this is a super-basic question but I am new to all this and a bit confused. I want to understand the process of radiation and other things like evaporation on the temperature of the Earth's surface. So to start, I want to truly understand what happens with the molecules in the ground.

But I am struggling already with the understanding of the incoming energy:

  1. If we look at the top of Earth's atmosphere, radiation arrives in various wavelengths [2].
  2. Not all of this radiation reaches the ground, but is absorbed by different gases in the atmosphere, so only parts of the radiation reaching the TOA finally reaches the surface [2]. Yep, makes sense.
  3. To calculate what happens on the ground, I now need to know what the total incoming radiation is, LW and SW. But Wikipedia [3] calls the solar radiation "shortwave" and relates longwave radiation solely to the outgoing radiation. Why?
  4. Once I understood how much LW and SW finally reaches the earth, the SW should have a higher effect in increasing the kinetic energy of the molecules in the ground, as SW radiation has a higher energy, right?

So now I am confused. I actually wanted to know how strong SW and LW radiation put the molecules in the ground in motion to understand the increase in temperature and thus via the Stephan-Boltzmann-Law compute again the outgoing radiation... But what is more important now, SW or LW? And why does Wikipedia call solar radiation only shortwave?

If someone could help me out here that would be so great.

Thank you all so much for helping this mathematician fail at learning something about physics haha.

Best regards!

[1] http://uv.biospherical.com/student/page3.html

[2] https://en.wikipedia.org/wiki/Solar_irradiance#/media/File:Solar_spectrum_en.svg

[3] https://en.wikipedia.org/wiki/Earth's_energy_budget

EDIT: I misread source [1] so one of my questions have been answered already so I removed that.


1 Answer 1


The Sun emits radiation over a broad spectral range. The photosphere emits most visible and near-infrared light and that is where most of the power (about 1.4 kW/m$^2$ if the Sun is at zenith) hitting the top of the atmosphere arises. There is plenty of power outside the visible band in the near-infrared part of the spectrum. The spectrum you show in your reference [2] seems quite clear to me.

There are also other wavelengths present which are emitted from the hotter chromosphere and corona of the Sun, which are at temperatures of $10^4$ to $10^7$ K. These regions are predominantly responsible for the UV and X-ray light hitting the top of the atmosphere. These contribute little to the overall power at the top of the atmosphere (less than 1%).

Most of the UV and X-ray radiation is absorbed by the atmosphere. Most of the visible light makes it to the ground. Some of the near-infrared radiation gets through, but certain bands are blocked by things like water vapour in the atmosphere (again, shown clearly in your reference [2]).

Outgoing radiation from the Earth's surface (or its atmosphere) are being emitted by a much cooler body than the Sun. There is a strong reciprocal relationship (a one-to-one relationship in the case of a blackbody) between the temperature of the emitter and the inverse of the wavelength peak of its emission spectrum. If the Earth is at $\sim 300$ K then most if its emission is in the mid and far-infrared (i.e. at $>5\ \mu$m, beyond the right hand side of your plot [2] and at longer wavelengths than most of the received energy from the Sun).

Individual photons have higher energies if they have shorter wavelengths. However, by and large, the heating effect of the radiation just depends on the total amount of energy received in any particular wavelength band. Of course, where the radiation is absorbed is important. For example, although UV and X-ray wavelengths form a minor part of the radiation power from the Sun, they are absorbed in the upper atmosphere and have a disproportionate effect there.

There is plenty of literature on all this that you can find on the internet. However, I would steer clear of anything that appears to be a student paper (your reference [1]) or anything written with the purpose of denying climate change.

  • $\begingroup$ Thank you for this answer. I edited my question already as I had misread reference [1]. So which radiation comes through is very clear to me now. But a lot of LW still hits the ground, does it not also contribute to the warming of the surface? $\endgroup$
    – konse
    Dec 11, 2020 at 10:11
  • 1
    $\begingroup$ @konse yes, some of it does, but it is a small fraction of the energy present in sunlight t the top of the atmosphere. $\endgroup$
    – ProfRob
    Dec 11, 2020 at 10:14
  • $\begingroup$ But all literature I read (and Wikipedia) when talking about incoming radiation always talks about SW, but the LW just seems not at all negligible? Reference [2] shows that there is still quite a lot LW hitting the ground. That just confuses me. Or is its effect on putting the molecules in motion so much lower? Oh and don't worry I know how bogus those denier-posts are. $\endgroup$
    – konse
    Dec 11, 2020 at 10:20
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    $\begingroup$ What theEarth emits - which I guess is what you are referring to as "LW" is at wavelengths longer than 5 micrometers and is beyond the right hand side of plot [2]. $\endgroup$
    – ProfRob
    Dec 11, 2020 at 11:07
  • $\begingroup$ Oooh yes! This is the reason why I didn't understand! Now it all makes sense, I had read that IR is generally considered longwave radiation (climate.ncsu.edu/edu/RadiationTypes) and this is what led to my confusion. I now understand that the terms SW and LW are not clearly defined, and that almost all of the incoming solar radiation is "SW" and the radiation emitted by the Earth has much much higher wavelengths. Thanks so much for bearing with me! $\endgroup$
    – konse
    Dec 11, 2020 at 13:44

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