Bridge surfaces often become icy long before surrounding roadways. This phenomenon, known as preferential icing, presents a safety concern for motorists. Understanding the scientific principles behind why bridge decks cool and freeze more rapidly than ground-level roads helps explain this difference.
The Role of Air Exposure
Bridge decks are exposed to cold air, which significantly accelerates their cooling. Unlike a typical road surface that only has its top exposed to the atmosphere, a bridge deck is elevated, allowing cold air to circulate both above and below the structure. This dual exposure means that heat can escape from the bridge’s surface and its underside simultaneously.
Heat loss from bridges occurs primarily through convection and radiation. Convection involves the transfer of heat by the movement of air, and with air flowing freely around all sides of a bridge, heat is continuously carried away. Additionally, the bridge radiates heat into the colder surrounding environment and the night sky, contributing to a more rapid temperature drop. Bridge materials like steel and concrete are effective conductors of heat, which allows any warmth within the bridge structure to quickly transfer to its surfaces and dissipate into the air.
Lack of Ground Insulation
Ground-level roadways benefit from the earth’s natural insulating properties, a significant advantage that bridges lack. The ground beneath a road acts as a thermal reservoir, storing heat from the sun during the day and from the earth’s interior. This stored heat is then slowly released, warming the road surface from below and helping to keep its temperature above freezing for a longer duration.
Bridges, being elevated structures, are completely isolated from this insulating and warming effect of the ground. Consequently, bridges cannot retain heat in the same way as ground-level roads, making them much more susceptible to rapid temperature drops when the ambient air cools. This absence of a thermal mass beneath the structure means bridges will quickly reflect the surrounding air temperature.
Evaporative Cooling and Wind Chill
Evaporative cooling and wind chill also contribute to bridges freezing before roads. When moisture, such as rain or melting snow, is present on the bridge surface, it can evaporate. This process of evaporation requires energy, which is drawn from the bridge surface itself in the form of latent heat, causing the surface to cool. This “evaporative cooling” effect can lower the temperature of the bridge deck even if the air temperature remains above freezing.
Elevated bridges are more exposed to wind than ground-level roads, which are often shielded by terrain or surrounding structures. Wind increases the rate at which heat is transferred away from the bridge surface through convection. This accelerated heat loss creates a “wind chill” effect on the bridge deck, making its surface temperature drop faster and potentially fall below freezing even when the air temperature is slightly above that point. While wind chill is often associated with how cold the air feels to skin, for inanimate objects like a bridge, it indicates a more rapid rate of heat removal, hastening ice formation.