Just thought I'd throw this in - it's taken from a sign outside the Bureau of Meteorology in our building:

"Imagine a system on a rotating sphere that is 8000 miles wide, consists of different materials, different gases that have different properties (one of the most important of which, water exists in three different states), heated by a nuclear reactor 90 million miles away. Then, just to make life interesting, this sphere is oriented such that, as it revolves around the nuclear reactor, it is heated differently at different times of the year. Then, someone is asked to watch the mixture of gases, a fluid only 20 miles deep, that covers an area of 250 million square miles, and to predict the state of that fluid at one point on the sphere two days from now. This is the problem that Weather Forecasters face every day."