The common perception that ground temperature closely matches air temperature is fundamentally inaccurate, especially when dealing with dark, dense materials like asphalt. Ambient air temperature is measured in the shade, representing the temperature of the surrounding atmosphere, not a surface exposed to direct sunlight. Asphalt absorbs and retains solar energy far more effectively than the surrounding air. This difference means that on a pleasant day, the pavement can become hot enough to pose a hazard.
The Surface Temperature on a 70°F Day
When the air temperature registers 70°F, the asphalt surface exposed to direct, midday sun is significantly hotter. Under sunny conditions, the pavement temperature typically reaches a range of 110°F to 130°F, demonstrating a difference of 40 to 60 degrees from the air temperature. This significant heat differential is a measurable effect of solar radiation striking a dark surface. For instance, studies have shown that when the ambient air is 77°F, asphalt can easily register a surface temperature of 125°F.
The Science Behind Asphalt’s Heat Absorption
Asphalt’s extreme temperature differential is rooted in its physical properties. It has a low albedo, meaning it reflects little solar radiation. Because asphalt is dark, it absorbs nearly all incoming shortwave solar energy, quickly converting this light energy into heat and raising the surface temperature rapidly. Asphalt also possesses a relatively low specific heat capacity compared to materials like water. This means asphalt heats up quickly once solar energy is absorbed, allowing it to store heat and slowly radiate the thermal energy back into the environment long after the ambient air has cooled.
Variables That Influence Asphalt Temperature
The surface temperature of asphalt is constantly influenced by dynamic environmental factors, explaining why the temperature on a 70°F day falls within a range. The angle of the sun and the time of day are primary factors, with the pavement typically reaching its peak temperature in the mid-afternoon due to the time required to absorb maximum solar energy. Wind speed is another modifying variable; higher speeds facilitate greater convective cooling, while a calm day allows heat to build up. Cloud cover significantly affects the surface temperature by blocking incoming solar radiation. Even moisture, such as recent rain, can slightly mitigate the surface temperature by providing an evaporative cooling effect.
Health Risks and Safety Thresholds
The high surface temperatures generated by asphalt pose direct health risks to humans and pets. For pets, walking on asphalt registering 125°F can cause damage to their paw pads within a minute. For human skin, the risk of injury occurs at even lower temperatures with prolonged contact. First-degree burns can begin to occur at surface temperatures of 111°F or higher. More severe second-degree burns can happen in a matter of seconds when the pavement reaches temperatures around 147°F. A simple practical method to check for safety is the five-second rule: if you cannot hold the back of your hand or bare foot on the pavement for five seconds, it is too hot for a pet’s paws. Walking pets during cooler hours or sticking to grassy areas can mitigate these risks.