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Winter Road Conditions – for Better or Worse

 

January 14, 2017

Most people consider winter weather forecasting to be difficult, but I think that predicting the winter road conditions is even more difficult.  That prediction requires not only an accurate weather forecast – but a lot of additional information.  Weather conditions are somewhat predictable with respect to timing and intensity, but many other factors — for better or worse — combine to create particular road conditions.  These factors are often subtle and not easily foreseen.

First, I want to emphasize so strongly the fact that visibility is always impacted significantly by falling and accumulating snow.  Road conditions become only a secondary problem when the road is hard to even see from your windshield!  Even if visibility is a couple miles from your window at home, visibility on the road will often be much more limited.  Windy conditions will make falling snow even more of a visibility hindrance.  I have found that if snow is NOT falling, winds need to be unusually strong and combined with very fluffy or powdery snow cover for a significant visibility issue to exist.

Now to the factors which affect the road surface

The first factor will be how the road has been treated — or not.  Interstate highways and major streets within cities are often pre-treated to prevent snow and ice from bonding to the pavement.  This is often effective in delaying the onset of slippery conditions – at least briefly.  The next level of maintenance is provided for state or US highways which get more attention during a storm than county roads and side streets.  More frequent attention from road crews does not always guarantee safer travel conditions.

That is because the second big factor involves traffic conditions.  Well-traveled roads have significant visibility issues during dry, fluffy snowfalls because the snow is lifted into a cloud by passing traffic.  Steady traffic also tends to favor the formation of black ice when temperatures are well below freezing as salt losing its melting power and hundreds of tires are packing and polishing the snow which falls or blows onto the road.

A third big impact comes from the road surface temperature.  The closer that surface is to 32 degrees, the more icing trouble can result.   Some of the worst icing occurs when rain or snow and cold air arrive quickly after a mild spell warms the roadway.  Falling snow may melt on contact with the warm ground and then refreeze due to the cold air, especially in late afternoon and early evening.  The opposite pattern (below freezing ground surfaces and warmer air) also is hazardous.  In certain situations (such as seen with this weekend’s Midwest Ice Storm) warmer air moving in aloft allows freezing rain to occur.  In the central US, you’ll often see highway signs warning of bridges icing first.  This graphic from the Kansas Turnpike Twitter feed explains why this is so.

Bridge Freezes Before Road

When the ground is unfrozen, but air temperatures are below the freezing mark, bridges will be the first part of the road to turn icy.

Farther north, where subfreezing temperatures are the rule in winter, the ground is cold enough that bridges often don’t turn icy any faster than other parts of the roadway.  Here in the northern US even without precipitation, roads and bridges may turn slick as a cold spell is ending.  This happens when more humid air arrives in the low levels and causes frost to form on the roadway.

Less-anticipated troubles may arise on a windy day when the sun comes out.  The impact of an asphalt roadway warmed by the sun to near or above the freezing mark can be large – and not always favorable.  That is especially true when loose snow blows onto the road, sticks and turns to slush or ice as depicted in the picture below.  If temperatures have been well below 32 degrees for some time, this blowing snow may occur hours or even days after the snow has fallen.  It also catches even local drivers unaware because the location of the icing is highly dependent on wind direction.   Short stretches can have glare ice while long stretches of road stay dry.

Slush created by blowing snow on a sunny afternoon.

Slushy roadway near Marshall, MN created by strong southwest winds on the afternoon of January 15th, 2017 (click on image to view at full size).

Wind speed and direction also play an important role during a snow event.  Snow will tend to collect on the roadway if the wind blows along the direction of the road.  Snow will blow across the road if wind is blowing at right angles to the direction of the road.  In this latter instance, much of the road may be clear of snow, except for snow collecting in sheltered areas and drifts in open areas encroaching onto the windward-facing lane.  Because drifted snow is almost always more dense than fresh, undisturbed snow, it should be treated as a hazard.  Hitting a drift at high speed can feel like hitting a solid obstacle.

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