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A cyanometer is an instrument for measuring the color intensity of the blue sky. It was invented by the Swiss naturalist Horace-Bénédict de Saussure in 1789 and consisted of squares of paper dyed in 52 graduated shades of blue arranged in a color circle that could be held up and compared to the color of the sky.

This web app provides a digital cyanometer for smartphones. Turn away from the sun, point the center to the sky and adjust the color using the red slider. The number indicates the current shade of blue.

(ȼ) Thomas Weibel, Basel