Sprout Authority NOAA 1991-2020 Frost Dates by ZIP
=========================================================

Version: 1.0.0
Published: 2026-08-01

This transformed dataset assigns each listed ZIP to the nearest NOAA station
with 1991-2020 annual and seasonal freeze-date normals. It combines the current
USDA 2023 hardiness-zone ZIP listing, Census 2024 ZCTA representative points,
and the NOAA station normals used by Sprout Authority's planting tools.

Coverage
--------
- 40,502 ZIP rows
- 40,472 ZIPs with a station assignment
- 30 ZIPs without a station assignment
- 6,720 distinct NOAA stations referenced
- 33,474 assignments from a ZIP's Census ZCTA representative point
- 6,998 assignments from the documented prefix-centroid fallback

Method and limits
-----------------
For a ZIP represented by a Census ZCTA, the station is the nearest NOAA station
to that ZCTA representative point. A ZIP without its own ZCTA uses the mean
centroid for the same 3-digit ZIP prefix, or a nearby prefix only when the
existing USDA-zone consistency guard accepts it. The dataset does not estimate conditions between stations.
Thirty ZIPs remain unassigned because no
climate-consistent fallback was available.

Station distance is the mapped great-circle distance in kilometers. These are
station normals, not an address-level forecast, and they do not establish local frost outcomes.
Topography, water, urban heat, and other microclimates can shift actual dates.

Date values use MM/DD on a non-leap calendar. "frost-free" preserves NOAA's
source state that the freeze does not occur at that threshold and probability.
Empty fields mean the source value or station mapping is unavailable.

Growing-season days are derived only when the matching 32 F last-spring and
first-fall values are both dates. A fall date on or before the spring date wraps
into the following calendar year. A matching frost-free pair yields 365 days.

Columns
-------
- zip_code: five-digit ZIP string; leading zeros are significant
- usda_zone_2023: current USDA half-zone, when present
- noaa_station_id: NOAA station identifier
- noaa_station_name: station name from the NOAA normals source
- station_distance_km: stored ZIP-to-station distance, to one decimal place
- mapping_method: zcta-centroid or prefix-centroid-fallback
- last_spring_freeze_32f_p10/p50/p90: NOAA 32 F last-freeze normal
- first_fall_freeze_32f_p10/p50/p90: NOAA 32 F first-freeze normal
- growing_season_days_32f_p10/p50/p90: directly derived matching-season length

Sources and versions
--------------------
- USDA Plant Hardiness Zone Map ZIP listings, 2023:
  https://prism.oregonstate.edu/phzm/
- NOAA NCEI U.S. Climate Normals, 1991-2020 v1.0.1 c20230404:
  https://www.ncei.noaa.gov/data/normals-annualseasonal/1991-2020/
- U.S. Census Bureau ZCTA Gazetteer, 2024:
  https://www2.census.gov/geo/docs/maps-data/data/gazetteer/2024_Gazetteer/

The underlying U.S. Government data is public domain. This transformed table is
not an official NOAA, USDA, Oregon State University, or Census Bureau product.
No agency endorsement is implied.

Integrity
---------
CSV: /data/noaa-frost-dates-by-zip-1991-2020.csv
Rows: 40,502
SHA-256: 22032060118a3eb5808ec070ddce8d5c6a052f13cb52b27fa79aa7bb191004e5

Suggested citation
------------------
Sprout Authority. NOAA 1991-2020 Frost Dates by ZIP. Version 1.0.0
(2026-08-01). Derived from NOAA NCEI U.S. Climate Normals, USDA 2023
hardiness-zone ZIP listings, and Census 2024 ZCTA representative points.
https://sproutauthority.com/tools/frost-dates