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Consider a shiny block of metal, and a dull block of concrete, both in direct sunlight. The metal will reflect more of the sunlight than the concrete, due to its reflective surface. However, when touching the two blocks, the metal will feel hotter. How is it that the metal is absorbing more energy from the sun, even though it is reflecting more back?

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  • $\begingroup$ Metal is not a perfect blackbody. Hence, it doesn't reflect every single photon that hits it. It does absorbe some amount (check Photoelectric effect) $\endgroup$ – Yashas Jul 31 '16 at 14:36
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    $\begingroup$ 'Feeling' hotter means the metal has higher thermal conduction. In equilibrium with the sunlight, both bodies will be at the same temperature. $\endgroup$ – Jon Custer Jul 31 '16 at 15:07
  • $\begingroup$ Absorbing more than emitting in an non-equilibrium property. But as Jon Custer said, in equilibrium the temperatures are equal. Again, when touching, that is an non-equilibrium transport problem, but this time, of thermal kind instead of radiation mediated. $\endgroup$ – Mikael Kuisma Jul 31 '16 at 15:14
  • $\begingroup$ One has to understand that temperature 'doesn't care' about mechanism of transport. As long as any means of transport exists, the temperature will equilibrate. It might take longer for the reflecting surface to heat up (again non-equilibrium property), but in the end, it will gain the same temperature as in equilibrium. $\endgroup$ – Mikael Kuisma Jul 31 '16 at 15:17
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The feeling depends on three factors:

  • Obviously the temperature
  • the specific heat
  • the thermal conductivity

What happens is that once your finger is on the material, heat is exchanged. The molecules at the surface of your finger are warming up, the molecules at the surface of the material are cooling down until both materials have the same temperature.

Now stone has a smaller specific heat, in fact six times less than water (the main material your finger consists of). The contact area of your finger and the material are obviously the same. But that means for every degree your finger molecules are warmed the stone must lose six degrees, so even if we start with the same temperature, water will always feel hotter. It also explains why water burns are so dangerous: Water can transmit much more heat to your body until the temperature comes down to safe levels.

The second thing is thermal conductivity. I can in fact touch the hot surface of a tile stove without burning myself. Why ? The thing is that heat is continously replenished or dissipated from the surrounding molecules of the contact surface, but the speed depends on conductivity. Touching a 200°C tile will cool it down from my 37°C finger to somewhat 70°C (remember the 1:6 factor), but stone has abysmal conductivity, so the stone molecules heat is not replenished, but my finger's are dissipated. Not so fast, but still more than the stone, so the resulting temperature will be something like 50-55 °C. (In fact, those purists which use cooled "whiskey stones" because they do not want water dilution by ice are deluding themselves; the cubes are not working). Metals on the other hand have a very good conductivity, so energy is easily replenished. I can therefore touch a very, very thin metal plate, but touching a 200°C thick plate with the same temperature as the stone tile will result in severe burns. So metal feels hotter (and colder) than other materials.

The last important thing is temperature. The more temperature, the hotter, but the equilibrium temperature of materials in sun under the same conditions depends on their color: Black materials have higher surface temperatures than bright materials. Unpolished metal should have higher reflectance than concrete and should be cooler, but the two other factors are dominant.

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