He was a professor at the Massachusetts Institute of Technology when he came up with the scientific concept that small effects lead to big changes, something that was explained in a simple example known as the "butterfly effect." He explained how something as minuscule as a butterfly flapping its wings in Brazil changes the constantly moving atmosphere in ways that could later trigger tornadoes in Texas.Link to AP obituary, here is the New York Times piece, and here is more about Lorenz at the MIT website. (thanks, gATO)
His discovery of "deterministic chaos" brought about "one of the most dramatic changes in mankind's view of nature since Sir Isaac Newton," said the committee that awarded Lorenz the 1991 Kyoto Prize for basic sciences. It was one of many scientific awards that Lorenz won. There is no Nobel Prize for his specific field of expertise, meteorology.
Jerry Mahlman, a longtime friend, noted that the man who pioneered chaos theory was "the most organized person I ever knew."