Method and accuracy
How Goldilocks Map counts just-right days
Every number is computed by our own model from public-domain climate data, then checked against real weather: for a typical city, our count lands within about 10 days a year of what 28 NOAA airport stations recorded from 2011 to 2020.
What "just right" means
A day counts when three things are true at once: the daily high falls inside your window, the air isn't muggy (the dew point stays at or under your limit), and the sky is sunny enough. Our default, used on every guide page, is 70 to 80°F highs, a dew point at or under 60°F, and cloud cover at or under 60%. That's narrower than NOAA's 64 to 86°F definition of a mild day, on purpose. On the map you can set your own.
Where the data comes from
- Daily highs and humidity: TerraClimate monthly normals for 1991 to 2020 (University of Idaho Climatology Lab), released to the public domain (CC0). Humidity is converted from vapor pressure to dew point.
- Cloud cover: CHELSA V2.1 climatologies for 1981 to 2010 (WSL), public domain (CC0), calibrated to match ERA5 daily cloud cover.
- Places: GeoNames (CC BY 4.0), towns and cities over 15,000 people.
- Map: © OpenStreetMap contributors, OpenFreeMap, OpenMapTiles.
How days are counted
Climate normals are monthly averages, so the model turns each month into a realistic spread of days. Daily highs vary around the monthly average by an amount fitted to real station data (more in continental climates, less on coasts), plus the seasonal drift within the month. Dew point varies the same way, and daily cloud cover follows a distribution matched to observed clear and overcast days. The chance that a day meets all three tests, summed over the year, gives the expected number of just-right days. The grid is about 35 km wide.
How accurate it is
| Check (2011 to 2020 daily records) | Typical error per city | Average bias |
|---|---|---|
| Goldilocks Map vs 28 NOAA stations, "just right" | ±10.5 days a year | +1.5 days |
| Goldilocks Map vs 28 NOAA stations, runner's weather | ±12 days a year | +1.7 days |
| ERA5 reanalysis vs the same stations, "just right" | ±20 days a year | −14 days |
Places rank in nearly the same order as reality (rank correlation about 0.93). The model is least precise on narrow coastlines and steep mountains, where weather changes faster than a 35 km grid can show, and it describes a typical year, not any particular year or forecast.
What we don't do
No AI writes our numbers. Every figure on every page is produced by code from the sources above. We don't copy other sites' rankings, scores or databases.
Notices: TerraClimate (Abatzoglou et al., 2018); CHELSA (Karger et al.); GeoNames (CC BY 4.0); © OpenStreetMap contributors; © OpenMapTiles. Accuracy checks use NOAA GSOD station records and ERA5 via Open-Meteo.com (CC BY 4.0), which contains modified Copernicus Climate Change Service information. Numbers are climate averages, not forecasts. All credits. Page data updated 2026-10-07.