wind can be a problem when leaving overpasses

So what SHOULD you do during severe weather when out on the roads?

Pull over and take shelter in any sturdy building that you can find. If you CANNOT avoid a tornado while out on the road, exit your car and lie in the nearest low ditch away from cars and trees.

Always remember to pay attention to the weather before you head out on the roads. You can always stay updated with the forecast on WQAD’s weather page.

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Roadway Icing and Weather:

Roadway Icing:  The Most Serious Weather Hazard in Washington State

     Ice on roadways is probably the most serious meteorological hazard faced by Washington State citizens and causes hundreds of serious injuries and several tragic deaths a year (click to view some recent media stories on ice-related accidents).  For example, in 1996, for the region including the Cascade passes and eastern Washington, roughly 25% of the total accidents were related to ice on the road.  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 reflects light or sparkles when illuminated.

Reducing the Threat of Ice-Related Accidents

    Roadway icing occurs under conditions that are generally well understood and often predictable.  Thus, a little knowledge, coupled with access to current weather information, can help motorists and transportation personnel to evaluate the threat and be prepared for ice formation.  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.  Key points to be reviewed include:

• Be prepared for icing when skies are clear or nearly clear and air temperatures fall below the upper 30s.
• Be wary of low areas and valleys, since cold air tends to pool in such areas.
• If temperatures are near freezing and fog is in the vicinity, heavy icing is possible.

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

 

 

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

Frost

    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 call 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.  Furthermore, wet roads often freeze up very rapidly when the air above is dry.  The reason for this is cooling from evaporation.  We all experience the chill of evaporative cooling when we exit from a shower or bath–this cooling is greatest when the air around us is dry.  Thus on a cold, dry night evaporation from a wet road can cause the surface to cool much more rapidly that dry road surfaces nearby, resulting in localized icing.  Thus, for all of these reasons, it is important for road maintenance personnel to be familiar with wet road areas, and to check them frequently on cold nights when air temperatures drop towards the mid 30s.  The fact that wet areas are often in low areas where cold air tends to pool, makes then doubly dangerous.