A cyanometer [sahy-uh-nom-i-ter] (from cyan and -meter) is an instrument for measuring ‘blueness,’ specifically the color intensity of blue sky. It is attributed to Swiss aristocrat, physicist, and mountaineer Horace-Bénédict de Saussure. It consists of squares of paper dyed in graduated shades of blue and arranged in a color circle or square that can be held up and compared to the color of the sky. The blueness of the atmosphere indicates transparency and the amount of water vapor.

De Saussure is credited with inventing a cyanometer in 1789 with 53 sections, ranging from white to varying shades of blue (dyed with Prussian blue) and then to black, arranged in a circle; he used the device to measure the color of the sky at Geneva, Chamonix and Mont Blanc. He concluded, correctly, that the color of the sky was dependent on the amount of suspended particles in the atmosphere.

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