The air temperature reported by a weather app rarely reflects the temperature of the surfaces we walk on during a summer day. Understanding how materials like concrete absorb and retain solar energy is important for summer safety. This is especially relevant for protecting children and pets, whose lower proximity to the ground exposes them directly to high surface temperatures.
The Difference Between Air and Surface Temperature
The primary reason concrete surfaces become far hotter than the surrounding air involves solar radiation and thermal mass. Air is largely transparent to the sun’s energy, meaning it does not absorb much incoming radiation. However, the solid, dense material of a concrete slab readily absorbs this radiant energy. Concrete possesses a high thermal mass, allowing it to absorb and store large amounts of heat energy. As the sun beats down, the concrete converts this energy into heat, causing its surface temperature to climb steadily. The air then heats up only secondarily through conduction from the hotter surface.
Estimated Concrete Temperatures
On a typical summer day with an air temperature of 90°F, the surface temperature of unshaded, standard concrete can easily be 40°F to 55°F higher. This places a sidewalk or driveway within a temperature range of 130°F to 145°F. This significant difference illustrates that the surface actively collects and retains solar energy at a rate much greater than the ambient air. This temperature increase shifts the thermal environment from merely hot to potentially hazardous.
Key Variables Affecting Surface Heat
The exact surface temperature is influenced by several environmental and material characteristics. The most significant factor is whether the surface is exposed to direct sunlight, as shade can keep the surface temperature close to the air temperature. Surface color also plays a large role; darker materials reflect less light and absorb more solar radiation.
While concrete is generally lighter than asphalt, small variations in its pigment affect energy absorption. Additionally, the material type determines the heat capacity; concrete remains significantly warmer than natural surfaces like grass. Humidity levels in the air influence heat transfer, though this effect is less pronounced than direct solar exposure.
Identifying Dangerous Heat Levels
The temperatures reached by concrete on a hot day are high enough to pose an immediate risk of injury to both humans and animals. Temperatures above 120°F cause discomfort and can lead to skin destruction with prolonged exposure. At 140°F, a temperature easily reached by concrete, it takes only about three to five seconds of contact to cause a first-degree burn on human skin. This rapid transfer of heat is especially dangerous because many people do not realize how hot the surface is until it is too late.
Pet safety is a major concern, as their paw pads are unprotected and constantly in contact with the hot surface. The “seven-second rule” is a practical guide: if you cannot comfortably hold the back of your hand on the concrete for seven seconds, the surface is too hot for a dog’s paws. Surfaces exceeding 125°F can cause burns on a dog’s paws in as little as 60 seconds. To protect pets, walks should be scheduled for the early morning or late evening when the surface has cooled, or they should be kept to natural, grassy areas.