Step 1
Read NOAA freeze normals
Use the 32°F first- and last-freeze probability fields from the 1991-2020 U.S. Climate Normals v1.0.1 archive.
Open calculation method
How we calculate frost dates from NOAA data
The calculator does not invent a frost date or call an undocumented third-party service. It combines published NOAA freeze probabilities with Census ZIP-area locations, then shows the station and distance behind each result.
Step 2
Keep qualified stations
Require all six spring and fall probability dates, NOAA S completeness and at least 24 years of observations for every included field.
Step 3
Approximate the ZIP area
Use the internal point for the matching 2025 Census ZIP Code Tabulation Area. A ZCTA is an approximation, not a USPS delivery boundary.
Step 4
Match a nearby station
Find the nearest qualified station by great-circle distance and return no result when that station is more than 100 miles away.
Current data build
What is actually in the calculator
- Qualified stations
- 2,393
- Mapped ZCTAs
- 33,474
- Minimum record length
- 24 years
- Maximum station distance
- 100 miles
The build dated July 17, 2026 intentionally leaves 317 ZCTAs unmatched because no qualified station is within 100 miles. Coverage is not filled with guessed dates.
How to read the three planning choices
| Choice | Spring interpretation | Fall interpretation |
|---|---|---|
| More cautious, 10% risk | A later date that leaves a larger margin against another 32°F freeze. | An earlier date that leaves a larger margin before the first 32°F freeze. |
| Middle estimate, 50% risk | In about half of historical years, the last spring freeze happened after this date. | In about half of historical years, the first fall freeze happened before this date. |
| Higher risk, 90% risk | An earlier planning date with less protection from a late freeze. | A later planning date with less protection from an early freeze. |
Checks built into the data pipeline
The spatial search is compared with brute-force nearest-station calculations for a deterministic sample of 250 ZCTAs. Automated integrity checks also reject missing station fields, distance-limit violations and frost windows in the wrong order.
What the method cannot know
A station normal cannot predict this year's weather or capture every hill, frost pocket, paved heat island and sheltered bed. The result is a planning reference. Use the current local forecast and observations from your own garden before acting.
Inspect a result instead of trusting a hidden number
Every successful lookup shows the NOAA station name and ID, distance from the ZIP-area center, record length, climate-normal edition and all three probability dates. Results can also be downloaded as CSV.