A cyanometer (from cyan and -meter) is an instrument for measuring ‘blueness’, specifically the colour intensity of blue sky.
It is attributed to Horace-Bénédict de Saussure and Alexander von Humboldt. 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.
De Saussure is credited with inventing the cyanometer in 1789. De Saussure’s cyanometer had 53 sections, ranging from white to varying shades of blue (dyed with Prussian blue) and then to black, arranged in a circle.
De Saussure concluded, correctly, that the color of the sky was dependent on the amount of suspended particles in the atmosphere.