Quote:
Originally Posted by barx1963
Here's a question for physics people out there. I have had a discussion at work today about whether the temperature in side a greenhouse is hotter than the outside temperature. My work colleague argued that the temp inside would never exceed the highest ambient outside temp on any one day, and that they only retain heat to keep the inside temp steadier.
I argued that they actually retain heat and so end up warmer than the outside temp and even exceed the peak temp on any particular day. I cannot find any documentation to support either point so I thought better refer it to the panel of experts that is IIS!! 
I am certain I am right as the same effect should apply to cars and I know I have heard that cars can exceed 50deg C on some days, but again I have no documentation to prove it.
Plus my protagonist in all this is an annoying Kiwi and I need to prove him wrong!! 
Malcolm |
Malcolm,
Your friend is right... if the greenhouse wasn't a green house but an insulated masonry building.
Glass has the property of being transparent to visible wavelengths of light but not to infra-red wavelength (i.e. heat). The greenhouse affect works by allowing visible light through, which is absorbed by the matter inside the greenhouse. When light is absorbed it is converted to heat.
Heat from the matter inside the greenhouse is then re-radiated as infra-red, but the infra-red cannot escape directly. Some is reflected inside the greenhouse, and some is absorbed by the glass. Of the infra-red light (heat) absorbed by the glass, Some is re-emitted inside the greenhouse, and some is re-emitted outside the greenhouse. The amount emitted inside and outside depends on the difference in temperature on the inside and outside surfaces of the glass and the thermal conductivity of the glass, but it will always be more on the hot side.
Keep in mind also that the peak intensity of the solar spectrum is in the yellow/green part of the visible spectrum. Also the energy of each photon is proportional to its frequency, so a lot of energy falls in the small visible band of the spectrum, but the infra-red band is wider.
Al.