Guide
How to read a weather forecast without being fooled
Two apps show different weather for the same location and the same hour. Neither of them is necessarily wrong – here's why.
A forecast isn't an observation but the result of a calculation. Weather data from satellites, balloons, ships and ground stations is fed into a model that computes the state of the atmosphere step by step. Different institutes run different models at different resolutions, which is why the answers differ – especially for local phenomena like showers and fog.
Precipitation probability doesn't mean what you think
"40% chance of rain" doesn't mean it rains 40 percent of the time, nor that 40 percent of the area gets wet. It means that in four cases out of ten, measurable precipitation falls at your exact spot during the stated period. In summer shower weather, the probability can be low while whoever happens to be caught under the shower gets ten millimetres in twenty minutes.
How far ahead is a forecast useful?
| Horizon | Reliability | Use it for |
|---|---|---|
| 0–6 hours | Very high | Showers, timing, same-day decisions. |
| 1–2 days | High | Outings, boat trips, planning outdoor work. |
| 3–5 days | Reasonable for wind and temperature | Rough planning, keep an eye on changes. |
| 6–10 days | Trend, not detail | General character: mild, windy, wet. |
| Longer | Statistics | Seasonal tendency, nothing more. |
Read the right number
Always check whether the wind is stated as mean wind or gusts – the difference is often 50 percent. Temperature applies in the shade at two metres above ground; in the sun on a terrace it can feel ten degrees warmer, and on a clear night the ground temperature can sit several degrees below the forecast air temperature, which is why frost turns up at "plus two". Precipitation is sometimes given as a total for the whole period and sometimes as intensity per hour; a bar chart showing 6 mm over six hours is something quite different from 6 mm per hour.
Why the apps disagree
Many apps show a single model, often a coarse-resolution global one, while national institutes combine several models and add the meteorologist's judgement. Point forecasts are also interpolated from grids that can be several kilometres wide, so a coastline, a lake or a range of hills in the middle of the grid cell gets averaged out. If you live somewhere with strong local contrasts – near the coast, in a valley, by a large lake – the forecast will almost always be slightly off.
The best approach is to look at how the forecast has changed over the past few days. If it's holding steady, the uncertainty is small. If it jumps back and forth between updates, the situation is uncertain, and you should plan with a margin regardless of what the latest figure happens to show.
From number to picture
Once you know which number to look at, you can pull the forecast straight into the visualiser, choose day and hour, and see what it actually means for wind, sea and rain.
Open the visualiser