Each city guide identifies its NOAA station, reference period, temperature threshold and freeze probabilities. This page explains how the data is selected and used.

Climate source

Freeze probabilities come from NOAA / NCEI’s 1991–2020 U.S. Climate Normals, annual/seasonal dataset. We use the ANN-TMIN-PRBLST-T32FP10/30/50 and ANN-TMIN-PRBFST-T32FP10/30/50 fields. Each guide links directly to the original station CSV. Source snapshot imported 2026-10-16.

How a city gets a station

Population-center coordinates come from GeoNames. We consider populated places with at least 1,000 residents in this source snapshot. We select the nearest station present in the NOAA annual/seasonal inventory, within 35 km, and reject matches with more than 200 m elevation difference where both elevations are known.

Several localities may share the same nearest station. Their historical freeze dates remain identical; each guide shows its own location, station distance and local forecast link. We require complete, ordered 10%, 30% and 50% spring and fall dates. We do not silently replace a rejected station with a more distant one. This produces 8,227 locality guides using 3,868 distinct stations; it is deliberately not exhaustive coverage of every town or ZIP code.

ZIP-code lookup

Our GeoNames postal snapshot covers 41,488 ZIP codes; 37,362 have a station covered by this site within 35 km of the approximate postal point. We select the nearest of our published stations, which is not necessarily the nearest NOAA station in the full network. The result opens the closest covered locality using that station, preferring a locality in the same state when available.

Postal coordinates are approximate, ZIP areas can be large, and this lookup does not test elevation differences or terrain. The shown distance is from the postal point to the station, not your home. Unmatched codes do not receive extrapolated dates. No street address or remote geocoding service is used.

Missing and exceptional data

Missing dates, NOAA special-value codes, year-round freezing patterns and records without an ordered spring-to-fall interval are excluded. Places with no meaningful annual freeze date are not assigned a neighboring city’s date. Absence from this site does not imply that a place has no frost risk.

Reading the probabilities

The spring percentage is the probability of a freeze on or after that date. The autumn percentage is the probability of a freeze on or before that date. A lower percentage creates a narrower planning window. Dates use the 32°F threshold; visible ground frost and station air-temperature freezes are different phenomena.

How the planner works

The planner places the selected month/day into the chosen calendar year. It adds your chosen delay to obtain an outdoor target, then subtracts your chosen number of weeks for an indoor start. Its defaults are editable examples, not crop-specific advice. The spring date remains a climate normal regardless of the year selected. Planning-window lengths use a fixed non-leap reference year.

Crop windows and fall calculations

Indoor durations are sourced from University of Maryland Extension. City tables subtract the duration range from an illustrative outdoor target seven days after the selected spring date. Presets fill the duration only; they do not prescribe an outdoor planting date. The fall calculator subtracts packet maturity days and an editable slowing-growth allowance from the selected autumn boundary, within the proposed start year. It is calendar arithmetic, not a growth or weather forecast.

Limitations

Station conditions may differ from your garden. Historical distributions do not predict a specific year, and the climate can change. City-level matching cannot represent all urban heat, topography, cold-air drainage or exposure. Always check the current forecast and your local extension guidance.

Credits & reuse

  • Climate data: NOAA / NCEI. This independent site is not affiliated with or endorsed by NOAA.
  • Location data: GeoNames, CC BY 4.0. We filter populated places, create URL slugs, match coordinates to climate stations and derive a postal-code lookup; this is a derived subset, not an unmodified GeoNames distribution.
  • Map: US Atlas, based on U.S. Census Bureau boundaries.
  • Garden photograph: Unsplash source image, used under the Unsplash license.
  • Typography: DM Sans, locally hosted under the SIL Open Font License.