Walking past a street on a sunny day and feeling a wave of intense heat rising from the pavement is a common experience. Asphalt surfaces frequently become much hotter than the surrounding air, sometimes reaching dangerously high temperatures. This phenomenon is a direct consequence of the physical properties and composition of the material used to create roads and parking lots. Understanding why asphalt absorbs and retains so much solar energy requires looking closely at the specific thermal mechanics involved.
Albedo and the Absorption of Solar Energy
The primary reason asphalt pavement grows hot is due to a property called albedo, which measures the fraction of solar radiation a surface reflects. Surfaces with a high albedo, like snow or light-colored concrete, reflect most sunlight, staying relatively cool. Conversely, surfaces with a low albedo absorb most of the incoming solar energy.
Asphalt is a dark-colored material composed of mineral aggregates bound together by black, petroleum-based bitumen. This dark composition gives standard asphalt a very low albedo, typically ranging from about 0.10 to 0.15. This low number signifies that the pavement absorbs between 85% and 90% of the sunlight hitting its surface. The absorbed energy is converted directly into heat, causing a rapid and significant rise in the material’s surface temperature. This high absorptivity is why asphalt can be tens of degrees hotter than the ambient air temperature during peak daylight hours.
How Thermal Properties Lead to High Temperatures
Once solar energy is absorbed, two other thermal properties of asphalt govern how quickly its temperature rises and how that heat is distributed. The first is specific heat capacity, which is the amount of energy required to raise the temperature of a material mass by one degree. Asphaltic concrete has a relatively low specific heat capacity, often falling in the range of 879 to 963 Joules per kilogram per degree Celsius.
Compared to water, which has a specific heat capacity over four times greater, asphalt requires much less absorbed energy to increase its temperature significantly. This low capacity means the pavement heats up rapidly under solar load. The pavement’s temperature can skyrocket within a short period after the sun begins to shine directly upon it.
The second property is thermal conductivity, which describes how efficiently heat moves through a material. Asphalt possesses relatively high thermal conductivity, with reported values for asphaltic concrete ranging from approximately 0.74 to 2.88 Watts per meter per Kelvin. This high conductivity ensures that the heat absorbed at the surface is quickly transferred deeper into the pavement structure, creating a large reservoir of stored thermal energy.
This efficient transfer also allows the stored heat to be readily conducted to the layer of air immediately above the pavement surface. The combination of low specific heat capacity and high thermal conductivity means asphalt both heats up quickly and effectively distributes that heat into its surroundings.
Asphalt’s Contribution to the Urban Heat Island
The widespread use of asphalt and other impervious materials in cities creates the Urban Heat Island (UHI) effect. This effect describes how urban areas experience significantly higher air temperatures compared to the surrounding rural environments. Vast expanses of pavement absorb and store enormous amounts of solar radiation throughout the day due to their low albedo and high thermal conductivity.
The impervious nature of most asphalt pavements prevents cooling that would otherwise occur through the evaporation of water, a process common in natural, vegetated landscapes. This lack of evaporative cooling further contributes to the buildup of heat in the urban environment. The stored heat is contained within the large thermal mass of the pavement structure.
As the sun sets, the pavement begins to re-radiate this stored energy back into the atmosphere in the form of long-wave radiation, a process measured by its high emissivity, which is around 0.94. This slow release of heat continues well into the evening, preventing the air from cooling down substantially after dark, maintaining elevated nighttime temperatures, a defining characteristic of the UHI effect.