El color externo de un vehículo no afecta a la cantidad de calor que acumula cuando se expone al sol, según comprobó el año pasado Sanford Klein, del Laboratorio de Energía Solar de la Universidad de Wisconsin Madison. Los coches se calientan debido al efecto invernadero: la luz solar que atraviesa la ventana es absorbida por las superficies del interior, y reflejada de vuelta al aire en forma de calor.
El aire interior de un coche se calienta más que el exterior que porque no se le permite la convección. El aire pegado al techo exterior sí la tiene y se puede observar a simple vista por diferencia del índice de refracción del aire muy caliente. Pero parte de la radiación se emite al interior, que lo calienta y no puede cederlo. Por los cristales, es cierto que entra también una cantidad importante de radiación que es absorbida por los materiales del interior, que al no ser reflectantes, se calientan y por contacto, el aire del interior.
Pero no olvidemos lo más importe, la temperatura que debería alcanzar un coche al sol debería de ser de más de 120ºC.
El efecto invernadero en un vehículo es ridículo. Para ello, observen que le pasa a un vehículo a la sombra. A penas está un poco más caliente que el aire de los alrededores. Eso sí es efecto invernadero.
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It is not difficult to measure the ground-air temperature differences, and get at least a qualitative impression of what goes on in a “real” greenhouse. We make 7 wells; 10 cm diameter and 4 cm deep, in a piece of EPS (Expanded Polystyrene), fix a temperature sensor on - Ordered List Item the center of each bottom, and cover some of the wells with windows that have a very different IR transparency. We lay a wet cloth on the bottom of half the wells. We make 7 temperature measurements every half minute, log the results, and see what happens night & day.
Red: 2 mm PMMA cover, dry bottom 1
Blue: 2 mm PMMA cover, wet bottom 2
Orange: 6 µ PE cover, dry bottom 3
Light blue: 6 µ PE cover, wet bottom 4
Green: No cover, dry bottom 5
Brown: No cover, wet bottom 6
Violet: Aluminum mirror above well, not closed 7
The experiment begins the 10th of May, 2008, on an open lawn at Fiwihex, Wierdensestraat 74 in Almelo. The sky is clear, it has been a sunny day, and the following day is sunny too. Quite un-Dutch weather. The experiment ends the 11th 13:00, because my daughter in law wanted to mow the grass by then.
Now, what do we see: After equilibration from installation, 21:00 in the evening, the PMMA [Perspex, or poly methyl methacrylate] cover, red and dark blue points, being black in the relevant IR wavelengths, keeps the well relatively warm. It does not matter if the bottom is wet or dry, however, because the temperature is so low, and the humidity so high outside and inside, that the wet-bulb temperature is equal to the dry bulb temperature.
We see also that, notwithstanding the greenhouse effect of the PMMA cover, the open dry well is slightly warmer that the PMMA greenhouse, and much warmer that the PE greenhouse. The warmest is the well with the mirror above it. How come?
This is, because the air, being composed of mainly a-polar gases like O2 and N2, and cannot radiate as strongly as a solid body, stays warm longer than any radiating solid or liquid. So, a sensor that is in open contact with the air, stays warmer too.
Now the coldest spot is the PE covered little greenhouse, wet or dry, because PE is transparent for IR radiation, so we have there a connection with the cold heavens, and insulation from the warm air by the cover. The well with the Al mirror is the opposite: here we have contact with the air, but the upward radiation is reflected back by the shiny metal. The difference is large; the PE greenhouse is about 10 °C colder than the mirror well.
Now the day comes, and we see a radically different pattern:
All covered closed greenhouses are very hot. We see a slight difference between the hottest, PMMA, 61 °C in the early afternoon, and the PE covered, 57 °C. The only well which is substantially less warm is the open well with the wet bottom. This well is able to cool itself by evaporation. We see also that a wet canopy, as every greenhouse gardener knows, lowers the temperature quite a bit in a greenhouse, because it is never completely closed, so the water vapor from the plants finds its way outside. In the blue case, the water was almost completely evaporated by 13:00, dry spots begin at 12:00 already.
Conclusion: what makes the surface, or climate, warm is the hindered convective heat transfer to up in the atmosphere. Not the hindered radiation heat transfer, this is much smaller.
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Conclusión: Lo que hace que una superficie o clima, se calientes es el obstaculizado de la transferencia de calor convectivo hacia arriba en la atmósfera.
Comentarios
Creo que es repe: ¿Se calienta más al sol un coche negro que uno blanco?
¿Se calienta más al sol un coche negro que uno bla...
muyinteresante.esEs falso lo que se indica.
El aire interior de un coche se calienta más que el exterior que porque no se le permite la convección. El aire pegado al techo exterior sí la tiene y se puede observar a simple vista por diferencia del índice de refracción del aire muy caliente. Pero parte de la radiación se emite al interior, que lo calienta y no puede cederlo. Por los cristales, es cierto que entra también una cantidad importante de radiación que es absorbida por los materiales del interior, que al no ser reflectantes, se calientan y por contacto, el aire del interior.
Pero no olvidemos lo más importe, la temperatura que debería alcanzar un coche al sol debería de ser de más de 120ºC.
El efecto invernadero en un vehículo es ridículo. Para ello, observen que le pasa a un vehículo a la sombra. A penas está un poco más caliente que el aire de los alrededores. Eso sí es efecto invernadero.
http://landshape.org/dokuwiki/doku.php?id=introduction
_______________________________________________________________
It is not difficult to measure the ground-air temperature differences, and get at least a qualitative impression of what goes on in a “real” greenhouse. We make 7 wells; 10 cm diameter and 4 cm deep, in a piece of EPS (Expanded Polystyrene), fix a temperature sensor on - Ordered List Item the center of each bottom, and cover some of the wells with windows that have a very different IR transparency. We lay a wet cloth on the bottom of half the wells. We make 7 temperature measurements every half minute, log the results, and see what happens night & day.
Color cover, mirror, wet or dry, # of well
Red: 2 mm PMMA cover, dry bottom 1
Blue: 2 mm PMMA cover, wet bottom 2
Orange: 6 µ PE cover, dry bottom 3
Light blue: 6 µ PE cover, wet bottom 4
Green: No cover, dry bottom 5
Brown: No cover, wet bottom 6
Violet: Aluminum mirror above well, not closed 7
The experiment begins the 10th of May, 2008, on an open lawn at Fiwihex, Wierdensestraat 74 in Almelo. The sky is clear, it has been a sunny day, and the following day is sunny too. Quite un-Dutch weather. The experiment ends the 11th 13:00, because my daughter in law wanted to mow the grass by then.
Now, what do we see: After equilibration from installation, 21:00 in the evening, the PMMA [Perspex, or poly methyl methacrylate] cover, red and dark blue points, being black in the relevant IR wavelengths, keeps the well relatively warm. It does not matter if the bottom is wet or dry, however, because the temperature is so low, and the humidity so high outside and inside, that the wet-bulb temperature is equal to the dry bulb temperature.
We see also that, notwithstanding the greenhouse effect of the PMMA cover, the open dry well is slightly warmer that the PMMA greenhouse, and much warmer that the PE greenhouse. The warmest is the well with the mirror above it. How come?
This is, because the air, being composed of mainly a-polar gases like O2 and N2, and cannot radiate as strongly as a solid body, stays warm longer than any radiating solid or liquid. So, a sensor that is in open contact with the air, stays warmer too.
Now the coldest spot is the PE covered little greenhouse, wet or dry, because PE is transparent for IR radiation, so we have there a connection with the cold heavens, and insulation from the warm air by the cover. The well with the Al mirror is the opposite: here we have contact with the air, but the upward radiation is reflected back by the shiny metal. The difference is large; the PE greenhouse is about 10 °C colder than the mirror well.
Now the day comes, and we see a radically different pattern:
All covered closed greenhouses are very hot. We see a slight difference between the hottest, PMMA, 61 °C in the early afternoon, and the PE covered, 57 °C. The only well which is substantially less warm is the open well with the wet bottom. This well is able to cool itself by evaporation. We see also that a wet canopy, as every greenhouse gardener knows, lowers the temperature quite a bit in a greenhouse, because it is never completely closed, so the water vapor from the plants finds its way outside. In the blue case, the water was almost completely evaporated by 13:00, dry spots begin at 12:00 already.
Conclusion: what makes the surface, or climate, warm is the hindered convective heat transfer to up in the atmosphere. Not the hindered radiation heat transfer, this is much smaller.
_______________________________________________________________
Conclusión: Lo que hace que una superficie o clima, se calientes es el obstaculizado de la transferencia de calor convectivo hacia arriba en la atmósfera.