Abstract: In this paper, we present SkyFinder, an interactive search system of over a half million sky images downloaded from the Internet. Using a set of automatically extracted, semantic sky attributes (category, layout, richness, horizon, etc.), the user can find a desired sky image, such as "a landscape with rich clouds at sunset" or "a whole blue sky with white clouds." The system is fully automatic and scalable. It computes all sky attributes offline, then provides an interactive online search engine. Moreover, we build a sky graph based on the sky attributes, so that the user can smoothly explore and find a path within the space of skies. We also show how our system can be used for controllable sky replacement.