How Can Anyone Predict Tomorrow?

Step 1 of 5

Air follows rules that can be written down.

Warm air rises, pressure differences drive wind, and vapour condenses when cooled. These are equations, not folklore.

Step 2 of 5

The world is divided into a grid of boxes.

The atmosphere is chopped into millions of boxes, each with its own temperature, pressure, humidity and wind, stacked in layers.

Step 3 of 5

Every box is filled in from real measurements.

Balloons, aircraft, ships, ground stations and satellites all report constantly, and their readings set the starting state.

Step 4 of 5

The computer steps the whole grid forward.

Each box is updated using its neighbours and the rules, one short step at a time, repeated until it has reached tomorrow.

Step 5 of 5

Small errors grow, which limits how far ahead.

Tiny gaps between boxes grow with every step, so forecasts are reliable for days rather than weeks no matter how good the computer is.

Forecast skill improves by roughly one day per decade — today's five-day forecast matches the accuracy of a one-day forecast in the 1980s.