Sixteen shades of blue; the use of a digital camera as a cyanometer

During his ascent of Mont Blanc in 1787, de Saussure used a cyanometer comprising sixteen shades of blue painted on cards to quantify the color of the sky. Simultaneous measurements gave a whiter shade of 7 at Geneva but a dark blue of 1-2 from the summit. We explore the use of a digital camera as a cyanometer. It was hoped to see the variation with altitude during a short ski holiday, but this was overambitious. However, results show that, for clear skies, the percentage of blue varies little, and the perception of blueness is primarily determined by the brightness.