A good part of the rest of this book is concerned with the processes that

A good part of the rest of this book is concerned

This preview shows page 52 - 56 out of 73 pages.

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.
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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.
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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.
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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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