Why can weather prediction be so correct?

I've done a test using ARMA model on some financial series. It turns out the prediction rate is really very bad – close to half time correct and half time wrong…

I am new to ARMA model so what I tried is very simple, following the textbook of deciding (p,q) for ARMA model by ACF and PACF first, and then use half data to do regression and half data to do test.

I often heard that weather forecasting is used time series model. And in my daily experience, I feel it is pretty good forecasting.

I wonder why the weather forecast could be done so great?

Also, if I want to push my toy ARMA model to next level, what is the direction that I should put into effort ?

Following up:
There are good answers below, so I am summarize it a little up :

1. From a systematic point of view, market as a system is evolving, while weather is more stable from one year to another (truth doesn't change).
2. Market is more complicated in some sense, while weather is ruled by physics laws (more understood structure).
3. Weather is more periodic, which adds to the predictability.
• I thought weather forecasters were using models similar to those used in general circulation models for predicting climate change. Hence they are modelling the system via the known physics, which will help. – Gavin Simpson Jul 24 '13 at 22:39
• I think this worded a bit too loosely, what you mean about correct?. Broadly speaking, weather prediction for two months ahead is pretty much sci-fi; it is mostly based on historical data and a lot of simulation (if they have a lot of money). Climate-change models (so long-term weather predictions if you like) are really a hot-bed for Statistics now. For your second question: Play with a GARCH model. – usεr11852 Jul 24 '13 at 22:49
• "My approach is rather naive ... why the weather forecast could be done so great?". Simply: because it's not so naive. – Glen_b Jul 24 '13 at 23:12
• As a quick follow on, weather predictions based on physics are generally considered to have some added value out to about 14 days (10 is probably a better answer). Beyond that, either climatology (e.g. the average in some place at some point in the year) or persistence (e.g. the same weather as the day before) will generally beat physical forecast models on average. – Thursdays Coming Jul 25 '13 at 1:16
• I just wanted to add that if you want to know a bit more about weather prediction, there's a whole chapter on it in Nate Silver's The Signal and the Noise (a pop science book.) – Flounderer Jul 25 '13 at 7:18