The Marked Blue project records the weather each day. Due to daily variations in UV index and humidity, each cyanotype produces a unique result.

Cyanotype is a camera-less photographic printing process. When iron salts coated on paper are exposed to ultraviolet light, they form a deep blue compound known as Prussian blue. Washing the paper with water stops the reaction and reveals the blue image.

Under cloudy weather, the image tends to appear bluish-gray, while sunny days produce more vivid blues.