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

2 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

3 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

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

5 While the absorption spectrum is strictly 100%, the emission spectrum of a blackbody has a sort of asymmetric bell shape Energy Emitted Wavelength The total energy emitted (E) and the wavelength of peak emission ( max ) both vary with the object's temperature - Warmer bodies emit more total energy (larger E) and have shorter peak wavelengths (smaller max )

6 While the absorption spectrum is strictly 100%, the emission spectrum of a blackbody has a sort of asymmetric bell shape Energy Emitted warmer body Wavelength The total energy emitted (E) and the wavelength of peak emission ( max ) both vary with the object's temperature - Warmer bodies emit more total energy (larger E) and have shorter peak wavelengths (smaller max )

7 While the absorption spectrum is strictly 100%, the emission spectrum of a blackbody has a sort of asymmetric bell shape warmer still Energy Emitted Wavelength The total energy emitted (E) and the wavelength of peak emission ( max ) both vary with the object's temperature - Warmer bodies emit more total energy (larger E) and have shorter peak wavelengths (smaller max )

8 While the absorption spectrum is strictly 100%, the emission spectrum of a blackbody has a sort of asymmetric bell shape Energy Emitted and a colder body Wavelength The total energy emitted (E) and the wavelength of peak emission ( max ) both vary with the object's temperature - Warmer bodies emit more total energy (larger E) and have shorter peak wavelengths (smaller max )

9 While the absorption spectrum is strictly 100%, the emission spectrum of a blackbody has a sort of asymmetric bell shape max peak wavelength decreases with temperature Energy Emitted Wavelength The total energy emitted (E) and the wavelength of peak emission ( max ) both vary with the object's temperature - Warmer bodies emit more total energy (larger E) and have shorter peak wavelengths (smaller max )

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

11 Some facts about blackbody radiation: The total energy emitted by a blackbody (per unit surface area) is given by the Stefan-Boltzman law E = T 4 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 E S E E 4 T S = ~ T E 4 160,000

12 Some facts about blackbody radiation: The wavelength of peak emission is given by Wien's law max = b T 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: max = 3000 m K 300 K ~ 10 m Sun: max = 3000 m K 6000 K ~ 0.5 m

13 Some facts about blackbody radiation: The wavelength of peak emission is given by Wien's law max = b T 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: max = 3000 m K 300 K ~ 10 m (IR) Sun: max = 3000 m K 6000 K ~ 0.5 m

14 Some facts about blackbody radiation: The wavelength of peak emission is given by Wien's law max = b T 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: max = 3000 m K 300 K ~ 10 m (IR) Sun: max = 3000 m K 6000 K ~ 0.5 m (visible)

15 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)

16 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

17 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

18 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

19 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

20 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

21 incoming solar no atmosphere To see how this works consider a simple thought experiment: First, suppose we take a completely cold planet with no atmosphere, and we expose it to the same solar radiation the Earth receives (here taken to be 3 units). What happens?

22 incoming solar Well, initially the planet warms up, and as it warms, it begins to radiate IR.

23 incoming solar outgoing IR Well, initially the planet warms up, and as it warms, it begins to radiate IR.

24 incoming solar outgoing IR Well, initially the planet warms up, and as it warms, it begins to radiate IR.

25 incoming solar outgoing IR Well, initially the planet warms up, and as it warms, it begins to radiate IR. Until eventually...

26 incoming solar outgoing IR Well, initially the planet warms up, and as it warms, it begins to radiate IR. Until eventually...the incoming solar and outgoing IR reach a balance.

27 incoming solar outgoing IR This balanced state is called the radiative equilibrium state--- i.e., the state in which there is no net energy gain or loss. Note that the planet will only warm up to the point where it radiates away as much energy as it receives from the sun (here 3 units).

28 incoming solar selectively absorbing atmosphere Now let's repeat the experiment, but this time we'll include a selectively absorbing atmosphere---i.e., one that absorbs IR but not visible.

29 incoming solar As before, the incoming solar radiation passes straight through to the ground, and the planet begins to heat up and emit IR. But this time...

30 incoming solar IR absorbed As before, the incoming solar radiation passes straight through to the ground, and the planet begins to heat up and emit IR. But this time...the IR is mostly absorbed by the atmosphere...

31 incoming solar IR absorbed IR emitted As before, the incoming solar radiation passes straight through to the ground, and the planet begins to heat up and emit IR. But this time...the IR is mostly absorbed by the atmosphere...and then re-emitted, half up and half down.

32 incoming solar IR absorbed IR emitted But note that now, the Earth's surface receives 5 net units of radiation (3 solar and 2 IR). So to be in balance, it must actually be emitting 5 units as well (some of which gets through to space).

33 incoming solar IR absorbed IR emitted But note that now, the Earth's surface receives 5 net units of radiation (3 solar and 2 IR). So to be in balance, it must actually be emitting 5 units as well (some of which gets through to space).

34 incoming solar IR absorbed IR emitted So in the end, the case with the selectively absorbing atmosphere has 2 extra units of radiation reaching the ground...which causes it to warm up a bit more (and emit more radiation) until everything is in balance again---i.e., until we have a new radiative equilibrium.

35 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.

36 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

37 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.

38 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.