söndag 1 augusti 2010
BlackBody: Transformer of Radiation
The Earth as a blackbody transforms incoming high-frequency radiation from the Sun into outgoing low-frequency infrared radiation with total energy in = total energy out. The incoming spectrum peaks at 0.5 reflecting a Sun surface temperature of 5778 K, and the outgoing at 10 (mikrometer) corresponding to an Earth surface temperature of 288 K. As seen in the above figure, there is little overlap.
The amplitude of the incoming radiation is down-scaled compared to the blackbody spectrum at the Sun surface given by Stefan-Boltzmann's Law, depending on the ratio between the Sun radius and the distance from the Sun to the Earth, while the amplitude of the outgoing radiation from the Earth surface is directly given by Stefan-Boltzmann's Law.
In other words, the Earth acts as a transformer of radiation, transforming both frequency and amplitude of the Sun surface radiation. High-frequency low-amplitude radiation is transformed into low-frequency high-amplitude radiation.
The effect is in particular that the Earth emits more infrared radiation than what is coming in from the Sun. This effect does not come from any "backradiation" from the atmosphere, because the effect is there even without an atmosphere.
Does this mean that the atmosphere has no influence on the temperature of the Earth surface?
Of course not! As we have seen in previous posts, the atmosphere acts as a retailer of outgoing radiation between the Earth surface and empty space, and as such has an influence on the temperature/price at the Earth surface (assuming the atmosphere is transparent to incoming radiation). But there is no "backradiation" with the retailer sending goods back to the producer (because a sufficient reason is lacking).
A mathematical analysis of blackbody radiation transformation is presented in Computational Blackbody Radiation.
This posts responds to a comment by ScienceofDoom to the previous post AGW Myth of Backradiation, where the above figure is presented. Recall that "backradiation" leads into a fiction of "radiative forcing from greenhouse gases" which has no physical counterpart. The forcing is the incoming radiation from the Sun and nothing else.