gray body
1
9.9
- noun
- /ɡreɪ ˈbɑdi/
- Specialized
:
cuerpo gris
- When heated, a gray body emits radiation at a level lower than that of a black body at the same temperature.
Examples
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Our atmosphere is a grey body -- in that there is a small amount of black-body type absorption and emission.
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So without any GHGs but with an atmosphere, the earth will radiate as before—as a grey body.
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We then end up with a warming gray body.
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And temperature will stabilize when gray body radiation out = radiation in.
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If we now add the other GHGs into the atmosphere, then the grey body radiation is reduced and temperature rises.
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At some point, the grey body radiation out will equal the incoming radiation, and we have temperature "stasis."
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That would put air's gray body radiation at about two magnitudes less than something like water.
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Given the emissivity of helium, it will have a gray body radiation spectrum appropriate to its temperature.
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The Earth is a "grey" body radiator, and if the atmosphere were suddenly lost, radiation away would be determined by the various emissivities of the different terrain.
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Scientists often use the concept of a gray body to explain how different materials lose heat through radiation.
How Emissive
- Specialized
2
8.5
- Specialized
1
9.9
Surface Forms
Morphology
Although the words 'gray' and 'body' have clear literal meanings, a B1 learner who knows those constituents would most likely interpret 'gray body' as an object that is gray in color rather than a technical radiative concept. The specialized meaning about emission at every wavelength and comparison to a 'black body' is not derivable from the parts and requires prior exposure, so the expression is opaque.
Etymology
Gray body comes from a simple picture: imagine a warm object that is not black but gray, so it only soaks up and gives off some light and heat instead of everything. So a gray body means an object that gives off 'less radiation' than a 'black body' but in the same steady way for all colours.