Asphalt is significantly hotter than concrete when exposed to direct sunlight. This temperature difference is driven by the fundamental material properties of the two pavement types, specifically how they interact with solar radiation and store thermal energy. This substantial thermal gap affects everything from road longevity to the urban climate. Understanding the composition and thermal behavior of these ubiquitous materials explains why one becomes a searing heat source while the other remains comparatively temperate.
The Direct Answer and Measurement
Under typical summer conditions, asphalt surfaces consistently reach higher temperatures than adjacent concrete surfaces. While ambient air temperature might hover around 90 degrees Fahrenheit, asphalt can easily surge to surface temperatures between 140 and 170 degrees Fahrenheit. This means the blacktop can be 50 to 80 degrees hotter than the surrounding air and approximately 10 to 25 degrees Fahrenheit hotter than a nearby concrete slab. Scientists quantify these surface temperatures using specialized tools like infrared thermometers, which measure thermal radiation remotely. Researchers may also embed thermocouples within the pavement layers to track the temperature profile and heat transfer throughout the material’s depth.
Albedo: How Surface Color Determines Heat Gain
The primary reason for asphalt’s higher temperature is albedo, which describes a surface’s ability to reflect solar radiation. Albedo is measured on a scale from 0 (perfect absorber) to 1 (perfect reflector). Dark-colored materials like asphalt possess a low albedo because the black bitumen binder readily absorbs incoming solar energy.
New asphalt typically has an albedo value ranging from 0.05 to 0.10, meaning it absorbs 90 to 95 percent of the sun’s energy. This high absorption rate converts light energy directly into heat, leading to rapid surface warming. The dark color is effective at capturing the entire spectrum of solar radiation, including visible and near-infrared light.
In contrast, concrete is a light-colored material, often gray or white, giving it a much higher albedo. New gray-cement concrete can have an albedo value between 0.35 and 0.40, reflecting over one-third of the sun’s energy back into the atmosphere. This reflection prevents a significant portion of solar radiation from being converted into thermal energy within the material. The difference in surface color is the primary determinant of initial heat gain during the day.
Thermal Properties and Heat Retention
Beyond solar absorption, the materials’ thermal properties dictate how they store and release absorbed heat. Specific heat capacity and emissivity determine how long the pavement remains hot. Specific heat capacity refers to the amount of energy required to raise the temperature of a given mass of the substance.
Asphalt and concrete exhibit different thermal masses, affecting their ability to hold onto heat after sunset. The dense composition of both materials allows them to store substantial amounts of absorbed heat. The high thermal conductivity of these dense pavements means that heat rapidly transfers into the subsurface layers.
Emissivity is the material’s efficiency in radiating stored energy back out as long-wave infrared radiation. Asphalt has high heat retention, meaning it continues to emit warmth long after sunset, despite its high emissivity. This stored energy keeps the pavement hot throughout the evening, contributing to warmer nighttime temperatures in urban areas.
The Role in Urban Heat Islands and Health
The vast areas of paved surfaces, especially those made of asphalt, contribute significantly to the Urban Heat Island (UHI) effect. This phenomenon causes metropolitan areas to be noticeably warmer than surrounding rural landscapes. The extreme heat absorbed and slowly released by pavements elevates the ambient air temperature, particularly overnight, with differences sometimes reaching up to 22 degrees Fahrenheit.
This heat retention has direct public health consequences, particularly during summer heat waves. When pavement temperatures reach 140 degrees Fahrenheit, they pose a burn risk to exposed skin. Surfaces can cause second-degree burns in just a few seconds of contact, posing a danger to small children or pets.