shamelessplug: Actually MODIS scans Earth using different wave lengths. The near infrared channel can be used to detect clouds and even to distinguish them from ice. Using a little recursive AviSynth Script I was able to produce an animation of daily changes in the European part of the Arctic with last year's data. It starts with all the clouds and then adds each day's cloud free pixels. The goal was to visualize how Kara Sea melted out within a few days. Enjoy: http://www.youtube.com/watch?v=j-EDFs8f_78 /shamelessplug