Blackbody Radiation. A substance that absorbs all incident wavelengths completely is called a blackbody.

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1 Blackbody Radiation A substance that absorbs all incident wavelengths completely is called a blackbody. What's the absorption spectrum of a blackbody? Absorption (%) UV Visible IR Wavelength And why do we care? - From a radiative standpoint, the Sun and Earth are both nearly blackbodies

2 While the absorption spectrum is strictly 100%, the emission spectrum of a blackbody has a sort of asymmetric bell shape Energy Emitted Wavelength

3 The Sun emits as a blackbody, with peak wavelength around 0.5 m

4 Some facts about blackbody radiation: The total energy emitted by a blackbody (per unit surface area) is given by the Stefan-Boltzman law where T is in Kelvin and is a constant. For the Earth we have T E ~ 300 K, and for the Sun T S ~ 6000 K. The energies emitted (per unit area) then satisfy

5 Some facts about blackbody radiation: The wavelength of peak emission is given by Wien's law where T is again in Kelvin and b is a constant given by b ~ 3000 m K. Plugging numbers for the Earth and Sun, we then have Earth: Sun:

6 So the Sun emits most strongly in the visible range, while the Earth emits mainly IR radiation - As a shorthand, the wavelengths emitted by the sun are often called shortwave radiation, while the wavelengths emitted by the Earth are called longwave emission spectra of the Sun and Earth (note the different scales on the axes)

7 Selective Absorbers and the Greenhouse Effect An object that absorbs some wavelengths better than others is a selective absorber. CO 2 is a selective absorber

8 Selective Absorbers and the Greenhouse Effect An object that absorbs some wavelengths better than others is a selective absorber. CO 2 is a selective absorber In fact, most atmospheric gases are selective absorbers - Longwave (IR) is absorbed effectively, while shortwave (visible) is barely absorbed at all

9 absorption spectra for the two main greenhouse gases (CO 2 and H 2 0), along with the absorption spectrum for the atmosphere as a whole

10 Selective absorption by the Earth's atmosphere produces a warming effect for the Earth's surface Incoming shortwave radiation from the Sun passes straight through and is absorbed by the ground The outgoing longwave radiation from the surface is then largely absorbed by the atmosphere, and part of the longwave is re-radiated back to the ground Result: The net incoming radiation at the surface is increased, resulting in higher surface temperatures

11 Selective absorption by the Earth's atmosphere produces a warming effect for the Earth's surface Incoming shortwave radiation from the Sun passes straight through and is absorbed by the ground The outgoing longwave radiation from the surface is then largely absorbed by the atmosphere, and part of the longwave is re-radiated back to the ground Result: The net incoming radiation at the surface is increased, resulting in higher surface temperatures This increase in temperature due to partial absorption is referred to as the greenhouse effect

12 To see how this works, consider a simple thought experiment in which we consider two planets: one with no atmosphere, and another with a selectively absorbing atmosphere. In both cases, the planet starts out completely cold, and we then expose the planet to the same amount of radiation received by the Earth. What happens?

13 The extra warming caused by the selectively absorbing atmosphere is called the greenhouse effect. For the Earth, the radiative equilibrium temperatures for the two cases are roughly T ~ 255 K (no atmosphere) T ~ 288 K (selectively absorbing atmosphere) That is, the greenhouse effect for the present-day Earth contributes something like 33 K in warming.

14 Scattering and Reflection In addition to absorption and emission, incoming sunlight can also be scattered or reflected. Scattering refers to light that's deflected in all directions (but not necessarily equally) - In the visible range, air particles are most effective at scattering blues and violets, which is why the atmosphere appears blue Reflection is similar to scattering, but the light is mainly sent backwards - On average, about 30% of the incoming solar radiation is scattered or reflected back to space. This called the Earth's albedo.

15 When we look away from the sun, the light we see is mostly scattered light, which is why the sky appears blue. But at sunrise and sunset, most of the blue light has already been scattered away, and all that's left is the reds and oranges.