Although Eq. (8.35) specifies the contrast of radiance, this equation is a good approximation to the luminance contrast if we take t to be in the middle of the visible spectrum.
All else being equal, contrast decreases as p increases. And as we show in Section 3.5, p is more sharply peaked in the forward direction the larger the scatterer. Thus we expect the details of a distant scene to be less distinct when looking toward than away from the sun if the optical thickness of the line of sight has an appreciable component contributed by particles comparable with or larger than the wavelength. Indeed, on many occasions we have observed marked improvements in contrast on a distant ridge or mountain to the east from morning to late afternoon despite no obvious change in particle concentration.
The misconception that water vapor is a powerful scatterer of sunlight is probably largely a consequence of the common observation that on humid, hazy days, visibility is often de-pressingly poor. But haze is not water vapor, rather water that has ceased to be vapor. At high relative humidities, but still well below 100%, small soluble particles in the atmosphere accrete liquid water to become solution droplets. Although these droplets are much smaller than cloud droplets, they markedly diminish visual range because of the sharp increase in scattering with particle size (see Fig. 3.11). Because of coherence, the same number of wa ter molecules when aggregated in haze scatter vastly more than when apart, increasing the scattering coefficient and therefore decreasing visual range.
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