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.