Many people consider the weather as a system which exhibits chaotic behaviors ('the butterfly effect' introduces weather as an example for chaos). But what are the reasons for that? Why are we so sure that weather is chaotic? Is it because people can't manage to predict it a long time in advance, or is there a deeper explanation for this? As I understand a chaotic systems is a systems which exhibits dynamics that are highly sensitive to initial conditions. Which means that the ratio between initial uncertainty and the uncertainty after a period of time is very small (which also gives a possible measurable quantity for chaos, or rather the inverse of chaos). Does this definition seem valid?