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I need to understand some basic concepts about the cooling characteristics of a black "polymer"-and-zinc-alloy handgun (specifically, this one).

If this gun was left on the ground under daytime sunlight, I assume that both the ground and the gun would get warmer. I assume, also, that after the sun went down, both the ground and the gun would begin to cool. My questions are:

1) would the gun warm under sunlight to a higher temperature than the ground, because it is black and/or because of what it's made of?

2) after sundown, would the gun cool at the same rate as the ground? Or, would the rate of cooling be different?

Edit: I should clarify that the surrounding ground is essentially brown dirt/mud.

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  • $\begingroup$ See "Planck's Law" and "Black Body Radiation" $\endgroup$ – Carl Witthoft Feb 16 '17 at 16:06
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There are three main heat transfer mechanisms: conductive, convective, and radiative.

Conductive transfer means direct transfer from one item to another it's in contact with. The transfer rate depends on the empirically determined thermal conductivity of both materials.

Convective refers to heat transfer between an object and a moving fluid such as wind. This depends heavily on the total surface area available (such as fins on a radiator).

Radiative means absorption or emission of photons. The energy transfer rate depends on the surface properties, called "emissivity" or equivalently "absorptivity" and is wavelength-dependent for any given material.

OK, so with that in mind, yes a black object generally will absorb more sunlight than a lighter shade of ground/dirt. This is a generality, as the absorptivity in the infrared may be different than in the visible. At the same time, the conductive heat loss/gain to the ground depends on the total contact area, and further will be much greater if the ground is damp, because water is good at conducting a lot of heat 'fast.' After dark, the radiative processes are sort of reversed. It's not an exact reversal because the spectral radiation rates depend on the material AND on the temperature of the object. (actually the radiation rates apply all the time; it's just that in daylight each object absorbs more light than it radiates until it reaches a thermal equilibrium).

In sum, the black object probably will get cold faster, and get colder than the ground. As a practical example, come to New England in the late Fall. On cool nights, black stuff such as roof shingles often frost over when other things like white walls don't, because of radiative cooling.

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It would likely be a higher temperature than the ground, as it would be heated by the suns radiation, and black generally absorbs better (if the surface finish were too smooth it may change this). The material may also feel warmer than the ground, even if they were at the same temperature. This relates to the conductivity of the materials and how much heat they would let off into your hand.

The rates of cooling would be very different. The black of the gun will also make it emit it's radiated heat faster once the surroundings drop below the surface temperature due to radiation. It will also cool down a lot faster than the ground because it has a much smaller mass, and higher relative surface area. The ground is a big thermal mass which slowly gains and loses heat. A small object generally gains and loses heat fast.

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  • $\begingroup$ Sorry, I need clarification: you wrote that the black gun will cool down a lot slower than the ground; but, that small objects generally lose heat fast. Did you mean that the black gun will cool down a lot faster than the ground? Or am I completely misunderstanding? $\endgroup$ – protasm Feb 16 '17 at 16:51
  • $\begingroup$ Meant faster, sorry. Sometimes it gets confusing comparing rates and forgetting which one you're referring to so I'll make small mistakes like that. Thankfully it was also apparent to you why it didn't make sense. $\endgroup$ – JMac Feb 16 '17 at 17:00

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