1. Pluto's atmospheric composition is similar to Earth's. It is predominantly nitrogen, with a little carbon monoxide and methane. At its closest approach, Pluto is 30 times further from the sun than Earth, and its surface temperature is cold enough to condense nitrogen gas into nitrogen ice. Pluto's orbit is peculiar. Its spin axis lies nearly in its orbital plane around the sun, so at solstice one hemisphere receive nearly all of the sunlight and the other is virtually all dark (See our textbook Fig 4-15, but tilt the axis 90 deg, instead of 23.5 deg). Naturally the lit hemisphere is warmer than the dark hemisphere, so nitrogen ice tends to sublimate slowly from the warm hemisphere and condense in the cold one. a) Estimate the surface temperature on Pluto's lit and dark sides using planetary energy balance, but include sublimation and evaporation as follows: Lit side (receives all of the incoming sunlight, surface is sublimating): S*(1-A)/2 = sigma * Tlit4 + Fsub Dark side (receives no incoming sunlight: surface is condensing): Fsub = sigma * Tdark4 All fluxes are averaged over one hemisphere. There is a factor of 2 (rather than a 4) in the first equation, because the sunlight is only distributed over one-half of the sphere for long periods of time. Let Fsub = .2 W/m2 be the energy flux leaving the surface on the lit side and received by the surface on the dark side due to sublimation and condensation, respectively. Let the albedo A = 0.45 and the solar constant for pluto S = 1.5 W/m2 (We guessed values for Fsub and A and estmated S from the inverse square law S= So(ro/r)2 = 1370 (1/30)2 , also note that sigma = 5.67 X 10 -8 W/m2/K4 for all planets. We can ignore the greenhouse effect because the concentration of greenhouse gases is very low.). ANSWER: S*(1-A)/2 = 1.5(1-0.45)/2 = 0.49 W/m2 sigma * Tlit4 = 0.4125 - 0.2 Tlit4 = 0.2125 / 5.67 X 10 -8 = 3.748 X 10 7 Tlit = 44.0 K Tdark4 = 0.2 / 5.67 X 10 -8 = 3.527 X 10 7 Tlit = 43.34 K b) In about 30 words summarize your results, regarding temperature and the heat transport via sublimation and condensation. ANSWER: Heat loss from sublimation is nearly half of the absorbed shortwave radiation. Therefore the dark side is kept nearly as warm as the lit side via heat transfer. Check out a recent newspaper article about pluto sublimation for fun/more information 2. Sketch a map of Africa (hand drawn is fine) and referring to Fig 4-26 discusss which areas you think have high and low rainfall (include the time of year) as a result of the position of the ITCZ, monsoons, prevailing winds and location of mountains. Explain your reasoning. ANSWER: In July, high rainfall in western Africa at about 10S results from a monsoon circulation. A surface low pressure develops over land because it warms more quickly in summer, the ocean surface to the south is cooler and has higher surface pressure. The ITCZ is further north at this time, but produces little rainfall due to lack of moisture supply. Warm moist air is drawn toward Africa from the ocean along the equator below Ivory Coast, but it is lifted by the monsoon circulation before it reaches the ITCZ. High precipitation over Ethiopia (to the east) results from the prevailing easterly winds traveling up the mountains there. In January, the ITCZ, which has moved south together with the monsoon circulation, creates high precip in southern Africa. 3. Refer to Fig 4-17 in your textbook and your knowledge about what causes cloud formation to describe where you are most likely to see clouds develop in the figure panels. ANSWER: Vigorous lifting over the land and surface onshore winds create clouds during the day. In contrast. at night there is gentle lifting over the ocean (necessary for mass continuity, as mentioned in class), which can produce offshore clouds. 4. In your own words: Explain why Earth experiences different seasons throughout the year. Which parts of Earth experience the greatest seasonal range in temperature (see Fig 4-18c), and which parts experience the least? ANSWER: Everyone got the first part right so I won't write it up. The maximum seasonal range is in the high northern latitudes, Siberia especially because it is very continental too. The tropics experience the least seasonal range. (extra - Many of you noted that the northern hemisphere generally had greater seasonal variations than the southern hemisphere.) 5. Imagine that a low pressure system is due south of Seattle, and a high pressure system is due north. Assuming straight isobars and perfect geostrophic balance, which way is the wind blowing in Seattle? Include a force-balance diagram like in Fig 4-13. ANSWER: Take fig 4-13 and rotate it so the low pressure points down (south). VG then points to the east, and so to a good approximation the winds in Seattle do too. (extra - friction would tend to cause the winds to be deflected slightly to the south, converging into the low.) 6. In your own words describe water vapor feedback: (a) what is it? (b) Why is it a positive feedback? (c) Why isn't there a runaway greenhouse effect on Earth? ANSWER: Water vapor feedback describes the relationship among temperature, water vapor concentration in the atmosphere, and the greenhouse effect. Each component has a positive coupling with the next, therefore the feedback is positive. If the temperature increases, water vapor concentration increases, the greenhouse effect increases, and finally the temperature increase is reinforced further. This doesn't cause a runaway greenhouse effect on Earth because other stronger negative feedbacks act like a governor. The main negative feedback is the temperature-IR feedback. Venus is not so lucky. Its temperature-IR feedback, although negative, is insufficient to prevent a runaway. |