Why can data models predict the future climate but not the weather?
Why are weather forecasts often wrong but modelling of the future climate highly accurate? I would presume a more complex system over a longer period would be harder to predict.
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Imagine tracking a molecule of air as it moves around inside a balloon: its speed, trajectory, spin, etc. Now try to predict how that molecule will behave as it moves through that balloon as it bounces off of the rubber and into other air molecules.
I hope you can understand that this would be incredibly, INCREDIBLY hard to do with accuracy given how many variables you’d need to track, and how small variations can throw off your estimates very rapidly.
BUT… what if instead of tracking the behavior of individual molecules, we track the behavior of all those trillions of air molecules as a whole: temperature, pressure, volume, etc. This is much easier, because the individual behaviors of all those particles will average out. This is how we get the ideal gas law: PV = nRT.
Another way to think of it is to consider predicting the results of a coin flip: how many times will it land on heads? You’ll only be able to get it right 50% of the time. But what if we consider 10 coins, and consider the results as a whole: how many times will the heads come up between 40% and 60% of the time? That’s a lot easier, and can be mathematically tracked along what’s called a [normal distribution](https://www.fourmilab.ch/rpkp/experiments/statistics.html). Furthermore, when you increase that number to 100 coins, or 1000 coins, that normal distribution gets narrower and narrower, because as you add more coins the system “averages out” more and more.
The larger a system is, the more accurately you can gauge its “average state.” Tomorrow’s weather is a much smaller system than the overall climate (which can be roughly seen as an average of a region’s weather patterns over a long period of time). The idiosyncrasies of daily changes to the weather effectively average out over a long period and is easier to predict, and trends are easier to observe.
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