Asphalt, a common material in modern infrastructure, forms the foundation of roads, parking lots, and paved surfaces. While durable, asphalt absorbs significant solar radiation, leading to elevated surface temperatures. Understanding this thermal behavior is important for mitigating its environmental and practical implications.
Why Asphalt Absorbs So Much Heat
Asphalt’s capacity for heat absorption stems from its dark color and material composition. Bitumen, the dark black binding agent derived from petroleum distillation, contributes to this darkness. This dark pigmentation allows asphalt to absorb almost all wavelengths of the light spectrum. A material’s albedo quantifies absorption efficiency by measuring the fraction of solar radiation reflected. Asphalt typically exhibits a low albedo, around 0.1, reflecting only about 10% of incident solar energy while absorbing most. In contrast, concrete has a higher albedo of 0.4. The absorbed solar energy converts into thermal energy, increasing the asphalt’s surface temperature. Asphalt also possesses a high emissivity, approximately 0.93, efficiently radiating this absorbed heat back into the environment.
How Hot Asphalt Can Get
Asphalt surfaces can reach temperatures far exceeding ambient air temperature, particularly under direct sunlight. On days when the air temperature is around 38°C (100°F), asphalt can easily surpass 65°C (149°F). This significant difference occurs because asphalt absorbs and stores solar energy. Peak pavement temperatures typically occur in the afternoon as solar radiation accumulates. In Phoenix, Arizona, conventional asphalt has been recorded at surface temperatures up to 66.7°C (152°F) at midday. Under extreme conditions, asphalt in direct sunlight can reach 82°C (180°F). Another study documented asphalt reaching 74.4°C (166°F) when the surrounding air temperature was 48.9°C (120°F). Generally, asphalt surface temperatures can be as much as 24°C higher than the air temperature. The maximum temperature of asphalt is influenced by several factors, including ambient air temperature, the intensity and duration of direct sunlight, and meteorological conditions such as wind speed and humidity. Newer, darker asphalt absorbs more heat than older, weathered surfaces, which tend to lighten over time.
Consequences of Extreme Pavement Temperatures
Hot asphalt surfaces present various practical implications and potential hazards. For pedestrians, especially those barefoot, asphalt temperatures exceeding 60°C (140°F) pose a burn risk. Exposure for as little as one second to pavement at 70°C (158°F) can cause skin burns. Pets are also vulnerable, with their paw pads susceptible to burns from hot asphalt. Elevated temperatures also affect the asphalt material itself. High temperatures soften asphalt, increasing its susceptibility to rutting and plastic deformation from heavy vehicles. This can reduce the evenness of the pavement and affect its service life. Beyond direct contact and material integrity, extensive areas of hot asphalt contribute to the Urban Heat Island (UHI) effect. This phenomenon causes urban areas to be noticeably warmer than surrounding rural areas, with temperature differences potentially ranging from 0.5°C to 1.5°C. The stored heat in asphalt is released into the atmosphere, particularly at night, which further exacerbates urban warming. This increased urban temperature leads to higher energy consumption for cooling buildings and can contribute to air pollution.
Cooling Down Asphalt Surfaces
Efforts to mitigate asphalt’s heat absorption have led to the development of “cool pavement” technologies. These strategies aim to reduce the urban heat island effect by increasing the pavement’s reflectivity or enhancing its evaporative cooling capabilities. One approach involves using reflective coatings or lighter-colored materials and aggregates. These reflective pavements work by increasing the albedo, reflecting more sunlight rather than absorbing it, thus lowering the surface temperature during the day. Another method employs permeable pavements, such as porous asphalt or permeable concrete. These materials allow water to penetrate and be retained within their structure, which then evaporates, cooling the pavement and the surrounding air. Simple, yet effective, strategies also include increasing shade cover over paved areas through the strategic planting of trees or installation of structures. Additionally, spraying water onto asphalt surfaces can provide a temporary cooling effect.