WITH A CYANOMETER, of course. That was an easy question and because it was so easy I announce this fantastic object to be the OBJECT OF THE MONTH. September is a good one to check the different colors of the sky.

By the way the cyanometer is an instrument for measuring ‘blueness’, specifically the colour intensity of blue sky. It is attributed to Horace-Bénédict de Saussure and Alexander von Humboldt 1789.