Deterministic Chaos

The finding that a fully deterministic system, one with no randomness in its equations, can still be practically unpredictable over time because arbitrarily small differences in starting conditions grow into large differences in outcome. Edward Lorenz found it by accident in 1961, rerunning part of a toy weather model from a mid-point value he had rounded for a printout, and published the result in 1963 as Deterministic Nonperiodic Flow. He later described the effect with an image that gave it its popular name, asking whether the flap of a butterfly's wings in Brazil could set off a tornado in Texas. In meteorology, it explains why a numerical forecast's skill degrades with the length of the forecast even when the model and the data are both good.