Asphalt concrete, commonly known as blacktop, is the material used to pave most roads, driveways, and parking lots. Its dark color is highly efficient at absorbing solar energy, making it a powerful heat collector. Blacktop surfaces can reach extreme temperatures far exceeding the surrounding air temperature, creating a localized thermal hazard. This phenomenon is a significant consideration for public safety and urban planning, especially during the summer months.
The Core Temperature Range
The surface temperature of blacktop consistently rises much higher than the ambient air temperature due to its solar absorption properties. A general rule is that blacktop can be 40 to 60 degrees Fahrenheit (20°C to 35°C) hotter than the air above it. For example, when the air temperature is a moderate 77°F (25°C), the surface of asphalt exposed to direct sun can already climb to 125°F (51°C). On a hot day with an air temperature of 95°F (35°C), blacktop surfaces often exceed 140°F (60°C). This intense heating contrasts sharply with other common surfaces, as grass temperatures typically remain much closer to the air temperature. The difference in heat retention between blacktop and lighter surfaces highlights the severity of the temperature spike.
Factors Driving Surface Heat
The primary reason blacktop becomes so hot lies in its low albedo, which is a measure of a surface’s reflectivity. Traditional black asphalt has a low albedo, meaning it reflects very little sunlight and instead absorbs 80 to 95% of the incoming solar radiation. This absorbed energy is then converted into heat, which the surface retains due to the material’s thermal properties.
The intensity of solar radiation is the most significant variable, as direct, unimpeded sunlight causes the highest temperatures. Other environmental factors also play a part in determining the final surface temperature. Low wind speed reduces convective cooling, allowing the trapped heat to build up at the surface. Conversely, high humidity levels can slightly moderate the surface temperature by facilitating evaporative cooling.
Safety Implications of Extreme Heat
The extreme temperatures reached by blacktop pose a serious risk for both humans and animals. Surfaces exceeding 120°F (49°C) can cause damage to sensitive tissues, like human skin or pet paw pads, in as little as 60 seconds. This temperature threshold is often met even on days with relatively mild air temperatures.
The danger increases dramatically as temperatures climb higher, with surfaces reaching 140°F (60°C) or more capable of causing second-degree burns almost instantly. Pets are particularly vulnerable because their paw pads lack the protective layers of shoes and can quickly blister and crack from the thermal stress. The elderly, young children, and those with limited mobility are also at heightened risk, as they may be unable to quickly move away from the hot pavement.
Reducing Blacktop Heat
Several practical methods exist for mitigating the intense heat absorption of blacktop surfaces. The most effective approach involves increasing the surface’s albedo, or reflectivity, to bounce solar energy away. This can be achieved through the application of specialized surface sealants or coatings that are designed to be lighter in color.
Strategic shading also provides a non-material method for temperature reduction. Planting trees or installing structures to cast shade over the blacktop can significantly lower the surface temperature. Temporary cooling can be achieved by hosing down the surface with water, as evaporation will draw heat away, though this effect is short-lived. Modifying the asphalt mix itself with reflective aggregates or using permeable pavements can also reduce heat absorption and surface temperature.