and the underlying surface. A good part of the rest of this book is concerned with the processes that determine how much radiation is 9 Atmospheric refraction allows the sun to be visible from a location on the earth’s surface when it is actually about 0.5 ◦ , or approximately the diameter of the sun’s disk, below the horizon. Thus, the sun rises somewhat sooner and sets some- what later than would be predicted from geometric considerations alone. There- fore, the length of continuous daylight at the North Pole (for example) is actually somewhat longer than the expected six months.
Applications 53 SP 60S 30S EQ 30N 60N NP JAN FEB MAR APR May JUN JUL AUG SE P OCT NOV DEC Latitude Daily Average Insolation [W m -2 ] 300 100 200 400 0 500 300 100 200 400 0 300 100 200 400 0 500 500 MA Y Solar Declination 24 hr darkness 24 hr darkness 24 hr darkness 24 hr light 24 hr light 24 hr light Fig. 2.9: Daily average solar flux at the top of the atmosphere, as a function of latitude and time of year. Contour values are given in units of W m − 2 . absorbed and how much is reflected. Problem 2.20: Compute, and compare with Fig. 2.9, the daily aver- age top-of-the-atmosphere insolation [W m − 2 ] for the following two cases: (a) the North Pole at the time of the Northern Hemisphere summer solstice; (b) the equator at the time of the equinox. Assume that the solar flux normal to the beam is a constant 1370 W m − 2 , and note that the North Pole is inclined 23 ◦ toward the Sun at the time of the solstice.
54 0 100 200 300 400 500 600 SP 60S 30S EQ 30N 60N NP Insolation [W m -2 ] Latitude Insolation Annual 21 June 21 December Fig. 2.10: Daily average solar flux at the top of the atmosphere as a function of latitude, for the two solstice dates and averaged over a year.
CHAPTER 3 The Electromagnetic Spectrum In the previous chapter, we examined how electromagnetic radia- tion behaves on a purely physical level, without being concerned yet with its detailed interactions with matter. One important obser- vation was that we can treat an arbitrary radiation field as a super- position of many “pure” sinusoidal oscillations. The clearest every- day example of this is the rainbow: white sunlight interacting with raindrops is decomposed into the constituent colors red through vi- olet, each of which corresponds to a narrow range of frequencies. Radiation associated with a given frequency and trajectory in space may be analyzed completely independently of all the others. We also saw that there is no fundamental constraint on the fre- quency that EM radiation can exhibit, as long as an oscillator with the right natural frequency and/or an energy source with the mini- mum required energy is present (recall from Section 2.6 that a single photon has a specific energy determined by its frequency and that an oscillator cannot emit less than that minimum amount). In a vacuum, the frequency or wavelength of a photon is of lit- tle practical consequence, as it cannot be absorbed, scattered, re- flected, or refracted but rather is condemned to continue propagat- ing in a straight line forever, regardless. In the presence of matter however, the frequency becomes an all-important property and, to 55
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