Roadway Icing and Weather: A Tutorial

    Icing on roadways is probably the most serious meteorological hazard faced by Washington State citizens and causes hundreds of serious injuries and several deaths a year.  Sometimes called "black ice" when not clearly evident at night, roadway ice is not black at all, but is made up of frozen water that reflects light or sparkles when illuminated. This tutorial will describe a variety of weather conditions that can result in roadway icing and how one can determine when roadway icing is a threat.  Some important meteorological principles that control surface icing will be reviewed as well.

How can roadway ice form?

To get ice on a roadway requires freezing temperatures (below 32F) and moisture (water) at the surface, a combination that can occur in a number of ways:

  • 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 road surface.
  • Freezing rain.
  • More than one of these icing mechanisms can occur at the same time!  Let us consider these icing processes one at a time.


        Frost tends to occur on cold, relatively clear 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 infrared radiation.  The warmer the object the more infrared radiation it emits.  We all have some experience with infrared radiation; for example, when you sit across a room from a fire you can feel the infrared radiation it emits.  Some objects emit infrared radiation better than others.  For example, the earth's surface is far more efficient in emitting radiation than the gases in the atmosphere.  Clouds are very good at emitting and 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. Therefore the surface and the air near it cool quickly.   Away from the cold surface, the air tends to be warmer.  Thus, on cold, clear nights temperature actually warms with height--the opposite of the normal situation!  Temperature increasing with height is called an inversion.  On overcast nights, clouds act like meteorological blankets that slow or prevent the loss of infrared radiation (heat) from the surface, and thus cooling is far less.  Blankets keep us warm by slowing the loss of heat from our bodies--clouds work similarly, except that they reduce the amount of heat leaving the surface.  Strong winds also tend to work against surface cooling since windy conditions "stir up" the atmosphere and mix some 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 condenses into water droplets or ice crystals. The amount of water vapor air can "hold" varies with temperature, with warmer air having the ability to hold more water vapor.  If we cool air down sufficiently (to the dew point temperature), it can no longer hold the moisture it started with, forcing the water vapor to condense out into water droplets or ice crystals.  Dew point temperature (or dew point as it is often called) is reported at many weather observing stations.

        During the day when the sun is out and is warming the surface, the air temperatures near the surface  are usually above the dew point temperature, and water in the atmosphere remains in the form of invisible vapor.  However, as the sun sets on cold, clear nights, the surface temperature plummets (as the earth radiates heat into space), and the air near the surface can cool to the dew point temperature. If the roadway temperature and dew point temperature are above freezing, liquid water forms on the surface (dew), but if the temperature is below freezing, frost forms instead.

        A moist atmosphere, with lots of water vapor, encourages frost since it can supply more water for freezing.  Such moist conditions are often found after a period of precipitation or in wet locations, such as near swampy areas or rivers.  Lots of water vapor also raises the dew point temperature, since with more water vapor in 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 mixing brings warmer, drier air from above down to the surface.  Frost is often more prevalent in valleys and low areas, into which cold air tends to drain and pool (more details on this below), and where wind speeds are generally less.

        Frost generally accumulates slowly and rarely accumulates more than 1/16 of an inch. For that reason, frost can 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 occurs on relatively clear nights when winds are light. Strong winds work against frost formation. As noted below, most weather stations report temperatures at about 5 feet, where temperatures can be considerable warmer than at the surface.  So if the sky is relatively cloud-free and nighttime air temperatures are dropping towards the mid 30's, frost may be a real threat.

    Fog and Icing

        Although the icing threat from frost is 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. Fog contains large amounts of liquid water, and if a fog bank passes over a roadway that has cooled to a temperature below freezing, icing can be rapid and severe, with a thick coat of ice being deposited in minutes.

        A number of serious icing accidents have occurred in Washington State as a result of fog-related icing. A typical scenario 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 form over a moist surface. The fog drifts over the road, and as it passes over the road surface a thick coating of ice is deposited. Thus, both motorists and road maintenance crews must be extra vigilant when fog forms on cold evenings when temperature drops below freezing at the surface or the mid-30s in the air immediately above.

    Freezing of Roadside Melted Snow and Groundwater Seepage

        Dangerous icing conditions frequently occur when road surface temperatures are above freezing during the day, but fall below freezing at night. Even if the road is snow free, snow is often found along its sides. This is particularly true of roads that are actively plowed, with piles of snow adjacent to the open lanes. The roadside snow melts during the day, particularly in those areas adjacent to the relatively warm road. (Roads, especially dark blacktop roads, readily absorb heat from the sun.) The melt-water runs over the road during the day, and then freezes at night, particularly if the sky has few clouds.

        A similar situation can occur without snow, 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 freezes on the road surface. Thus, it is important for road maintenance personnel to be familiar with such wet road areas, and to check them frequently on cold nights when air temperatures drop towards the mid 30s.

    Refreezing of Melted Snow on the Road

        A particularly dangerous type of icing occurs early in the winter season or after a period of warm weather. At such 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 the warm road surface. If the air temperature continues to fall rapidly (perhaps after the passage of the arctic front from the north or a push of cold eastern Washington air into the passes), warming from the road surface and the warm ground below is overwhelmed by the cold air above and the slush mixture turns to ice.

    Freezing Rain

        Freezing rain or drizzle is relatively rare over Washington State, being most prevalent in the Columbia Gorge area, the passes, near Bellingham, and in parts of eastern Washington. Freezing rain occur when there is a layer of below-freezing air near the surface with warmer (above freezing) air aloft. Rain from aloft falls into the cold layer and gets cooled to temperatures below freezing-and remains liquid! Amazingly, water doesn't necessarily freeze  immediately in the atmosphere when temperatures are below 32 F-such subfreezing water is termed "supercooled." When such supercooled rain hits the surface, it freezes immediately into a clear glaze ice. Such freezing rain often results in ice storms that can make travel treacherous and that down trees and powerlines.

    The Columbia Gorge area can experience freezing rain when it is filled with cold, westward-moving low level flow originating in eastern Washington and Oregon. Warm rain from an incoming Pacific weather system is cooled below freezing as it travels through the cold Gorge air and then freezes on contact with the surface. The mountain passes can get freezing rain when cold air from eastern Washington is drawn into the passes when the atmospheric pressure is higher in eastern Washington than in western Washington. As a relatively warm Pacific weather system approaches the coast, warm rain falls into the cold air resident in the mountain passes, causing freezing rain. After the Pacific system passes through the area the temperatures decrease aloft and thus precipitation generally changes over to snow.  The area near Bellingham is also at risk for freezing rain when cold air from the interior of British Columbia passes through the 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 warm Pacific system approaches the State at this time, freezing rain can spread over Bellingham and vicinity.  Finally, eastern Washington occasionally experiences freezing rain. Eastern Washington is really a topographic basin, in which cold air frequently pools. As warm, rain-bearing systems pass through the region above cold air trapped near the surface, freezing rain can be a serious threat.

        The Puget Sound region generally escapes freezing rain and resultant icy road conditions, but it can happen. For example, a severe freezing rain case struck the southern Puget Sound area during 26 December 1996, resulting in hundreds of accidents, the closure of Sea-Tac Airport for days, and numerous power outages.

    The Effects of Local Terrain and Land Use on Icing

        Cold air is denser and heavier than warm 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. The cool surface then cools the air above. As the air cools, it becomes denser and heavier and tends to sink to lower elevations. Thus, valleys or the lower portions of slopes tend to be cool compared to adjacent regions of higher elevation. Changes in temperature with elevation can be quite large in such cases, with even shallow valleys (100-200 feet deep) being 2-5F cooler than higher regions only tens or hundreds of feet away.

        Surface temperatures are also warmer in 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. Water surfaces also tend to be much warmer than land on cold winter nights, and thus freezing is far less likely near large bodies of water such as Lake Washington, Puget Sound, or the Pacific Ocean.

        Variations in temperature due to local terrain 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 vulnerable to freezing. These complex, terrain-related variations in temperature are often called "thermal signatures" and can be measured with instrumented cars or using remote sensing on aircraft.

    Why Bridges Often Ice Up First and
          Why Doesn't the First Snow Of the Season Often Doesn't Stick or Produce Ice?

        The temperature of a road surface is impacted by a number of factors such as radiational cooling to space and the amount of heat coming up from the ground below---to name only few. Temperature changes at the surface take days or weeks to extend more than few inches into ground, and at night the roadway surface is often cooler than the ground underneath. Heat conducted from below the road surface lessens nighttime temperature falls, and thus reduces the potential for icing. Bridges have air (a good insulator) underneath them and thus do not receive heat from the ground below.  Thus, bridges are much more vulnerable to roadway icing at night compared to normal road surfaces, particularly early in the winter when the ground is relatively warm.  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, preventing icing even when air temperatures fall below freezing. This works only up to a point, since very cold air temperatures eventually 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 solid as much colder air moves into the region.

    Surface Temperature and Air Temperature Observations: How Are They Related?

        Temperature measurements are generally taken with thermometers in a sheltered enclosure at about 5 feet above the ground, usually above a vegetated surface. It is absolutely crucial to understand that the "official" temperatures are reported by thermometers located at around 5 feet whose readings can be very different from road surface temperatures. On clear nights when winds are light, the surface radiates heat to space much more effectively than the air above. On such nights, temperature at ground level can be 2-5F cooler than air temperature only a few feet above. Thus, frost can be occurring at the surface even when official temperature observations are reporting temperatures of 35 to 37F. During the day, the opposite situation can occur, with the road surface several degrees warmer than the air temperature at 5 feet. Air temperature readings on trucks and cars are similarly problematic...they often are colder (night) or warmer (day) that the road surface. The moral of this story is that motorists and maintenance personnel must be wary of icing as air temperatures drop below around 37F.

    The Effects of Trees and Other Objects Near and Above a Roadway

        Shading of road surfaces by trees, hills, and other objects greatly influence the potential for, and longevity of, roadway ice. At night, overhanging trees or other road covers can lessen the 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, if an area does frost up or get covered with ice, shading due to trees or hillsides can delay melting well into the late morning or allow ice to remain all day. A number of fatal accidents have occurred on State roadways when drivers hit unexpected areas of ice protected by shade. Areas shaded by hillsides can start to cool rapidly hours before sunset, resulting in icing before dark. Such icing was associated with a recent fatal accident on Interstate 90 near the town of Thorp.

    How Temperature Changes the Slipperiness of Ice

        The slipperiness of ice changes with 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 on the outside--even when temperatures are below freezing.  This layer of liquid water is what makes ice slippery.  The thin veneer of liquid water on the outside of ice is thickest when the temperature of the ice approaches freezing, and thins substantially when temperatures cool well below 32F.  Thus, ice is most slippery when temperatures are near freezing (25-32F) and is much less slippery when temperatures reach the single digits and below.  So if the air temperature is just below freezing and ice is on the roadway, extra care is warranted.

    Practical Tips for Predicting the Threat of Roadway Icing

        Applying some of the basic ideas described above, there is a great deal a motorist or those responsible for highway maintenance can do to predict the potential for roadway icing. Several important points must be kept in mind:

    1. Clear or nearly clear skies promote rapid cooling at night. If the skies are nearly cloud free or clearing rapidly, one must expect rapid nighttime temperatures falls near and after sunset.  Weather satellite pictures available on the Washington State Road Weather Web site, among other web locations, can show where the skies are clear and how cloudiness may change during the next few hours.

    2. The temperatures at official weather observing sites generally provide air temperatures at  approximately 5 feet, at which height temperatures are generally warmer than road surface temperatures on cold, clear nights. Thus, if air temperatures drop below approximately 37F  icing conditions may already be occurring at the surface! State maintenance personnel and motorists can monitor air temperatures at hundreds of locations throughout the state on the Washington State Road Weather Web site. This site provides real-time surface observations in text and graphical form. Using this site it is easy to see if any observing locations in your area have air temperatures below or near freezing, and plots of weather observations with time (called meteograms) let one evaluate the temperature trends during the past few hours.

    3. If air temperatures are below the mid-30's and fog is in the vicinity, extreme caution is prudent. Fog passing over a below-freezing roadway surface can deposit large amounts of ice quickly. Because fog and heavy frosts are most prevalent in wet or swampy areas, such as in the vicinity of river valleys, these areas should be approached with considerable caution on cold nights.  If the road is curvy as well, extreme caution is called for.

    4. Areas shaded from the sun can remain icy well into the morning, and in some cases will not thaw out the entire day if daytime temperatures remain in the 30s.

    For Further Information:

        If you have any further questions about the weather conditions producing roadway icing or suggestions how this tutorial might be improved, please contact one of the University of Washington staff working on the Washington State Department of Transportation Road Weather Project:

    Professor Clifford Mass, Department of Atmospheric Sciences, University of Washington
    email:, (206) 685-0910

    Mr. Richard Steed, Department of Atmospheric Sciences, University of Washington