Why different apps show different forecasts
Which model an app uses, how it picks a point from the grid, and what it adds on top.
Eight reasons two apps disagree
| Reason | What happens |
|---|---|
| Different model | One app shows a global model at 13 km, another a regional model at 2 km, a third a blend. They start from different data and use different physics |
| Different run | One app updated an hour ago, the other has the previous run |
| Picking a point from the grid | Your position lies between grid points. An app may take the nearest one, or interpolate between several. Near a coast the nearest point may be on land |
| Corrections | Some apps adjust the model for known errors, for height, for distance from the coast or for typical bias of the station; others show raw output |
| Blending | Some average several models or pick the model that scored best recently; the result is smoother than any single one |
| Which wind | Mean wind or gust? 10 m or another height? Ten-minute mean or hourly? Without these the numbers are not comparable |
| Rounding and time | Different units and rounding; times in different time zones; hourly values that mean the hour before or the hour after |
| Presentation | Colours, icons and wording for rain probability or cloud differ even when the numbers do not |
What the services in the catalog do
Many of the popular tools for sailors are aggregators: they show output from public models, and some also run their own. We list them to track how soon a new run appears in each, not to rank them, for example Windy, PredictWind, Windguru, Windfinder, Meteoblue and Open-Meteo. Each decides which models to show and how. Read the app’s own description of its sources.
How to compare two apps fairly
- Pick the same model in both, if each allows it. If not, find out which model each is showing.
- Set the same time zone (UTC is simplest) and the same units.
- Check the same quantity: mean wind or gusts, at 10 m.
- Compare the same run time.
- Look at the same place: zoom in so that the point is not a different grid box.
- Apps that refresh only when you open them may show data several runs old; look for a “model run” or “updated” time.
- A single number for “wind” on a map is the mean wind from one model; the gust is usually available on a separate layer.