The cyanometer (from cyan and -meter) is an instrument for measuring 'blueness', specifically the colour intensity of blue sky. It is attributed to Horace-Benedict 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.
If you're feeling crafty you can make your own cyanometer. Instructions here.
Not feeling crafty? Just take a photo of the sky and there's an app that defines the colour.