Measuring the Blueness of the Sky

My background teaching color theory has been showing lately. I've been becoming more and more intrigued with things like this: a "Cyanometer" from over 225 years ago.

From ThisIsColossal:

The simple device was invented in 1789 by Swiss physicist Horace-Bénédict de Saussure and German naturalist Alexander von Humboldt who used the circular array of 53 shaded sections in experiments above the skies over Geneva, Chamonix and Mont Blanc. The Cyanometer helped lead to a successful conclusion that the blueness of the sky is a measure of transparency caused by the amount of water vapor in the atmosphere. You can learn more at the Royal Society of Chemistry.