How Are Weather Forecasts Made?

Weather stations, balloons, ships, aircraft, radar and satellites constantly measure temperature, pressure, humidity and wind around the globe. These observations set the starting state for numerical weather prediction models, which divide the atmosphere into a 3D grid. Supercomputers solve equations of fluid motion and thermodynamics to predict how each grid box will change over time. Forecasters run 'ensembles' of slightly different simulations to estimate uncertainty, which is where percentage rain chances come from. Modern five-day forecasts are about as accurate as one-day forecasts were in the early 1980s.