text: what are atmospheric chaos and the butterfly effect? and how do they affect forecasting?   in 1960 a meteoroligist named edward lorenz was researching into the possibilities of long term weather prediction. he created a basic computer program using mathematical equations which could theoretically predict what the weather might be. one day he wanted to run a particular sequence again, and to save time he started it from the middle of the sequence. after lettin gthe sequence run he returned to find that the sequence had evolved completely different from the original. at first he couldn't comprehend such different results but then realised that he had started the sequence with his recorded results to 3 decimal places, whereas the computer had recorded them to 6 d.p. as this program was theoretically deterministic we would expect a sequence very close to the original, however this tiny difference in initial conditions had given him completely different results.  this sensitivity to initial conditions became known as the butterfly effect. this is because it compares to the idea that a butterfly flapping its wings could produce a tiny change in the atmosphere, and trigger a series of changes which could eventually lead to a hurricane on the other side of the world. the butterfly effect is a key characteristic to a choatic system.  john