butterfly.jpgPerhaps you were as surprised as Gothamist when you saw a meteorologist mentioned in the Sunday Styles section of the Sunday Times. In the essay David Carr offers his explanation of how the "changed the world" genre of pop history books that have recently become popular. You know the kind, "How the Irish Saved Civilization"; "Mauve: How One Man Invented a Color That Changed the World"; and "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time" to name just a few. Along the way Carr comes to blame MIT meteorologist Edward Lorenz for this phenomenon, citing Lorenz's 1963 paper presented to the New York Academy of Sciences. In discussing his research Lorenz quoted a meteorologist as saying "if the theory were correct, one flap of a seagull's wings would be enough to alter the course of the weather forever", meaning that small changes in initial conditions can have enormous consequences later on. Lorenz later dropped the seagull in favor of a butterfly, in part because his calculations looked like a butterfly when graphed (you can watch the butterfly, or Lorenz attractor, in action). Carr may not have realized it but Lorenz's insight changed how meteorologists viewed the atmosphere and introduced the world to chaos theory. In his classic, for weather geeks, paper "Deterministic Nonperiodic Flow" Lorenz wrote

"When our results concerning the instability of non-periodic flow are applied to the atmosphere, which is ostensibly nonperiodic, they indicate that prediction of the sufficiently distant future is impossible by any method, unless the present conditions are known exactly. In view of the inevitable inaccuracy and incompleteness of weather observations, precise very-long-range forecasting would seem to be non-existent."

In other words, say you had the tens of thousands of global observations of temperature, pressure, wind speed, humidity etc. needed to make a forecast. Plug those numbers into a forecast model and the model will churn out future weather conditions minute by minute for as far out into the future as you would like. In effect what Lorenz said was, if just one of those observations was off by a fraction (say Central Park's temperature was not 81 F as reported, but 81.00000001 F degrees) and those corrected numbers were plugged into a model, the results of the two forecasts will slowly diverge until they no longer resemble each other. The time it takes for these forecasts to diverge has since been discovered to be about three weeks. This is the limit to deterministic (i.e. saying exactly where a high or low pressure system, or front, or rainfall may be) prediction. We can make longer-term predictions, but those are statistical, not deterministic, in nature.