ABSTRACT
The interactions between sunlight and the Earth’s atmosphere are both useful and aesthetically pleasing. The extra daylight due to bending of the light in the atmosphere has already been mentioned in chapter 2. Amore obvious effect of the atmosphere is the scattering of sunlight, which colours the sky and provides indirect illumination. For an astronaut on the surface of theMoon, the Sun shines out of a black sky. When it sets there is an abrupt transition from daylight to darkness and no twilight, because the Moon has virtually no atmosphere. Mars has some atmosphere, but it is very different from that on Earth. The Martian pressure is much lower and the major gas present is carbon dioxide. In addition, vigorous winds stir up a fine dust containing iron oxide. These particles absorb light at the short wavelength end of the visible spectrum, and reflect light of longer wavelength in all directions, giving the Martian sky a yellowish brown colour sometimes described as resembling butterscotch. (This was only established after some initial misinterpretations of images from the Viking Landers of 1976; the colour was later confirmed by the Mars Pathfinder.)
In the Earth’s atmosphere changes in the direction of light occur even when clouds are absent. The effect known as Rayleigh scattering arises from variations in the scattering medium on a scale smaller than the wavelength of light. Lord Rayleigh was the first person to explain the scattering and calculate the way it
depends on wavelength. In mathematical terms, the rate of scattering is inversely proportional to the fourth power of the wavelength. Figure 5.1 shows that this relationship implies a large change of the scattering rate over the wavelength range from 300 to 800 nm. Most of the ultraviolet light that avoids absorption by ozone in the upper atmosphere, and much of the violet and blue light, undergo scattering in a cloudless sky, which appears blue for most of the day. Most of the longer wavelength radiation reaches us directly, giving the Sun a yellowish colour. When the Sun is close to the horizon the path length through the atmosphere is extended so that the longer wavelengths also suffer appreciable scattering, shifting the colour of the Sun and of the sky around it towards red. Figure 5.2 (colour plate) is an example of the delightful colours that can be seen in skies around sunrise and sunset.
