Cyanometer

[2] The cyanometers were manually produced with a predefined recipe of watercolor concentration for each section, and then distributed to friends and fellow naturalists to gather more observations.

De Saussure used the device to measure the color of the sky at Geneva, Chamonix, and Mont Blanc (Col du Géant):[2][3][4] Alexander von Humboldt (1769–1859) was an eager user of the cyanometer on his voyages and explorations: during his trip across the Atlantic Ocean, he observed 23.5 degrees at noon; at the summit of Teide, a record 41 degrees; and, while climbing to the summit of Chimborazo, on 23 June 1802, Humboldt broke both the record of highest altitude ever reached by humans, but also of observed darkness of the sky, with 46 degrees on the cyanometer.

[1] In his satirical verse epic Don Juan (Canto IV, 112), Lord Byron alludes to this device as an ironical means of measuring the blue of bluestocking ladies, crediting Humboldt for its invention.

When looking through clear air toward the horizon, distant sunlight of all wavelengths (colors) will generally undergo Mie scattering from spherical suspended particles.

In an unpolluted sky, these spherical particles will primarily be liquid water condensed onto natural atmospheric dust grains.

[...] Humboldt, "the first of travellers," but not The last, if late accounts be accurate, Invented, by some name I have forgot, As well as the sublime discovery's date, An airy instrument, with which he sought To ascertain the atmospheric state, By measuring "the intensity of blue:" Oh, Lady Daphne!