Many people misunderstand when snow sticks to roads, often assuming air temperature provides the answer. This misconception can lead to dangerous surprises, as roads become slick even when the air temperature is slightly above freezing. Snow accumulation is governed by physics, and the true determinant is the temperature of the pavement itself, which often differs significantly from the surrounding air.
The Critical Factor: Road Surface Temperature
The most important measurement for determining snow accumulation is the road surface temperature (RST), not the air temperature. Snow only sticks and accumulates when the pavement temperature is at or below 32°F (0°C). If the RST is above this threshold, the snow melts immediately upon contact, forming a layer of water that prevents accumulation.
Thermal lag explains why roads can freeze even when the air temperature is 35°F (1.7°C). Road surfaces, particularly bridges and overpasses, cool faster than the surrounding air due to radiative cooling. Conversely, stored heat within the pavement and ground can keep a road warm, melting snow even if the air temperature drops below freezing. Drivers relying solely on their car’s ambient air temperature gauge may be misinformed about the actual road conditions.
The Physics of Accumulation and Melting
Whether snow melts or sticks depends on the energy exchange between the snowflake and the pavement. The principle governing this is the latent heat of fusion, which is the energy required to change water from a solid to a liquid state. For snow to melt, the road surface must supply this energy to the snowflake.
The latent heat of fusion is significant; the energy needed to melt ice is 160 times the energy required to raise its temperature by one degree Celsius. When the pavement is 33°F, it quickly transfers enough thermal energy to melt the falling snow, resulting in runoff. When the road temperature drops to 32°F (0°C) or below, the pavement acts as a heat sink, allowing the snow crystal to retain its solid form and bond with the surface. This energy balance dictates the accumulation rate.
Environmental Factors Affecting Road Temperature
The road surface temperature is constantly modified by several environmental variables. Solar radiation is a warming force, as dark asphalt absorbs radiation even on cloudy days, potentially keeping the pavement above freezing. Conversely, wind speed accelerates the cooling of the road surface through convection, quickly dropping the RST to the sticking point.
The type of pavement also plays a part, as the density and material affect how much heat is retained and released. Concrete and asphalt have differing thermal properties, altering their ability to store and conduct heat from the ground beneath. Residual heat from the earth or high traffic volume can also temporarily warm the pavement, pushing the effective sticking threshold slightly lower than 32°F.
Driving Safety and Snow Accumulation
Translating road temperature science into practical safety knowledge is essential for winter driving. The first surfaces to accumulate snow are typically bridges, overpasses, and shaded areas. These structures are exposed to cold air on both the top and bottom, lacking the residual ground heat that keeps regular roadways warmer. This double exposure causes them to cool and freeze faster than surrounding roads, leading to a sudden change in traction.
The nature of the snow accumulation also affects driving safety. Dry, powdery snow is easier to manage than wet, heavy snow. Wet accumulation occurs when the road surface is just below freezing, resulting in slippery slush or, if temperatures drop further, the formation of black ice. Drivers should pay extra attention to the road surface, slow down before entering elevated or shaded sections, and assume colder temperatures in these exposed areas.