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Be very, very careful what you put into that head, because you will never, ever get it out. Thomas Cardinal Wolsey (1471-1530) |
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Bad Greenhouse FAQ | |
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Click on the symbol for its explanation. |
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Preamble
This FAQ (Frequently Asked Questions) is written by Alistair B. Fraser. It is in response to questions posed over the years by readers of the Bad Meteorology pages. If you have arived on this page without having read those pages or the other Bad Science pages, then what follows, will probably make little sense.Issues discussed below (arising out of the Bad Greenhouse page)
Although the questions presented here are often ones asked by a specific person, each is chosen to characterize a group of similar questions which have been asked about the topic.
Temperature conversion
Questions arising out of Bad Greenhouse:
Temperature conversion
I just wished to point out a small mistake in the Celsius - Fahrenheit conversion in the Bad Greenhouse page: 50 Fahrenheit degrees should be [equal to] 10 Celsius degrees (not 30).
What the page said was, "In the absence of an atmosphere the earth would average about 30 degrees Celsius (about 50 degrees Fahrenheit) lower..." which is correct: a change of 30 degrees Celsius does correspond to a change of 50 degrees Fahrenheit. In short, I was not discussing the magnitude of the temperature, but the magnitude of a temperature change.
You make the silly assertion that “The surface of the earth is warmer than it would be in the absence of an atmosphere because it receives energy from two sources: the sun and the atmosphere.” That’s not an explanation, because the atmosphere just gets its energy from somewhere else.
The fact that the atmosphere gets its energy from somewhere else, does not preclude it from being a source of energy for the Earth’s surface. You would probably not object to the suggestion that you get (much of) your energy from food. Yet, the energy in that food came from elsewhere also. Indeed, you raised no objection to the suggestion that we get energy from the Sun. Yet, the energy from the Sun, comes from the Sun’s photosphere, and that photosphere gets its energy from elsewhere just as assuredly as does our atmosphere. So, why are you content with the one and not the other?
However, let’s just treat the issue empirically. If you sit on the Earth’s surface and take measurements with radiometers, one quickly discovers that the Earth’s surface is receiving energy predominately from two sources: the Sun (actually, the Sun’s photosphere) and the (Earth’s) atmosphere. The fact that you can make a theoretical argument which traces the energy from each back over many thousands of years to the Sun’s core, and back further over billions of years to the Big Bang is irrelevant. Empirically, the energy received by the Earth’s surface comes (overwhelmingly) from the two stated sources. And curiously, our atmosphere supplies more energy to the surface than does the Sun, so it is hardly an irrelevant source.
But, still the Sun is the basic source of the energy and any suggestion that the Earth’s atmosphere acts like the Sun misleads students.
But, the Earth’s atmosphere does act like the Sun (or at least that part of it we see and which sends us energy). Both the Earth’s atmosphere and the Sun’s photosphere (the part that sends us energy) emit radiation for the same basic reason. Although their temperatures differ, they are both emitting (what is nearly) Blackbody radiation because of those temperatures. And while the photosphere is hotter, our atmosphere occupies a much larger fraction of our sky so we get more energy from the latter.
You are missing the point: the Earth’s atmosphere only emits because it receives radiation from the Sun. It really has a different status.
The Earth’s atmosphere emits because it has a temperature, not because it received energy in the form of radiation from the Sun. If the Sun were turned off (as, effectively, it is every night), the Earth’s atmosphere continues to emit.
Actually, I think you are raising a slightly different issue: not one about why the Earth’s atmosphere is a source of energy (which measurements clearly indicate it is), but how it can maintain its temperature in the face of these losses. You are suggesting to me that in the absence of the Sun, the temperature of the Earth’s atmosphere would drop (as indeed it would), even though as the temperature dropped, the atmosphere would continue to be a source of energy.
So, how is the temperature of the Earth’s atmosphere maintained in the face of all of the energy it is emitting in the form of radiation (to both the Earth’s surface and to space).The answer, of course, is that it is receiving energy, mainly from below via radiation, latent heat, and convection. (Incidentally, while our atmosphere receives some energy directly from the Sun, most comes from elsewhere: below).
Curiously, that is an almost identical explanation for how the Sun’s photosphere maintains its temperature in the face of all the energy it is emitting in the form of radiation. It is receiving energy, mainly from below, via radiation and convection. Indeed, the energy emitted by the Sun’s photosphere is a very long time removed from the solar core. Estimates are that it takes on the order of a hundred thousand years for the energy in the Sun’s core to be passed up to the photosphere through many many convective and radiative exchanges.
So, the Earth’s atmosphere and the Sun’s photosphere play an amazingly parallel role as sources of energy for the Earth’s surface. Both supply a comparable amount of energy (although the atmosphere supplies more); both emit radiation for the same reasons; the temperature of each is maintained by (essentially) the same processes; each is separated by a great many thousands of years from the (thermonuclear) source of their energy.
You argue that one should not say the atmosphere reradiates because it is not the same photons which are sent back. This sounds perfectly correct to me. Yet, what would be wrong to say that it "reflects".
Well, in the infrared, there is very little reflection from natural substances (even snow, which is highly reflective in the visible, is highly absorbent in the infrared). Radiation which falls on an object (the Earth, a cloud, the atmosphere) is (largely) absorbed, so the word reflection, just does not describe what is known to be happening.
Saying that they were not the same photons emitted as received is correct (they are not). Yet, that was merely a way of making a point about something that confuses many students (largely because their teachers are equally confused). These concepts are (all too often) presented as if some radiation is absorbed and then it is subsequently emitted. The implication is that there is some conservation of radiation as if a glass were first filled with water and then the same water (and the same amount of water) were subsequently released. But this is not the way nature works. Neither the atmosphere nor the photosphere emits radiation because either receives radiation. Each emits because of the temperature (and other characteristics of the medium). The Stefan-Boltzmann Law does not say the emission depends upon the amount of radiation received.
The energy emitted by the atmosphere comes only in part from energy received in the form of radiation and the radiation emitted is not the same radiation, nor does it even have the same spectrum as that which might have been received. Yet, the term reradiate implies that some radiation is absorbed and then it is subsequently emitted. And we know that radiation is not conserved in this way. Clearly, I am not telling you anything new about the physical processes, but I might be telling you something new about the way school teachers and students interpret the word reradiation to imply a process that has no physical reality. Recall, my pages are pedagogical in nature.
