Shown above is a solargraph for a site near Tenerife, Spain (Canary Islands) from December 21, 2014, to June 21, 2015. Solargraphs give immediate information about sunshine and cloudiness. However, using the other property of the black-and-white photographic paper (used for solargraphs), some information about precipitation can also be gleaned. The paper increases its sensitivity when atmospheric conditions are humid, for example, during the rainy season in January. Additionally, after a rain event its reaction to sunlight changes. Note that the color gradient in the middle of the image changes from white to purple following the last rain event. Refer to the key at bottom left.
Weather from Solargraphy - EPOD - a service of USRA
http://epod.usra.edu/blog/2015/09/weather-from-solargraphy.html