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#16
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Sunburn [was Clivia/Kaffir Lily]
"Nick Maclaren" wrote in message ... [snip] [On reradiation] No, what I mean is surface heating. This is the effect by which the surface of an object can become much hotter than either the body of the object or the air temperature. Which of the objects under the glass will receive more reradiated heat from neighbouring bodies than others? Are you remembering that the body you have chosen to receive reradiated heat is itself also reradiating? Surely as time passes, all the objects in the enclosure will try to achieve the same temperature? One point is that glass reflects long (far) wavelength infrared well, That is not true. It absorbs infrared radiation. [snip] Franz Franz |
#17
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Sunburn [was Clivia/Kaffir Lily]
[ sci.bio.botany restored, in case anyone more knowledgable can comment. ] In article , "Franz Heymann" writes: | | Yes. I was referring to the known effect by which glass 'magnifies' | the strength of sunlight, as it affects plants. I believe that it | a reradiation effect. | | What does that mean? | The intensity of the UV per unit wavelength increment is almost | negligible compared to that in the yellow-green region of the spectrum | of sunlight reaching the surface of the earth. If I understand | correctly what you mean by reradiation, namely absorption of UV and | reradiating at a longer wavelength, then the absorbed UV will be so | littlle that it will not resulet in a measurable increase in the | intensity of the botanically active frequencies. Why did you think that I meant ultraviolet? I didn't. But, on that topic, ultraviolet is as effective at damaging plant cells as it is at damaging animal ones, and plants that grow in high ultraviolet locations have developed protection mechanisms. No, what I mean is surface heating. This is the effect by which the surface of an object can become much hotter than either the body of the object or the air temperature. One point is that glass reflects long (far) wavelength infrared well, though it transmits short (near), and that causes the greenhouse effect, but you can get it even with materials that transmit uniformly. What can happen is that an object under glass can receive the direct radiation, and a proportion of the reflected radiation from ALL of the objects under the glass (i.e. a focussing effect). This does not have to be a precise focus to double or even triple the total radiation it is receiving, and explains why the exact location is an important factor. Now, it might appear that this would raise the temperature of the leaf as a whole, but it is not necessarily so. Transpiration will keep the leaf cool, just as sweating does for humans, but that will not stop the surface cells between the pores from getting very hot. It is quite possible that a significant amount of human sunburn (under dry conditions, when sweat evaporates rapidly) is due to this, rather than purely to ultraviolet. Now, I have no PROOF of the above, but it is the only explanation that I can think of that matches the properties of the effect that I know about, and of course the biology and physics. Regards, Nick Maclaren. |
#18
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Sunburn [was Clivia/Kaffir Lily]
"Nick Maclaren" wrote in message ... In article , Franz Heymann wrote: [On reradiation] No, what I mean is surface heating. This is the effect by which the surface of an object can become much hotter than either the body of the object or the air temperature. Which of the objects under the glass will receive more reradiated heat from neighbouring bodies than others? Those that are an approximation to a focus of the glass structure. There is no such thing as "focus of the glass structure". Each light ray exits from the glass at the same angle as that a which it entered. At worst, it might be displaced sideways parallel to its original trajectory by a millimetre or two. The intensity distribution is then essentially the same as it would have been if there had been no glass. Are you remembering that the body you have chosen to receive reradiated heat is itself also reradiating? Of course. Surely as time passes, all the objects in the enclosure will try to achieve the same temperature? Er, no. That is FAR too simplistic a model. Err, no. Remember that there is an external source of energy, and therefore the most elementary steady state calculations do not apply. I know that. I am almost right. The short term temperature of each body in it will depend essentially only on its albedo. The various plant leaves will have very nearly equal albedos. One point is that glass reflects long (far) wavelength infrared well, That is not true. It absorbs infrared radiation. Please go and look it up. I suggest you do that. Glass absorbs infrared quite strongly, which is why infrared lenses have to be made of rather unusual materials, many of which are in fact black as far as visible light is concerned. The greenhouse effect is precisely that the short wavelength infrared emitted by the sun is transmitted, but the long infrared emitted by the earth is reflected. No. You misunderstand the greenhouse effect quite seriously. Yes, they are both absorbed, too, but everything is relative. You seem to be unaware of the fact that there is a relationxsship between the reflection coefficient and the absorption coefficient of any optical medium. A good absorber is a bad reflector, and glass is a very good absorber of infrared radiation. The greenhouse effect arises in fact because the glass absorbs essentially all the reradiated infrared quite close to the inner surface of the glass, whose temperature rises as a consequence. Most of this heat is returned to the enclosed volume by convection and reradiation. Franz |
#19
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Sunburn [was Clivia/Kaffir Lily]
"Nick Maclaren" wrote in message ... In article , "Franz Heymann" writes: | | There is no such thing as "focus of the glass structure". Each light | ray exits from the glass at the same angle as that a which it entered. | At worst, it might be displaced sideways parallel to its original | trajectory by a millimetre or two. The intensity distribution is then | essentially the same as it would have been if there had been no glass. I am talking about reradiation from inside and, perhaps even more importantly, other glass. Consider a greenhouse built like: --- / x \ A plant at point 'x' is, in some sense, at the focus of the structure. No. The intensity of reradiation by the glass which is received at any point inside the greenhouse is proportional to the solid angle subtended by the glass at that point. The value of this does not vary all that much from point to point, and it is very small in comparison with the incident radiation. Radiation from the glass plays a minor role compared to convective heat transfer. Remember that the temperature of the glass is only a fraction of a percent different from that of any other object in the greenhouse. See below. | Surely as time passes, all the objects in the enclosure will try to | achieve the same temperature? | | Er, no. That is FAR too simplistic a model. | | Err, no. I suggest that you take a few max./min. thermometers, calibrate them against each other, and place them at various parts of a greenhouse (properly shaded from direct sunlight). They won't all show the same values. Of course not, to the extent that warm air is lighter than cooler air. I have actually tried it, and there is not much to choose from point to point, except for a small increase in temperature with height above the floor | Remember that there | is an external source of energy, and therefore the most elementary | steady state calculations do not apply. | | I know that. I am almost right. The short term temperature of each | body in it will depend essentially only on its albedo. The various | plant leaves will have very nearly equal albedos. Er, no. You have forgotten convection and evaporation. Those can vary just as much as the albedo. Touche. I was considering only radiation effects. That was obviously wrong. | I suggest you do that. Glass absorbs infrared quite strongly, which | is why infrared lenses have to be made of rather unusual materials, | many of which are in fact black as far as visible light is concerned. I know that. It isn't the point, which is the RELATIVE transmission of near and far infrared. You have lost me there. | You seem to be unaware of the fact that there is a relationxsship | between the reflection coefficient and the absorption coefficient of | any optical medium. A good absorber is a bad reflector, and glass is | a very good absorber of infrared radiation. In physics, as in life, things are rarely in black and white. While what you say is true, it does NOT have an albedo of one for infrared radiation. I did not say, or imply that. | The greenhouse effect arises in fact because the glass absorbs | essentially all the reradiated infrared quite close to the inner | surface of the glass, whose temperature rises as a consequence. Most | of this heat is returned to the enclosed volume by convection and | reradiation. Ah. Cross-purposes. Yes, that is so. I was referring to the 'greenhouse effect', where reflection is more important. That is not correct. The greenhouse gases *absorb* the infrared radiation, they reflect little of it. The heat exchange effect is still primarily a convective mechanism. Do think of the fact that the temperature of the greenhouse gases will be less than around 90% of the temperature at the surface of the earth. Consider what that means in terms of relative amounts of radiated power per unit area. You might spare a moment to have a read at the Wikipedia entry under "Greenhouse effect". It is possible that the "plants burning under glass" effect is more due to reradiation from hot glass that reflection of the reradiated infrared from plants. Not so much of this "hot glass" lark! The temperature of the glass is unlikely to be more than a degree or two above the ambient temperature in the greenhouse. I hadn't thought of that one. In particular, it makes the 'focus of the structure' even more important. No. There is no "focus of the structure". I have spent a couple of hours playing with a shielded minimax thermometer in a greenhouse. There were no noticeable hot spots anywhere in it. There was only a gentle increase of temperature with distance, amounting to about 2 deg. C between floor highest point. Franz |
#20
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Sunburn [was Clivia/Kaffir Lily]
"Nick Maclaren" wrote in message ... [snip] [On reradiation] No, what I mean is surface heating. This is the effect by which the surface of an object can become much hotter than either the body of the object or the air temperature. Which of the objects under the glass will receive more reradiated heat from neighbouring bodies than others? Are you remembering that the body you have chosen to receive reradiated heat is itself also reradiating? Surely as time passes, all the objects in the enclosure will try to achieve the same temperature? One point is that glass reflects long (far) wavelength infrared well, That is not true. It absorbs infrared radiation. [snip] Franz Franz |
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