# Frost Dates Finder

> Last spring and first autumn frost at your place, with the chance of a later or earlier frost. Finds the nearest weather station with frost statistics for 1991–2020 — Deutscher Wetterdienst for Germany, NOAA for the United States — and gives the last spring frost and the first autumn frost at three risk levels, a hard frost and the length of the frost-free season.

Part of usefulweb.site (https://usefulweb.site/). Part of [Hearth](https://hearth.usefulweb.site/) (ownerApp: hearth). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://hearthfrostdates.usefulweb.site/ (interactive tool at https://hearthfrostdates.usefulweb.site/tool)
- JSON API: POST https://hearthfrostdates.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://hearthfrostdates.usefulweb.site/api/v1/tool)
- OpenAPI: https://hearthfrostdates.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://hearthfrostdates.usefulweb.site/mcp - tool `hearth_frost_dates`

## Inputs
- place (text, required): Place (latitude, longitude) - Decimal degrees such as “52.47, 13.40”. Rounded to 0.01° (about 1 km).

## Method
- Germany: the nearest of 265 Deutscher Wetterdienst stations (within 40 km) with daily minimum temperatures for at least 25 of the years 1991–2020, from a bundled table built by scripts/data/hearth-dwd-frost.py. Frost = a daily minimum at 2 m of 0 °C or lower; hard frost = −2.2 °C or lower (NOAA’s 32 °F and 28 °F).
- Per year, the last spring frost is the last such day from January to July and the first autumn frost the first one from August to December; a year counts when March–June (or September–December) miss at most five days. The 10 / 50 / 90 % dates are percentiles over the years (linear interpolation): after the 10 % spring date a frost came in only one year in ten.
- United States: the nearest NOAA NCEI station with 1991–2020 freeze-date probabilities (annual/seasonal normals ANN-TMIN-PRBLST/PRBFST at 32 °F and 28 °F, same 10 / 50 / 90 % meaning) found with the NCEI search service in a box of ±0.4° (then ±1.2°) around the place; answers are cached for 30 days.
- Frost-free days per level = days between the last spring and the first autumn frost of that level. The station’s own median frost-free season is shown as well.

## Limits
- Frost dates describe 1991–2020 at a weather station; hollows, slopes and city gardens can differ by weeks. Watch the forecast before planting out tender plants.