.

 

Freezing precipitation due to a warm front associated with a cyclone. Courtesy of University of Illinois Dept. of Atmospheric Sciences

[xx] 

 

 

Damage caused by the added weight of ice on a tree.  Courtesy of the Farmer’s Almanac

[xxi]

Cold-air damming

Freezing precipitation occurs around the Appalachian Mountains through processes know as cold-air damming.  Cold-air damming (CAD) results in freezing precipitation on the east side of the mountain range.  Ice storms due to CAD are hard to accurately predict.  For cold-air damming to occur there must be a sloping inversion (warm air on top of cold air) on the eastern side of a mountain range, warm moist air flowing from the east over the top of the cold air mass.  As cold air comes from the north and flows southward with the barrier jet around the high-pressure system over the northeastern U.S., it becomes dammed against the mountains. The role of the cold air is to lower the surface temperatures and form the cold layer needed to get freezing rain precipitation. [xxii] As warm moist air flows off of the Atlantic over the top of the cold air that is trapped or dammed against the mountain, clouds and precipitation form. As this precipitation falls, it eventually falls into this cold air providing the necessary conditions for supercooling the droplets and creating freezing rain. 

Diagram of cold-air damming.  Courtesy of METED

[xxiii]  

 

 

Measurement and Forecasting

Meteorologists use surface pressure maps to monitor fronts and patterns that are associated with freezing precipitation.  Soundings and data taken from ASOS stations are also very helpful.  A temperature inversion in the lowest levels of the atmosphere will appear on the sounding during an ice event. (See Figure 1.2). Meteorologists forecast an ice storm advisory based on how fast the fronts are moving.  Damage caused by ice storms can be measured in many different ways.  Aerial photography and satellite imagery can give pictures of surface damages.  People survey property damage and damage to landscapes to describe the severity of the storm.  The total cost of the damage to an area also helps determine how severe the ice storm was.  The number of icy days during a winter season gives an idea about the harshness of the winter.

 

 

Figure 1.3

The chart above (called a sounding) offers a thermodynamic profile of the atmosphere on Saturday Dec. 10 2007 during a freezing rain event near Norman, OK. On the sounding the 3-layer structure of the atmosphere is highlighted. Image courtesy of Eric Snodgrass

 

 

Destruction

 

Freezing precipitation has caused over 16 billion in property losses in the United States alone and accounts for 20 percent of all winter-related injuries. About  70% of these injuries result from vehicle accidents. The average victims susceptible to ice storms are males over 40 years old (xxiv). Heavy ice accumulation can bring down trees and topples utility poles and communication towers. Consequently, ice storms can disrupt communication between cities for days on end as they lose power and proper means of transportation. Road surfaces become damaged as well. Bridges and overpasses have a tendency to freeze before other types of surfaces and thus increase the risk of traveling during ice storms. Building infrastructures are vulnerable as well. Pipes can freeze and burst in poorly insulated homes. Insured property losses from ice storm events in the U.S. average 326 million dollars per year in damages (xxvi). The agricultural sector is punished by ice storms as well. Freezing temperatures can cause severe damage to citrus fruit, crops and vegetation with damages recorded as high as 1.6 billion dollars in total damages per agricultural season (xxvi).  Although there is no record of deaths per year due to ice storms, the affects of ice storms attribute to the total deaths per year equating to 47 deaths per year (U.S.) (xxv).

Damages

 

Ice storm damages are insurmountably high partly due to ice pillage. In perspective, the weight of ice on a power line  for a 300 ft span of power lines that are 1" thick coated with 1/4 inch of ice, adds  117 lbs. of weight to the object. Coated with 1/2 inch, the added weight is 281 lbs, coated with 1", the added weight is 749 lbs and coated with 2", the added weight is 2248 lbs. For a 1500 foot span of power line the added weight of 2 inches of ice is 11242 lbs! Overall, Accumulations of ice can increase the branch weight of trees by 30 times its original weight. The average annual property damage loss in ice storms based on an 8 year period is 226 million dollars and accounts for about 60 percent of winter storm damages (xxiv).

   

http://images.google.com/imgres?imgurl=http://farm3.static.flickr.com/2335/2108315551_2d52a66fcf.jpg&imgrefurl=http://crankyflier.com/2007/12/13/frozen-airplane-porn/&usg=__qcHSwKeFFo7XmzTJvTXD6YnBi9Y=&h=375&w=500&sz=109&hl=en&start=1&sig2=y-ti_jEQhYfeBjucUE-7xQ&tbnid=iASXGnBDwTGvbM:&tbnh=98&tbnw=130&ei=-OodSa3uEZrUMcSF2fsJ&prev=/images%3Fq%3Dfrozen%2Bairplane%26gbv%3D2%26hl%3Den%26sa%3DG

 

The accumulation of ice can also damage airplanes. If freezing drizzle droplets in clouds grow to above .04 mm, they collect on the wings of aircrafts. Ice can and has accumulated to crash airplanes.

 

 

http://farm3.static.flickr.com/2185/2078343483_cec8c70f56.jpg

 

Destructive Regions

Most ice storm catastrophes occur in the Northeast, Central Southeast, and South climate regions. The maximum and average ice thickness is relatively the largest in New England and in the general Northeast however the South’s ice storms are the most devastating. In contrast, Ice-thickness is the lowest in the upper Midwest and Pacific Northwest (xxviii). Ultimately, the greatest risk of ice storm damages is in the Northeastern United States followed by the Lower Midwest and the south. Storm-area sizes ranged from 205 to 796 000 km2 with the greatest area’s (per km2) being in the Eastern US (xxviii).

Preventative Measures

The National Climatic Data Center's (NCDC) Research Customer Service Group collaborated with the U.S. Army's Cold Regions Research and Engineering Laboratory (CRREL) to provide preventative steps in regard to freezing rain. Overall, Ice storms are economic catastrophes which can and have forcefully dismantled cities and parks and have capabilities of disrupting air and land transportation. Because of new technology, ice formation forecasts save our country 29 million dollars per year in damages while the Integrated Icing Diagnostic Algorithm saves 33.7 million dollars in plane damages per year (xxiv).

 

 

 

Ice Storm Vodcast

Fliqz has shut down their service. To access this video, email support with this video id: c2fe4b90425b4d59a6563e39e29918c2

 

Case Study: Ice Storm of 1998

Sources

Overview:

[i]

http://www.srh.noaa.gov/srh/jetstream/synoptic/precip.htm

[iv]

http://www.ci.savannah.ga.us/cityweb/disasterinfo.nsf/5ae9bd7c8938f28a85256b88006d8e15/973ecbe69ed086d585256c2400535911?OpenDocument

[vi]http://ols.nndc.noaa.gov/plolstore/plsql/olstore.prodspecific?prodnum=C00490-PUB-A0001

[x]http://imiuru.com/icestormdiary/1pages/factsfigures.html

 

Description:

[xi] Rauber, Robert, and John Walsh, and Donna Charlevoix. Severe and Hazardous Weather. Kendall/Hunt      Publishing Company, 2005.

[xii] http://www.srh.noaa.gov/srh/jetstream/synoptic/precip.htm

[xiii] http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/prcp/zr/cond/cyc.rxml

 

Formation:

[xiv] Freezing Precipitation and Ice Storms – Chapter 11.  Severe & Hazardous Weather written by:  Robert M. Rauber, John E. Walsh, Donna J. Charlevoix.

[xv]http://www.ems.psu.edu/~lno/Meteo437/Usaf1.jpg - ice on cloud

[xvi] “Ice Storms: Beauty Amid Destruction.”  Keith C. Heidorn. http://www.suite101.com/article.cfm/science_sky/86621

[xxiii]http://www.meted.ucar.edu/mesoprim/cad/webcast/frameset.htm

Destruction:

[xxiv]http://www.economics.noaa.gov/?goal=weather&file=events/snow 

[xxv] http://ams.allenpress.com/archive/1520-0477/80/6/pdf/i1520-0477-80-6-1077.pdf

[xxvi]http://www.nws.noaa.gov/om/winterstorm/winterstorms.pdf

[xxvii] http://web.aces.uiuc.edu/vista/pdf_pubs/ICESTORM.PDF

 

[xxviii] Stanley Changnon, Characteristics of Ice storms in the US PDF. U OF I.

 

 

Authors 

 

 

 

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