Why can a bridge surface freeze first?
During winter, when temperatures drop significantly, it is common to observe bridges covered in a thin layer of frost.
But, the interesting thing is that while the bridges may be completely frozen, the adjacent roads remain comparatively unaffected.
This perplexing phenomenon occurs due to a combination of distinct factors that are particular to bridges.
And that is when people ask, “Why do bridges freeze before roads?”
The likelihood of a bridge surface freezing first is influenced by various factors such as structural design, material composition, and exposure to the surrounding environment.
Do Bridges Freeze Before Roads?
Yes. Bridges freeze before roads because even in icy, cold weather conditions not all surfaces freeze at the same time.
Bridges always freeze before roads and wet bridges will freeze faster than dry.
You might have noticed the effect and taken advantage of clear ice-free sidewalks and avoided the snow- or ice-covered grass verges that run alongside them.
The same sort of thing happens with roadways and bridges.
It has to do with:
- How the different construction materials hold onto heat
- Its location
- What is underneath
Even if the bridge looks clear, there could be multiple patches of ice to add to the hazards of winter driving.
Why Can a Bridge Surface Freeze First?
Bridges freeze before roads because they span gaps.
They are positioned where they are exposed to the worst of the weather on all sides.
Roads are nowhere near as exposed. Unlike bridges with four faces, a road has only two and lies flat so only one surface is exposed.
Here are some factors explaining why bridges may freeze first:
Exposure to Cold Air from Above and Below
Since bridges are essentially suspended in the air, almost every part of the structure is exposed to the weather.
In winter, freezing winds pass over and below a bridge making them among the first of the winter driving hazards road users will face as temperatures fall towards zero.
Bridges freezing before roads are well established and it happens for a variety of reasons.
Even where pockets within the structure are more sheltered with nothing underneath the span, there is barely any protection from freezing rain, sleet or snow.
And because bridges are designed with a wide span that has minimal weight, there is little chance of trapping heat.
Consequently, bridges lose heat from all sides, although when and at what rate heat is lost will vary along and around the structure.
Fact: With no way to trap heat, bridges freeze almost at once when the air temperature reaches zero.
Limited Insulation and Ground Heat Transfer
Bridges lose heat to the atmosphere faster than roads because of the way they are constructed and the materials they are made from.
Bridges have a much greater surface area than roads. It means the heat is lost from all the faces of the exposed surfaces.
Roadways are typically made of black asphalt, a natural insulator that keeps the ground underneath it warmer.
Compared to a bridge, a road’s heat loss happens at a significantly slower rate even when the bridge and the road are at roughly the same location.
An Important Consideration
Unlike asphalt, metal and concrete bridges tend to be made of heat conductors. Whatever heat the structure gets, it finds it impossible to hang on to.
Fact: With most being high up and over water, bridges do not benefit from ground heat transfer and this contributes to a bridge surface freezing quickly.
Absence of Natural Ground Heat Sources
Many bridges are built to span rivers, creeks and bodies of water and have no access to significant natural ground heat sources.
But, the further away from the land you go, the greater the influence of the water and this includes its falling surface temperatures.
Around bridges, ice forms on the water’s surface around the piers and footings and under the width of the span where water is colder slow moving and shallow.
The growing collection of cold spots underneath it increases the likelihood of the bridge freezing.
How Does It Happen?
As heat is lost from the water and into the atmosphere and air temperatures fall to zero and below, water attempts to freeze.
But it does it against its currents and pockets of differing surface temperatures. Therefore, it freezes unevenly although ice always forms moving out from the shores.
This process creates cold spots that cool air around and above it adding to the falling ground-level temperatures.
Wind Speed and Its Impact on Heat Transfer
Large horizontal structures such as bridges, are designed and built to be flexible to limit their vulnerability to winds.
The strength of the structure depends on it being able to resist potentially damaging wind effects such as:
- Galloping
- Vortex excitation
- The flutter phenomena
The wind speeds also impact the efficiency of its heat-conducting materials, concrete and metal.
As cold air moves in, the bridge and the air are at different temperatures.
The bridge structures and the air immediately around it are warmer than the ambient air.
As the cold air currents develop around and between the elements of the structure, the cold air draws more of the heat into it.
The makes the wind blow faster and increases the likelihood of bridges freezing first.
Fact: Generally, the larger the temperature difference to start with, the faster the winds will be and with wind chill, the bridges are likely to feel colder.
Role of Wind Chill Factor
The wind chill is the temperature as felt on the skin so it is influenced by humidity and wind speed.
It is a measure developed to help understand how the human body loses heat on cold windy days. Wind chill also accounts for why bridges freeze first.
With such a huge amount of exposed surface area, the winds blowing over, under and through the structure takeaway huge amounts of the heat in the same way that a cold wind chills a human.
Since the bridge cannot store and hold on to the heat the way asphalt can, its ambient temperatures drop rapidly.
Shadowing Effect and Reduced Sunlight Exposure
Once ice starts forming it is not always instantly noticeable although heat will rise from within the bridge’s construction materials.
It means they will be cold at the surface resulting in the water vapor precipitating out of the cold air and freezing.
The process begins in colder parts of the bridge’s structure where the shadowing effect and reduced exposure to the sun means that part of the bridge was colder, to begin with.
It happens in patches along the road surface.
Fact: Black ice forms as the surface reacts unevenly to the colder temperatures but any areas in shadow will freeze first.
Increased Air Circulation Under Bridges
Primarily, the circulating air under bridges causes them to freeze, with temperature, pressure and conditions all linked.
Air currents are disrupted and trapped by the bridge’s structure, boxed in by its pylons and the nearby terrain. They become chaotic and unpredictable.
As temperatures drop, the bridge is continuously losing a proportional amount of heat but with its footings already in icy water is left with no way to replace it.
Temperatures fall noticeably quickly, and as a result, wind speeds increase as does the pressure gradient, so the circulating winds blow harder and cool the down bridge even faster.
Impact of Traffic Patterns and Vehicle Exhaust
Bridges close because of atmospheric icing. It cuts off communities, interrupts traffic flow and has implications for business and industry.
Ice also impacts the safety of the public. Lately, there have been more reports of ice falling from bridge cables onto morning traffic.
With global warming already influencing the severity of the weather around bridges, scientists have studied the impact of vehicle exhaust fumes.
It is already suspected that particulate matter expelled into the atmosphere gets deposited locally.
In the higher concentrations on a bridge, they contaminate the natural processes as the compounds interact with each other and the atmosphere.
Takeaway
Why can a bridge surface freeze first? The reason why bridge surfaces tend to freeze before regular roads can be attributed to various factors that distinguish them from one another.
Bridges are particularly vulnerable to heat loss and freezing temperatures due to their elevated position, lack of insulation, and exposed nature.
It is essential for engineers, maintenance crews, and drivers to comprehend these factors.
This understanding emphasizes the importance of implementing specialized measures to reduce the risk of icy conditions on bridges and guarantee safe passage for everyone.
