The metal becomes hot in the sun because it absorbs the sun's light energy and converts it into heat, which raises its temperature.
Metals are composed of atoms that form a kind of regular grid (known as a crystal lattice). These atoms have electrons that they share freely among themselves: these are called free electrons. When the metal is heated by the sun, the atoms start to vibrate more and more intensely, and these free electrons allow heat to circulate very quickly throughout the material. The faster the atoms vibrate, the hotter the metal feels under your hand. It is this particular atomic structure that makes metal an excellent thermal conductor.
In metals, heat primarily travels through free electrons. Essentially, these electrons are not bound to a specific atom but move freely from atom to atom. When a part of the metal heats up in the sun, the atoms start to vibrate more, and these mobile electrons help transfer this thermal agitation quickly to other parts of the metal. This is why a metal bar gets hot very quickly, even if only one end is exposed to the sun. The more free and available electrons a metal has, the better it is at conducting heat.
The surface of a metal often behaves like a mirror, reflecting a significant portion of the sunlight that strikes it. However, a part of the solar radiation that is not reflected is directly absorbed by the electrons present at the surface of the metal. When these electrons capture the sun's energy, they become more agitated and vibrate more. This agitation in turn generates heat inside the material, gradually increasing its temperature. Some dark-colored metals absorb solar radiation even better, which explains their tendency to heat up more in full sunlight compared to those with light or polished surfaces.
Metal heats up much faster than other materials commonly used around us (such as wood, plastic, or stone). Why? Because it has what is called a very high thermal conductivity: as soon as it absorbs a tiny bit of thermal energy, it quickly distributes the heat throughout its mass. In contrast, a piece of wood or plastic tends to conduct this heat slowly; therefore, the exposed surface can get hot without the entire mass becoming burning hot quickly. Furthermore, many metals have a shiny surface that reflects some of the solar radiation, but the amount that is absorbed is more than enough to cause a rapid and noticeable increase in temperature. This is why if you touch a metal car parked in the sun, it will feel hotter, more quickly, than a wooden bench in the same spot exposed to exactly the same light intensity.
The temperature of a metal exposed to the sun varies with several external conditions, including the presence of wind. Even a very light wind removes heat to the air around the metal, which cools it down. If the wind stops, the metal heats up faster. Ambient humidity also plays a role: humid air conducts heat better than dry air, so it can cool hot surfaces more effectively. Conversely, if the metal is located next to other hot objects, such as a concrete slab or a dark wall, it will warm up more through indirect radiation. A final essential factor is altitude. At higher elevations, due to the cooler and more ventilated air, the metal generally heats up a little less quickly than at lower altitudes, even though solar radiation is more intense there.
A white car can remain up to 15 degrees Celsius cooler under intense sunlight than a black car, thanks to the superior reflection of solar radiation by light-colored surfaces.
Some metals, like aluminum, reflect up to 95% of the solar radiation that hits them, which is why they are often used in the composition of emergency blankets.
The smoother and more polished the surface of a metal, the lower the amount of heat it will absorb compared to a dark and dull surface.
The metal feels colder to the touch than wood at the same temperature because it conducts heat away from our skin more quickly, creating a misleading mechanical sensation of cold.
Although they are all subject to heating, certain types of reflective metal surfaces, such as polished aluminum, remain relatively cooler by reflecting the majority of incident solar rays rather than fully absorbing them.
Yes, metal exposed to the sun can reach temperatures high enough to cause skin burns, especially with prolonged exposure. It is advisable to carefully check the temperature of metal surfaces before handling them, for example by bringing your hand close without touching them directly.
Thanks to their high thermal conductivity, metals quickly transfer their heat to the cooler environment when placed in the shade. This characteristic allows them to warm up quickly in the sun but also to cool down quickly in the absence of a direct heat source.
Dark colors and matte textures absorb solar rays better and heat up more compared to shiny and reflective surfaces, which reflect a significant portion of solar radiation, thus limiting the increase in temperature.
It mainly depends on their ability to absorb solar radiation, known as thermal emissivity, and their thermal conductivity. Dark or matte metals absorb light better and retain more heat than shiny metals, which reflect part of the radiation.
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