Imagine a rotating sphere that is 12,800 kilometers in diameter, has a bumpy surface, is surrounded by a 40-km-deep mixture of gases whose concentrations vary both spatially and over time, and is heated, along with its surrounded gases, by a nuclear reactor 150 million kilometers away. Imagine that this sphere is also revolving around the nuclear reactor and that some locations are heated more during one part of the revolution and other locations are heated during another part of the revolution. And imagine that this mixture of gases continually receives inputs from the surface below, generally calmly but sometimes through violent and highly localized injections. Then, imagine that after watching the gaseous mixture, you are expected to predict its state at one location on the sphere one, two or more days into the future. This is essentially the task encountered day to day by a weather forecaster.

On the difficulty of weather forecasting, Bob Ryan, Bulletin of the American Meteorological Society, 1982.