How SowClock finds your frost dates
Where the numbers come from, how we compute them, how we checked them, and what they cannot tell you.
Last updated · Sources checked on
The data
What the sources say
Daily minimum and maximum temperatures and precipitation come from NOAA's Global Historical Climatology Network daily (GHCNd), a database of land weather-station observations with quality control.
(source: NOAA NCEI, GHCNd; Menne et al. 2012)We downloaded the station files from the public copy on the AWS Open Data registry (source: AWS Open Data, GHCN), and town names, coordinates, elevations and time zones from GeoNames (towns over 15,000 inhabitants, licensed CC BY 4.0) (source: GeoNames). Data processed on 1 November 2026.
The method
What we do
- Frost night: a day whose minimum air temperature at the station is 0.0 °C or below (values with a quality flag are dropped).
- Last spring frost of a year: the last frost night from 1 January to 15 July (northern hemisphere) or from 1 July to 31 December (southern). First autumn frost: the first from 16 July to 31 December (north) or from 1 January to 30 June of the next year (south).
- A half-year counts only if each of its months has at least 24 days of data. A station is used only with at least 20 usable years out of 1991-2020.
- Average date = the median of those years: in half of the years the last frost came later. Careful date = about 1 year in 10 (90th percentile for spring, 10th for autumn).
- Each town gets the nearest such station within 40 km and 150 m of height difference (25 km if the town's height is unknown). Towns with no such station are left out rather than guessed.
- Monthly averages 1991-2020: mean daily high and low temperature (months with at least 24 days of data) and mean monthly rainfall (complete months only), kept when at least 20 years are usable.
How we checked it
Cross-check with official normals
For a random sample of 60 US stations, we compared our median dates with NOAA's official 1991-2020 normals (50 % probability dates of the last spring and first autumn 32 °F freeze), which use their own statistical method. Over 113 comparisons, the median difference is 3 days, 9 in 10 are within 7 days, and the largest is 30 days. Monthly temperature averages differ by a median of 0.2 °C. These checks run automatically in our tests, together with checks of the moon phases and sunrise times against U.S. Naval Observatory tables.
What the sources say
NOAA's normals give the dates after which the chance of a freeze drops below 50 %, 40 %, 30 %, 20 % and 10 %.
(source: NOAA Climate.gov; NOAA NCEI, U.S. Climate Normals)Coverage
Towns over 15,000 inhabitants with a matching station, by country. Elsewhere, or if your town is missing, type your own dates.
| Country | Towns with data | Towns in list | Share |
|---|---|---|---|
| Australia | 88 | 89 | 99 % |
| Austria | 24 | 29 | 83 % |
| Belgium | 111 | 223 | 50 % |
| Canada | 269 | 297 | 91 % |
| Czechia | 7 | 117 | 6 % |
| Denmark | 30 | 63 | 48 % |
| Finland | 76 | 78 | 97 % |
| France | 435 | 657 | 66 % |
| Germany | 945 | 962 | 98 % |
| Ireland | 21 | 40 | 52 % |
| Italy | 17 | 598 | 3 % |
| Luxembourg | 3 | 3 | 100 % |
| Netherlands | 234 | 234 | 100 % |
| New Zealand | 0 | 47 | 0 % |
| Norway | 36 | 41 | 88 % |
| Poland | 17 | 344 | 5 % |
| Portugal | 118 | 174 | 68 % |
| Spain | 322 | 545 | 59 % |
| Sweden | 98 | 99 | 99 % |
| Switzerland | 49 | 73 | 67 % |
| United Kingdom | 539 | 803 | 67 % |
| United States | 3048 | 3122 | 98 % |
Low coverage (for example Italy, Poland, Czechia, and none for New Zealand) comes from gaps in the open station records available to us for 1991-2020, not from the climate.
Sun and moon
Our explanation
Sunrise, sunset and day length are computed in your browser with the equations of NOAA's Solar Calculator (sun 0.833° below the horizon). Moon phases use the formulas of Jean Meeus's Astronomical Algorithms. Both are checked against U.S. Naval Observatory tables: phases within a few minutes, sunrise and sunset within about a minute or two.
(source: NOAA Solar Calculator; U.S. Naval Observatory)Gardening tradition, not science
Moon phases are shown as a gardening tradition. Scientific evidence that they affect plants is lacking. (source: Illinois Extension)
Good to know: the limits
- Station records describe 1991-2020, not the future, and the climate is warming. (source: Met Office)
- A station up to 40 km away and 150 m higher or lower can have a different climate from your garden: frost hollows, hills, coasts and city centres all differ. (source: Utah State University Extension; Bureau of Meteorology)
- City-centre stations are often milder than gardens on the edge of town.
- Ground frost can occur when the station records a little above 0 °C. (source: Bureau of Meteorology)
- In mild climates, frost may occur in only some years; SowClock then says that frost is rare instead of giving a date.
All the code and tests are in the project repository; the data can be rebuilt with tools/data/frost_pipeline.py.
Sources
- NOAA NCEI: Global Historical Climatology Network daily (GHCNd)
- Menne et al. (2012), An Overview of the Global Historical Climatology Network-Daily Database, J. Atmos. Oceanic Technol. 29(7)
- Registry of Open Data on AWS: NOAA Global Historical Climatology Network Daily
- GeoNames: free geographical database (cities with a population over 15,000), CC BY 4.0
- NOAA Climate.gov: Interactive map, average date of last spring freeze across the United States (April 2025)
- NOAA NCEI: U.S. Climate Normals (1991-2020)
- NOAA Global Monitoring Laboratory: Solar Calculator
- U.S. Naval Observatory, Astronomical Applications: Data services
- University of Illinois Extension: Lunar Influence on Gardening (The Garden Scoop, April 2022)
- Met Office: Why frost can still occur in late spring (blog, 11 May 2026)
- Utah State University Extension: Suggested Vegetable Planting Dates, Grand and San Juan Counties (2024)
- Bureau of Meteorology: About frost
Sources checked on 1 November 2026.