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Sunlight, perceived as mostly white, consists of a wide range of colours. The colours of sunlight are best seen when light is reflected from drops of rain, causing a rainbow. In a rainbow all the colours contributing to the white light, from red through yellow, green and blue to purple, are seen separately. A similar separation of the sunlight colours can be achieved by passing it through a prism. This range of colours is the spectrum of sunlight.

Rainbow in The Bilt, The Netherlands (photograph by A. Apituley)

Rainbow in The Bilt, The Netherlands (photograph by A. Apituley)

The sunlight that we perceive is filtered by the Earth’s atmosphere, and it is scattered by air molecules, water, and tiny atmospheric particles that we call aerosols. We perceive it as daylight when the Sun is above the horizon. When the Sun is at its highest point we see it as mostly white. The blue of clear skies is the result of the scattering of sunlight on molecules of nitrogen and oxygen. The orange or red color occurring at sunrise and sunset is the result of a longer light path through the atmosphere before it reaches the eye. The longer this path, the more the sunlight is scattered by atmospheric particles, which causes the blues and greens to be removed from its spectrum, leaving it to appear orange or red.

Each type of atmospheric particle has unique optical properties, its own scattering and absorption characteristics, such that they interact with sunlight in a unique way. Our Earth atmosphere contains many different types and sizes of particles, their occurrence and dynamics are highly variable, depending on many conditions, all of which affect the sunlight spectrum that can be observed in different ways. As such the observed spectrum contains information on these conditions. This is how measurements of the (scattered) sunlight spectrum can be used to monitor atmospheric particles.