Data sources and methods

Frost and freeze dates

NOAA National Centers for Environmental Information (NCEI), U.S. Climate Normals 1991–2020, annual/seasonal normals by station (v1.0.1). We use the agricultural normals: ANN-TMIN-PRBLST (last spring freeze), PRBFST (first fall freeze) and PRBGSL (growing season) at 36°F, 32°F and 28°F, and PRBOCC (share of years with a freeze). U.S. government data, public domain.

Reading the odds. Spring: the 90% date is when 9 years in 10 still have a later freeze; the 10% date is when only 1 year in 10 does. Fall: the 10% date is when 1 year in 10 has already frozen; the 90% date is when 9 years in 10 have.

Matching a ZIP to a station

Each ZIP is a Census ZIP Code Tabulation Area (ZCTA) with its internal point from the 2024 Census Gazetteer. We pick the nearest NOAA station that has published 32°F spring and fall dates at the 10/50/90% levels, preferring standard or representative records over estimated ones, and (where we have a USGS elevation for the ZIP point) switching to a nearby station at a closer elevation when the nearest one differs by more than 150 m. Pages show the station, its distance, and three nearby stations so you can judge local variation. County and place names come from the Census 2020 ZCTA relationship files.

ZCTAs approximate USPS ZIP codes. PO-box-only and single-business ZIPs have no ZCTA and no page.

Hardiness zones

2023 USDA Plant Hardiness Zone Map ZIP listing (phzm_us_zipcode_2023.csv), PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed Oct 17, 2026. Shown as text, unaltered. This site is not the official USDA Plant Hardiness Zone Map; see planthardiness.ars.usda.gov.

Limits

Normals describe 1991–2020. They are not a forecast, and local conditions (low spots, slopes, cities) can shift dates by days or weeks.