Because It’s the Weekend: Small Multiples of the Sky

The stunning video below, which I recommend you play full-screen and in HD, is a time lapse of the San Francisco sky every day for a year:

Not only is the movie aesthetically beautiful, it’s also a great example of the “small-multiples” principle of data visualization pioneered by Cleveland and popularized in Tufte’s first book. Because the days (each small square) are arranged in chronological order throughout the year, and all windows are synchronized to show the same clock time each day (ignoring Daylight Savings) the eye can pick up patterns from the repetition: the subtle changes in the colour of the blue sky at the same time of day over the course of a year, and the greater likelihood of cloudy days in the winter montha (in the middle rows of the grid; the sequence begins in late July). Also note that the fade-in and fade-out of the windows at the beginning and end isn’t artistic license, but rather reflects the shortening and lengthening of the days over the seasons.

Reliably collecting all of this data (almost 9000 images a day, about half a terabyte of JPGs over the course of a year) wasn’t a trivial process. The artist, Ken Murphy, set up an automated wide-angle camera rig on the roof of the Exploratorium in San Francisco (pointed north away from the sun, to avoid exposure issues), driven by a custom Linux box (with a remote-reboot capability) and protected by an uninterruptable power supply. The details are fascinating, which he recounts in this blog post and in this interview at the Atlantic at the Alantic.

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