What is a weather model?
A computer simulation of the atmosphere that is started from today's observations and stepped forward in time.
The idea in one picture
Air moves, warms, cools and carries water according to physical laws that are well known: Newton’s laws of motion, the conservation of mass and energy, and the behaviour of gases. A weather model writes those laws as equations and solves them on a computer. If you know the state of the atmosphere now, you can calculate what it will be a few minutes later, then repeat that step again and again to reach tomorrow, next week or the end of the passage.
The starting state is called the analysis. It is not simply a set of measurements, because measurements are sparse and uneven: there are many over land and in busy shipping lanes, few over remote ocean. The analysis blends all observations with the previous short forecast to build one complete, consistent picture of the whole atmosphere. Where the data comes from explains how.
Model, forecast and forecaster
- The model is the software and the equations. It produces numbers at every point of a grid, at fixed time steps.
- The forecast is what you read: a wind arrow, a rain symbol, a text bulletin. It may come straight from one model, from a blend of several, or from a forecaster who has looked at the models and added judgement.
- The forecaster adds what models handle badly: local knowledge, recent mistakes of each model, and uncertainty.
What a model gives you
| Output | Typical use | Comes from |
|---|---|---|
| Wind at 10 m, and gusts | Sail choice, reefing, routing | The atmospheric model |
| Pressure at sea level | Reading fronts and lows | The atmospheric model |
| Rain, cloud, temperature | Comfort, visibility, thunderstorm risk | The atmospheric model |
| Wave height, period, direction | Sea state, comfort, passage timing | A wave model driven by the wind forecast |
| Currents, sea level, ice | Tidal gates, harbour entries, ice limits | An ocean model |
What a model does not know
A model describes boxes of atmosphere, not your boat’s position. Anything much smaller than a few boxes, such as a sea breeze front, a gust off a headland or a single squall, is only approximated. It also cannot be better than its starting point: a small error in the analysis grows into a large error days later. These limits are explained in What km and NM mean on a model and How far ahead can you trust a forecast?