HDR (High Dynamic Range) IBL (Image Based Lighting) is a technique in which we use a texture to map the world (celestial sphere or sky dome) and, besides giving us a diffuse channel, it also emits light according to the texture itself. This produces very interesting and accurated results including shadows, most noticeable with cloudy or heterogeneous skies. Here, we used a free HDR by Peter Guthrie (www.peterguthrie.net/freesky2013) to illuminate this scene. The square frame have a semi-black transparent material so that the full range of color can be seen.
Download the .blend file here: cyclesHDR.blend