The seven day weather forecast defines hubris. The weatherboys and girls with their vacuous smiles, and pointy sticks pretend to tell you in minute detail what the weather will be in seven days time. I've got news for them. This is impossible!

The reason for this is that the weather is a chaotic dynamical system. This means that even if we assume that their mathematical model for the weather is perfect (a big "if" but let's be charitable) they must measure the initial conditions for the system accurately. But they can't do this unless they measure the pressure, temperature, and so on at every point of the earth's surface. Inevitably they can only make measurements at a discrete set of points. In between these points they estimate, making small errors, and as time progresses these small errors magnify into larger ones, and these larger errors... This idea (due to Edward Lorenz) is popularly known as the Butterfly Effect.