Chapter 6. Using the LED Photometer

One of the oldest bits of weather lore is "Red sky at night, sailor’s delight. Red sky in morning, sailors take warning." The sky is often red, or reddish orange, at sunrise and sunset. Reds, pinks, and oranges dominate at these times of day because the distance from your eye to the sun is greater at these times than when the sun is directly overhead. When sunlight travels through the atmosphere—and especially when it travels that extra distance from the horizon—an effect known as Rayleigh scattering takes place.

The British physicist John William Strutt (the third Baron Rayleigh) explained in 1871 that air molecules are so small they interfere with and scatter photons of light. The various molecules that make up the Earth’s natural atmosphere—mostly nitrogen and oxygen, with trace amounts of other gases—scatter blue light (shorter wavelength light) more efficiently than red, orange, or yellow light (longer wavelengths). So when you look up, you see more scattered blue light than red or orange light. This is why our sky appears blue during most of the daytime (Figure 6-1).

When the sun is on the horizon at dawn or dusk, however, its light must travel through more of the atmosphere before we see it than when it is overhead (Figure 6-2). This leads to more and more of the blue light being scattered away, leaving behind the red-orange-yellow light. As well, dust and aerosols in the atmosphere more ably scatter long wavelengths of light—leading to lurid ...

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