Roadway Icing: The Most Serious Weather Hazard in Washington State
Ice on roadways is probably themost serious meteorological hazard faced by Washington State citizens andcauses hundreds of serious injuries and several tragic deaths a year (clickto view some recent media stories on ice-related accidents).
For example, in 1996, for the region including the Cascade passes and easternWashington, roughly 25% of the total accidents were related to ice on theroad. Sometimes called "black ice" when not clearly visible at night,roadway ice is not black at all, but is made up of frozen water that reflectslight or sparkles when illuminated.Reducing the Threat of Ice-Related Accidents
Roadway icing occurs under conditionsthat are generally well understood and often predictable. Thus, alittle knowledge, coupled with access to current weather information, canhelp motorists and transportation personnel to evaluate the threat andbe prepared for ice formation. This tutorial will describe a varietyof weather conditions that can result in roadway icing and how one candetermine when roadway icing is a threat. Key points to be reviewedinclude:To get ice on a roadway requires freezing temperatures(below 32F) and moisture (water) at the surface, a combination that canoccur in a number of ways:
More than one of these icing mechanisms can occur at thesame time! Let us consider these icing processes one at a time.Frost Fog passing over a cold roadway surface Freezing of groundwater seepage or melted snow Freezing of snow that had initially melted on a warm roadsurface. Freezing rain.
Frost tends to occur on cold, relativelyclear nights when wind speeds are low (less than 10 mph in general).But why are clear skies and light winds important?
All objects give off or emit infraredradiation. The warmer the object the more infrared radiation it emits.We all have some experience with infrared radiation; for example, whenyou sit across a room from a fire you can feel the infrared radiation itemits. Some objects emit infrared radiation better than others.For example, the earth's surface is far more efficient in emitting radiationthan the gases in the atmosphere. Clouds are very good at emittingand absorbing infrared radiation.
On a clear night the surface emits infrared radiation,and with no clouds to stop it, most of this radiation is lost to space.
Thereforethe surface and the air near it cool quickly. Away from thecold surface, the air tends to be warmer. Thus, on cold, clear nightstemperature actually warms with height--the opposite of the normal situation!Temperature increasing with height is call an inversion. On overcastnights, clouds act like meteorological blankets that slow or prevent theloss of infrared radiation (heat) from the surface, and thus cooling isfar less. Blankets keep us warm by slowing the loss of heat fromour bodies--clouds work similarly, except that they reduce the amount ofheat leaving the surface. Strong winds also tend to work againstsurface cooling since windy conditions "stir up" the atmosphere and mixsome of the warmer air aloft down to the ground.
All air has some water vapor in it.Water vapor is an invisible gas; water can only be seen when it condensesinto water droplets or ice crystals. The amount of water vapor air can"hold" varies with temperature, with warmer air having the ability to holdmore water vapor. If we cool air down sufficiently (to the dew pointtemperature), it can no longer hold the moisture it started with, forcingthe water vapor to condense out into water droplets or ice crystals.Dew point temperature (or dew point as it is often called) is reportedat many weather observing stations.
During the day when the sun is outand is warming the surface, the air temperatures near the surfaceare usually above the dew point temperature, and water in the atmosphereremains in the form of invisible vapor. However, as the sun setson cold, clear nights, the surface temperature plummets (as the earth radiatesheat into space), and the air near the surface can cool to the dew pointtemperature. If the roadway temperature and dew point temperature are abovefreezing, liquid water forms on the surface (dew), but if the temperatureis below freezing, frost forms instead.
A moist atmosphere, with lots of watervapor, encourages frost since it can supply more water for freezing.Such moist conditions are often found after a period of precipitation orin wet locations, such as near swampy areas or rivers. Lots of watervapor also raises the dew point temperature, since with more water vaporin the air you don't have to cool the air as much to get dew or frost.As noted above, windy periods have less frost since wind-produced mixingbrings warmer, drier air from above down to the surface. Frost isoften more prevalent in valleys and low areas, into which cold air tendsto drain and pool (more details on this below), and where wind speeds aregenerally less.
Frost generally accumulates slowlyand rarely accumulates more than 1/16 of an inch. For that reason, frostcan be less of threat than fog-related and other forms of icing.But keep in mind, frost related roadway icing has caused plenty of accidents!
In summary, frost generally occurson relatively clear nights when winds are light. Strong winds work againstfrost formation. As noted below, most weather stations report temperaturesat about 5 feet, where temperatures can be considerable warmer than atthe surface. So if the sky is relatively cloud-free and nighttimeair temperatures are dropping towards the mid 30's, frost may be a realthreat.
Although the icing threat from frostis lessened by its slow accumulation and relatively minimal thickness,this is not true of fog-related roadway icing. Fog often forms on cold,clear nights as the temperatures drop to the dew point temperature. Fogcontains large amounts of liquid water, and if a fog bank passes over aroadway that has cooled to a temperature below freezing, icing can be rapidand severe, with a thick coat of ice being deposited in minutes.
A number of serious icing accidentshave occurred in Washington State as a result of fog-related icing. A typicalscenario starts with a clear, cold night in which the surface rapidly cools.A light frost might form on the roadway, but nearby fog begins to formover a moist surface. The fog drifts over the road, and as it passes overthe road surface a thick coating of ice is deposited. Thus, both motoristsand road maintenance crews must be extra vigilant when fog forms on coldevenings when temperature drops below freezing at the surface or the mid-30sin the air immediately above.
Freezing of Roadside Melted Snowand Groundwater Seepage
Dangerous icing conditions frequentlyoccur when road surface temperatures are above freezing during the day,but fall below freezing at night. Even if the road is snow free, snow isoften found along its sides. This is particularly true of roads that areactively plowed, with piles of snow adjacent to the open lanes. The roadsidesnow melts during the day, particularly in those areas adjacent to therelatively warm road. (Roads, especially dark blacktop roads, readily absorbheat from the sun.) The melt-water runs over the road during the day, andthen freezes at night, particularly if the sky has few clouds.
A similar situation can occur withoutsnow, if water drains over the road from a spring or other water source.During the day the water remains liquid, but during the night it freezeson the road surface. Furthermore, wet roads often freeze up veryrapidly when the air above is dry. The reason for this is coolingfrom evaporation. We all experience the chill of evaporative coolingwhen we exit from a shower or bath--this cooling is greatest when the airaround us is dry. Thus on a cold, dry night evaporation from a wetroad can cause the surface to cool much more rapidly that dry road surfacesnearby, resulting in localized icing. Thus, for all of these reasons,it is important for road maintenance personnel to be familiar with wetroad areas, and to check them frequently on cold nights when air temperaturesdrop towards the mid 30s. The fact that wet areas are often in lowareas where cold air tends to pool, makes then doubly dangerous.
Refreezing of Melted Snow on theRoad
A particularly dangerous type of icingoccurs early in the winter season or after a period of warm weather. Atsuch times the road surfaces are above freezing. If the weather turns cold,snow may start to fall, and initially is melted into a wet slush by thewarm road surface. If the air temperature continues to fall rapidly (perhapsafter the passage of the arctic front from the north or a push of coldeastern Washington air into the passes), warming from the road surfaceand the warm ground below is overwhelmed by the cold air above and theslush mixture turns to ice.
Freezing rain or drizzle is relativelyrare over Washington State, being most prevalent in the Columbia Gorgearea, the passes, near Bellingham, and in parts of eastern Washington.Freezing rain occur when there is a layer of below-freezing air near thesurface with warmer (above freezing) air aloft. Rain from aloft falls intothe cold layer and gets cooled to temperatures below freezing-and remainsliquid! Amazingly, water doesn't necessarily freeze immediately inthe atmosphere when temperatures are below 32 F-such subfreezing wateris termed "supercooled." When such supercooled rain hits the surface, itfreezes immediately into a clear glaze ice. Such freezing rain often resultsin ice storms that can make travel treacherous and that down trees andpowerlines.
The Columbia Gorge area can experience freezing rain whenit is filled with cold, westward-moving low level flow originating in easternWashington and Oregon. Warm rain from an incoming Pacific weather systemis cooled below freezing as it travels through the cold Gorge air and thenfreezes on contact with the surface. The mountain passes can get freezingrain when cold air from eastern Washington is drawn into the passes whenthe atmospheric pressure is higher in eastern Washington than in westernWashington. As a relatively warm Pacific weather system approaches thecoast, warm rain falls into the cold air resident in the mountain passes,causing freezing rain. After the Pacific system passes through the areathe temperatures decrease aloft and thus precipitation generally changesover to snow. The area near Bellingham is also at risk for freezingrain when cold air from the interior of British Columbia passes throughthe Fraser River Valley into northwest Washington. During such periods,strong northeasterly (from the northeast) winds of 30-70 mph are not unusual,with temperatures plummeting to the twenties or below. If a warmPacific system approaches the State at this time, freezing rain can spreadover Bellingham and vicinity. Finally, eastern Washington occasionallyexperiences freezing rain. Eastern Washington is really a topographic basin,in which cold air frequently pools. As warm, rain-bearing systems passthrough the region above cold air trapped near the surface, freezing raincan be a serious threat.
The Puget Sound region generally escapesfreezing rain and resultant icy road conditions, but it can happen.
Forexample, a severe freezing rain case struck the southern Puget Sound areaduring 26 December 1996, resulting in hundreds of accidents, the closureof Sea-Tac Airport for days, and numerous power outages.
The Effects of Local Terrainand Land Use on Icing
Cold air is denser and heavier thanwarm air. Thus, the coldest air tends to drain or move down slopes.
On nights with relatively few low clouds,the surface tends to cool due to loss of infrared radiation to space. Thecool surface then cools the air above. As the air cools, it becomes denserand heavier and tends to sink to lower elevations. Thus, valleys or thelower portions of slopes tend to be cool compared to adjacent regions ofhigher elevation. Changes in temperature with elevation can be quite largein such cases, with even shallow valleys (100-200 feet deep) being 2-5Fcooler than higher regions only tens or hundreds of feet away.
Surface temperatures are also warmerin cities at night since concrete tends to hold daytime heat from the sun,and buildings and businesses produce a great deal of heat as well. Watersurfaces also tend to be much warmer than land on cold winter nights, andthus freezing is far less likely near large bodies of water such as LakeWashington, Puget Sound, or the Pacific Ocean.
Variations in temperature due to localterrain features are very repeatable night after night, and thus "old timers"who live or work in an area for a while get to know the cold spots vulnerableto freezing. These complex, terrain-related variations in temperature areoften called "thermal signatures" and can be measured with instrumentedcars or using remote sensing on aircraft.
Why Bridges Often Ice Up Firstand
Why the First Snowof the Season Often Doesn't Stick or Produce Ice?
The temperatureof a road surface is impacted by a number of factors such as radiationalcooling to space and the amount of heat coming up from the ground below---toname only few. Temperature changes at the surface take days or weeksto extend more than few inches into ground, and at night the roadway surfaceis often cooler than the ground underneath. Heat conducted from below theroad surface lessens nighttime temperature falls, and thus reduces thepotential for icing. Bridges have air (a good insulator) underneath themand thus do not receive heat from the ground below. Thus, bridgesare much more vulnerable to roadway icing at night compared to normal roadsurfaces, particularly early in the winter when the ground is relativelywarm. In fact, heat from warm ground below a road during the fall(or after a warm period in winter) can greatly heat a road surface, preventingicing even when air temperatures fall below freezing. This works only upto a point, since very cold air temperatureseventually cause freezing to occur even when the ground is above freezing.In fact, some of the worst icing situations have occurred with warm ground:snow falls and starts melting on the road surface and later freezes solidas much colder air moves into the region.
Surface Temperature and AirTemperature Observations: How Are They Related?
Temperature measurements are generallytaken with thermometers in a sheltered enclosure at about 5 feet abovethe ground, usually above a vegetated surface.It is absolutely crucialto understand that the "official" temperatures are reported by thermometerslocated at around 5 feet whose readings can be very different from roadsurface temperatures. On clear nights when winds are light, the surfaceradiates heat to space much more effectively than the air above. On suchnights, temperature at ground level can be 2-5F cooler than air temperatureonly a few feet above. Thus, frost can be occurring at the surface evenwhen official temperature observations are reporting temperatures of 35to 37F. During the day, the opposite situation can occur, with the roadsurface several degrees warmer than the air temperature at 5 feet. Airtemperature readings on trucks and cars are similarly problematic...theyoften are colder (night) or warmer (day) that the road surface. The moralof this story is that motorists and maintenance personnel must be waryof icing as air temperatures drop below around 37F.
The Effects of Trees and OtherObjects Near and Above a Roadway
Shading of road surfaces by trees,hills, and other objects greatly influence the potential for, and longevityof, roadway ice. At night, overhanging trees or other road covers can lessenthe potential for frost by blocking the loss of infrared heat to space.This is why cars rarely frost up under carports. On the other hand, ifan area does frost up or get covered with ice, shading due to trees orhillsides can delay melting well into the late morning or allow ice toremain all day.
Anumber of fatal accidents have occurred on State roadways when driversh*t unexpected areas of ice protected by shade. Areas shaded by hillsidescan start to cool rapidly hours before sunset, resulting in icing beforedark. Such icing was associated with a recent fatal accident on Interstate90 near the town of Thorp.
How Temperature Changes the Slipperiness of Ice
The slipperiness of ice changeswith temperature, with ice being more slippery when it warms to near freezing.Recent research has found that ice has a thin layer of liquid water onthe outside--even when temperatures are below freezing. This layerof liquid water is what makes ice slippery. The thin veneer of liquidwater on the outside of ice is thickest when the temperature of the iceapproaches freezing, and thins substantially when temperatures cool wellbelow 32F. Thus, ice is most slippery when temperatures are nearfreezing (26-32F) and is much less slippery when temperatures reach thesingle digits and below. So if the air temperature is just belowfreezing and ice is on the roadway, extra care is warranted.
How do Temperatures Vary on a Typical Icing Night?
On cold, relatively clear nights duringwhich the earth rapidly loses heat to space, temperature typically fallsquickly during the first three to four hours after sunset; in fact, temperaturescan start falling before sunset, particularly if the road section in questionis in the shade.
Thus,icing problems can begin to develop during the evening during cold periods.If there are no clouds or fog, the temperatures will then slowly fall duringthe remainder of the night, reaching the lowest temperatures near sunrise,or occasionally shortly after sunrise. Therefore, on marginal nightsthe greatest icing threat is between 5 and 8 am. If fog develops,the temperature decline will be lessened since fog, like any low cloud,acts as a thermal "blanket," keeping heat from escaping from the surface.
Practical Tips for Predicting theThreat of Roadway Icing
Applying some of the basic ideas describedabove, there is a great deal a motorist or those responsible for highwaymaintenance can do to predict the potential for roadway icing. Severalimportant points must be kept in mind:
1. Clear or nearly clear skies promote rapid coolingat night. If the skies are nearly cloud free or clearing rapidly, onemust expect rapid nighttime temperatures falls near and after sunset.Weather satellite pictures available on theWashingtonState Road Weather Web site, among other web locations, can show wherethe skies are clear and how cloudiness may change during the next few hours.
2. The temperatures at official weather observing sitesgenerally provide air temperatures at approximately 5 feet, at whichheight temperatures are generally warmer than road surface temperatureson cold, clear nights. Thus, if air temperatures drop below approximately37F icing conditions may already be occurring at the surface!State maintenance personnel and motorists can monitor air temperaturesat hundreds of locations throughout the state on the WashingtonState Road Weather Web site. This site provides real-time surface observationsintext and graphical form. Using this site it is easy to see if any observinglocations in your area have air temperatures below or near freezing, andplots of weather observations with time (called meteograms) let one evaluatethe temperature trends during the past few hours.
3. If air temperatures are below the mid-30's and fogis in the vicinity, extreme caution is prudent. Fog passing over a below-freezingroadway surface can deposit large amounts of ice quickly. Because fog andheavy frosts are most prevalent in wet or swampy areas, such as in thevicinity of river valleys, these areas should be approached with considerablecaution on cold nights. If the road is curvy as well, extreme cautionis called for.
4. Areas shaded from the sun can remain icy well intothe morning, and in some cases will not thaw out the entire day if daytimetemperatures remain in the 30s.
For Further Information:
If you have any further questions aboutthe weather conditions producing roadway icing or suggestions how thistutorial might be improved, please contact one of the University of Washingtonstaff working on the Washington State Department of Transportation RoadWeather Project:
Professor Clifford Mass, Department of Atmospheric Sciences,University of Washington
email: mass@atmos.washington.edu,(206) 685-0910
Mr. Richard Steed, Department of Atmospheric Sciences,University of Washington
email: steed@atmos.washington.edu
