A cyanometer is a simple ring made of cardboard. Its segments show all possible shades of blue, from light to dark. If you hold the cyanometer against the sky, you will always find a blue that matches the current sky color.

The cyanometer was invented by the Geneva naturalist Horace Bénédict de Saussure in the 1760s. De Saussure realized that the blue value indicates the water content of the air: The bluer the sky, the less vapor, the whiter, the more. De Saussure applied the shades of blue with watercolor to a total of 53 strips of paper, which he glued to a cardboard ring, from white, which de Saussure labeled "0", through all shades of blue to black, which had the value "52".

De Saussure used to give cyanometers to his scientist friends with the aim of determining the blue of the sky in as many different places of the world as possible. In fact, the young Alexander von Humboldt carried such a cyanometer with him in 1802 when he climbed the 6263-metre-high Chimborazo in Ecuador, which was considered the highest mountain on earth at the time. The ascent was arduous and the mountaineers struggled with altitude sickness. It wasn't until they reached the summit that the weather cleared, revealing the darkest blue ever measured up to that point: 46 degrees on the cyanometer.

This Glamhack project will provide a freely available online cyanometer for smartphones.