Modern weather and climate prediction originated at the Institute for Advanced Study (IAS) in Princeton, New Jersey. Mathematician John von Neumann, a member of the IAS faculty, interacted with computing pioneer Alan Turing in the late 1930s and became involved in the construction of the first general-purpose digital computer, ENIAC, in the early 1940s. One of his goals was to use the computer to forecast weather using the equations of physics. He formed the Princeton Meteorology Group by hiring scientists with expertise in weather. In 1950, this group made the world’s first digital weather forecast. Two basic concepts from the philosophy of science – inductivism and deductivism – are introduced in the chapter to provide the context for the scientific developments being discussed. Von Neumann’s (thwarted) ambition of going beyond weather prediction to weather control is also discussed.

MIT professor Edward Lorenz made a serendipitous discovery about weather in the early 1960s, using a digital computer. He found that a small error in the initial conditions for a weather forecast could grow exponentially into a large error within days. This came to be known as the Butterfly Effect, one of the founding principles of chaos theory. To obtain initial conditions for weather forecasts, data from hundreds of weather balloons launched daily around the world are combined with satellite data through a procedure called data assimilation. The Butterfly Effect limits the useful period of weather prediction to about two weeks in advance. Carrying out an ensemble of weather forecasts, starting from different initial conditions, allows probabilistic predictions that extend beyond this limit. The philosophical concept of determinism is introduced, as is the concept of Laplace’s Demon, motivating the notion of a Climate Demon that predicts climate perfectly.