In short: Each flight is a single line; the top is midnight, moving downwards through the day, and left to right indicates the week of the year, 1987-2007. Orange is for longer flights, on average.
Description: This stacked image shows the number of planes in flight at any one time (summed over the week) from 1987 to 2007, color-weighted for the distance being traveled. Initially, the shape and broad areas of color make it appear almost featureless, with the only gradient being the deepening of the color left-to-right. However, this is a great example of the image evolving as the viewer takes longer to look at it, and features start to appear. The most pronounced are the sharp vertical stripes as airlines were in financial turmoil in the 90’s and the most stark of them all being the attacks of Sept 11 2001, and the effects on the aviation industry in the year that followed. The contrast of the blue and orange are, when forgetting the context of the data, pleasingly not without hints of a Rothko multiform painting.
Point of interest: Ignoring the vertical features there are horizontal areas where proportionally more planes were in the air at certain times, around 10am being a clear one. Later there are 4-5 lines that start to get washed out after the millennium, but not only that they start to broaden and are pushed later as the airspace (or rather airport space) fills up with more and, on average, shorter flights.
Technical: All domestic flight data from 1987-2007 available at http://stat-computing.org/dataexpo/2009/. The dimensions of the image are specific to denote a calendar-approach to the image, in so much that each pixel is a minute vertically and week of the year horizontally. Each flight is added to the image as a line from departure time to arrival time, so each flight adds to the image as a vertical line. The red channel is the number of planes in flight at the time, blue the average distance being traveled and green a combination of the two. Each channel is equalized.