About the Frost Date Data

Where our frost dates by ZIP code come from, how we match ZIPs to weather stations, and where the numbers can be off.

Sources

NOAA NCEI U.S. Climate Normals (1991–2020), with a comparison against the prior 1981–2010 normals — per-station probability dates for the last spring and first fall occurrence of 32°F and 28°F, at 10%, 50%, and 90% likelihood. ncei.noaa.gov/products/land-based-station/us-climate-normals
US Census Bureau 2025 ZCTA Gazetteer File — latitude/longitude for each ZIP Code Tabulation Area, used to place each ZIP and find its nearest weather station. census.gov 2025 ZCTA Gazetteer
USDA-ARS / Oregon State University PRISM Plant Hardiness Zone Map, 2023 and 2012 ZIP-code files — used for the hardiness zone shown for each ZIP and the zone-change comparison. 2023 zone file and 2012 zone file. This is not the official USDA Plant Hardiness Zone Map — it’s our own lookup built from USDA-ARS/PRISM’s published data.

How we match your ZIP to a station

Every ZIP is matched to the nearest NOAA weather station with usable Climate Normals data, using straight-line (haversine) distance between the ZIP’s coordinates and the station’s. We cap the search at about 30 miles — beyond that, distance from a real station starts to matter more than the average itself, so we flag the result as low-confidence instead of guessing.

Some ZIP codes — mostly PO boxes and other unique ZIPs that the Census Bureau doesn’t assign a ZCTA to — have no coordinates of their own. For those, we fall back to the average location of nearby ZIPs sharing the same 3-digit prefix, and mark the result as a prefix fallback. Every result on the tool shows its station name, distance, and any low-confidence flag so you can judge how much to trust it.

Limits

Microclimates.Elevation, slope, and nearby water can shift your yard’s actual frost timing by a week or more from the nearest station’s average.
Coastal and mountain areas. Frost timing changes quickly over short distances near coastlines and in mountainous terrain, so a station a few miles away may not represent your exact spot well.
Prefix-fallback ZIPs. ZIPs with no ZCTA location use a nearby-area average rather than their own coordinates, which is less precise.
Averages, not forecasts. These are 30-year statistical normals, not a prediction for this year. Actual frost can come earlier or later than any of the 10/50/90% dates shown.

How to cite this data

This dataset is CC BY (attribution requested) — free to use, embed, or republish with a credit and link back. Suggested attribution:

Frost date and hardiness zone data via Trowelful (trowelful.com/frost-dates), based on NOAA and USDA-ARS/PRISM climate data.

CSV download

Download the full dataset as CSV, free to use under the CC BY terms above:

National CSV (all ZIPs)
Individual states, e.g. California, Texas, and New York — all 50 states plus DC are available at /data/frost/csv/<2-letter-state>.csv.

Embed this tool

Want frost dates on your own site? Paste this snippet where you want the widget to appear, and swap in your ZIP or leave it blank for a live input.

<!-- Embeddable frost date widget — coming soon, not yet live -->
<iframe
  src="https://trowelful.com/embed/frost?zip=80202"
  width="100%"
  height="420"
  style="border:0"
  loading="lazy"
  title="Frost dates by ZIP — Trowelful"
></iframe>

Note: the embeddable widget isn’t live yet — this snippet shows the planned format. In the meantime, link to the frost dates tool directly.