Road Hazards during Winter Weather Conditions
By: Mark Bloomer
Snow and ice frequently results in slippery
road conditions during the wintertime.
Road surface slipperiness seems to be affected by a variety of factors,
mainly related to changes in precipitation type during a winter storm event and
changes in temperature before, during and after a winter storm. The most dangerous road conditions seem to
occur when water freezes on road surfaces.
This can either be in the form of freezing rain or in the form of frozen
precipitation that melts and refreezes.
Conditions that deteriorate rapidly or change quickly from one place to
the next can be especially hazardous since they may catch motorists my
surprise. The following are perhaps two
of the most extreme conditions that occurred in Northern Maine within the past five years. Each event resulted in hundreds of accidents.
Mid December 2000 brought an early winter
cold spell. For several days average
highs were only around 10 degrees and average lows were around 10 below. Road surfaces became very chilled, and the
low December sun angles did little to warm the surfaces. At the end of the cold spell a storm moved in
and brought an inch of fresh snow. The
snow changed over to rain resulting in slushy road conditions. A layer of ice formed beneath the slush as
the rain and melting snow refroze on very cold surfaces. The slipperiness of slush over freshly frozen
ice was extreme resulting in many cars sliding off the road. One of the factors that added to the extreme
slipperiness was the fact that this was an early season precipitation event and
there was little or no preexisting sand and salt on the roads.
Another extreme event occurred in late
February of 2003. Light snow was falling
and temperatures were near freezing.
Because temperatures were near freezing, a lot of the snow was melting
on road surfaces. An extremely strong
cold front moved through early in the morning.
Temperatures dropped from around 32 F down to around 0 F in just a
couple of hours while light snow continued to fall. The water on the roadways froze and light
snow quickly covered the fresh glaze of ice.
Blowing snow and a light hazy arctic snowfall reduced visibilities
considerably, and hundreds of accidents occurred on the roadways that morning.
Both of these extreme conditions occurred
when water on the roadways froze into a layer of ice. In each case the ice was hidden from view,
either by a top layer of slush as in the first case or by a top layer of snow
as in the second case. Being aware of
the temperature extremes in each case may have alerted travelers to the
potential for the presence of an icy layer.
In the first case, extreme cold preceded the event and in the second
case, extreme cold moved in during the event.
Drifting snow can also be especially
hazardous for travelers. Snow blowing
across the road can rapidly reduce visibility while adding snow to the
roadways. The combination of these two
frequently results in rear-ending type collisions in areas of drifting. Drifted snow is more densely packed than
fallen snow, and snow drifts that are deeper than the height of a vehicle above
the road can cause a vehicle to ride up onto the snow and become stranded as
its wheels are lifted off the road surface.
One of the most dangerous hazards resulting from drifting snow commonly
occurs in the late winter and very early spring. The higher sun angle later in the season
causes increased mixing in the atmosphere resulting in stronger wind
gusts. Likewise, the higher sun angle
begins to heat the roadway which causes snow blown onto the roads to melt upon
contact with road surfaces. Areas where
snow has been drifting over the roads and melting turn into dangerous patches
of ice as the cool dry wind refreezes the melted snow. These icy patches can catch motorists by
surprise since they can form in otherwise clear and dry road conditions.
In addition to freezing rain and refreezing
snow, sleet is typically more hazardous than plain snow. The density of sleet can cause tires to ride
up over the ice pellets and the ice pellets can behave like miniature ball
bearings causing the wheels to loose traction.
Road safety during the winter months involves
being aware of conditions including how the weather in recent days has affected
roads and how current weather changes are impacting road surfaces.
Freezing and refreezing conditions are the most hazardous and can occur
during a variety of situations. It
is advisable for both winter travelers and public works personnel to keep
in close touch with the weather and its changes during the winter months. The National Weather Service is here to help
keep everyone informed of the weather and its changes, and we wish everyone
a safe and comfortable winter season.
Weather Events Home
Maine-ly Weather Home