SIO 217C: Climate
Spring 2014
Gaseous Absorption Discussion Questions
1. In terms of aggregate impact, what is the strongest greenhouse gas in Earth’s atmosphere?
What is the second strongest greenhouse gas?
2. What are the three most abundant gases in Earth’s atmosphere? Why is their aggregate
greenhouse effect extremely weak?
3. A molecule of a chlorofluorocarbon (CFC) gas added to the atmosphere increases the
greenhouse effect by an amount ten thousand times larger than the amount when a CO2
molecule is added to the atmosphere? Why is this?
4. Why is the sky blue? Why are clouds white?
5. Why is the daytime clear sky typically more bluish overhead and more whitish towards the
horizon? Why does the sky have a darker color of blue at high elevation? Why does the sun
become more yellow or red near the horizon?
6. Photographs taken by landers on Mars indicate the sky is reddish. Why is that?
7. When humidity is low (e.g., Santa Ana day in San Diego or in the desert), it is often hot in
direct sunlight but cool in the shade. When humidity is high (e.g., eastern U.S. in summer), it
is often nearly equally hot in direct sunlight and shade. Why is this?
8. Since the emissivity of a gas at a particular wavelength is the same as the absorptivity of the
gas at that wavelength, why does ozone absorb radiation at UV wavelengths but not emit
radiation at UV wavelengths?
9. In ACC Fig. 2.3, CD Fig. 4.7, and GPC Fig. 3.2, there are very few and weak absorption
bands at visible wavelengths, several strong absorption bands at near IR wavelengths, and
numerous overlapping bands at thermal IR wavelengths. Why is this?
10. Explain how measurements by a satellite of upwelling radiation made at several specific
wavelengths could be used to construct a vertical profile of atmospheric temperature.
11. Unlike most terrestrial substances, bare metal is a relative weak absorber at thermal IR
wavelengths. Why then does bare metal get so hot on sunny summer days?
12. In CD Fig. 4.6, note how there is overall less upwelling radiance in the 650-700 cm-1
wavenumber interval (around 15 µm wavelength) and the 1000-1050 cm-1 wavenumber
interval (around 9.6 µm wavelength). Why is this? Note that in each interval there is a
narrow peak of greater upwelling radiance in the middle of the broad valley of less upwelling
radiance. What is the cause of the narrow peak of greater upwelling radiance?
Difficult Question
13. Consider a planet in orbit around a brown dwarf star with an effective emission temperature
of 900 K. If the planet had the same atmospheric composition as Earth, would it have a
vertical temperature profile qualitatively similar to that of Earth? Why or why not?