Mathematics · Explainer
Why weather forecasts stop working after about two weeks
Weather is deterministic, yet it cannot be forecast beyond about two weeks. Tiny errors in today's measurements grow so fast that they swamp the forecast, a limit found by Edward Lorenz in 1963 and confirmed with modern models in 2019. No better computer or denser sensor network can push much past that ceiling.
Not a technology problem
It is tempting to think forecasts fail because our computers or sensors are not good enough yet, and that one day we will predict the weather months ahead. We will not, and the reason is mathematical, not technological.
The atmosphere follows fixed physical laws, so in principle it is deterministic: the same starting state always leads to the same weather. The catch is that we can never know the starting state exactly. Every measurement has a tiny error, and in weather those tiny errors do not stay tiny.
How a rounding error reroutes the future
In 1963 Edward Lorenz was rerunning a weather simulation and typed in a number rounded to a few decimal places instead of its full precision. The forecast that came out was completely different. A difference far too small to measure in the real world had rerouted the entire simulated weather.
This is sensitive dependence on initial conditions, the heart of chaos. In a chaotic system, two nearly identical starting points pull apart exponentially fast. The gap between them roughly doubles over and over until, after a couple of weeks of atmosphere, an unmeasurably small initial difference has grown into two totally different forecasts. The name that stuck for this is the butterfly effect.
Living with the limit
A 2019 study using today's best models put a firm number on it: even with near-perfect data and physics, midlatitude weather has a hard predictability ceiling of about two weeks. That is a property of the atmosphere, not of the forecaster.
So forecasters stopped trying to make one perfect prediction. Weather centers now run ensembles: dozens of simulations started from slightly different conditions. Europe's ECMWF runs 51 at once, and the US GEFS runs 31. Where the simulations agree, the forecast is confident; where they scatter, it is not. Instead of pretending to know the exact future, they measure how uncertain it is.
Sources & further reading
Free to start. Sign in with Google, no card.