The Earth's atmosphere scatters sunlight (comprised of all colours), and it’s the blue light that gets scattered the most. Simply put – that's why the sky is blue.
So we know the sky is blue, but what shade of blue?
Well, there’s a tool for that. A cyanometer (from cyan and -meter) is an instrument used to measure “blueness", specifically the colour intensity of blue sky.
Horace-Bénédict de Saussure, a Swiss physicist and avid mountain climber, is credited with inventing the cyanometer in 1789, having concluded that the blueness of the sky is influenced by both moisture and the amount of suspended particles in the air.
De Saussure's cyanometer was divided into 53 coloured, numbered sections, arranged in a circle, ranging from white (0) to gradually darker shades of blue and then black (52). The cyanometers were manually produced with a predefined recipe of Prussian blue watercolour concentration for each section, and then distributed to friends and fellow naturalists to gather more observations.
The user simply holds up the cyanometer against the sky and visually matches the colour in the centre hole to the closest shade on the scale. The deeper the blue, the clearer and less humid the atmosphere.
Though it may seem quaint, the cyanometer is still relevant in scientific and educational contexts. Also, some artists, photographers and designers reference cyanometer scales when trying to capture accurate sky tones in paintings or digital works. An Italian photographer Paolo Saccheri has even developed one (with 50 shades) that you can buy. His website HERE. And there's even an app for that. Search for Cyanometer.
Finally, the Cyanometer by Martin Bricelj Baraga is a tribute to the blueness of the sky. It is both a physical monument that celebrates the original invention, and software that periodically collects images of the sky at a number of locations: Ljubljana in Slovenia, Wroclaw in Poland, Geneva in Switzerland and Grenoble in France. Check out the cyanometer.net website for current and archival readings from those locations.
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References
wikipedia.org/wiki/Cyanometer
saccheri.it/portfolio/cyanometer
cyanometer.net
facebook.com/groups/explorersoftheblue
Images
1. A cyanometer by Horace-Bénédict de Saussure (from the collection of Musée d'histoire des sciences de la Ville de Genève
2. Portrait of Horace Bexedict de Saussure, in the Library at Geneva
3 & 4. Photo credit: Paolo Saccheri
5. Paolo Saccheri cyanometer. See: saccheri.it/portfolio/cyanometer
6. Cyanometer app screenshot
7. Cyanometer by Martin Bricelj Baraga
8. Geneva reading from cyanometer.net
