Question 1

How is this different from the average last frost date?

Accepted Answer

The 'average last frost date' is just the 50th percentile: it means half the years will freeze later. This tool shows the full distribution. For tender crops, you really want to plant on the 10% date (90% of the time it won't freeze after that), not the average. Garden guides that only publish the average are technically correct but practically misleading.

Question 2

Where does the 30 years of data come from?

Accepted Answer

Open-Meteo's historical archive pulls from the ECMWF ERA5 reanalysis. The same global dataset NOAA climatology tables are built from. It covers every location on Earth at ~9 km resolution since 1940. The 32°F threshold is the standard light-freeze definition used by NOAA NCEI and cooperative extension services.

Question 3

Why only February through June?

Accepted Answer

This tool looks specifically at the last spring frost for planting-out decisions. The fall first-frost question is symmetrically important but belongs in a separate tool, so we fetch a narrower window and keep it fast. A fall frost probability tool is on the roadmap.

Question 4

Does this account for microclimate?

Accepted Answer

Only indirectly. ERA5 is a 9km gridded product, so it captures regional climate but not your specific south-facing brick wall or frost pocket. A south-facing slope with full sun and good drainage typically beats the ZIP average by 1-2 weeks on the last frost side. Urban heat islands can add 1-3 weeks of extension. Use the tool's number as a floor, then adjust for your site conditions.

Question 5

My zone didn't change after 2023. Is the historical data still relevant?

Accepted Answer

The 2023 USDA zone map update pushed about half the country a half-zone warmer because the climate normals shifted. This tool always uses the most recent 30 years, so the data automatically reflects the warming trend without you having to adjust. That's actually an advantage over published NCEI 1991-2020 tables, which are already three years stale.

Question 6

The tool said 90% safe is May 15, but I've seen frost on May 20. Is it wrong?

Accepted Answer

Not wrong, just stating probability. 90% safe means roughly 3 out of every 30 years WILL frost after that date. If you were gardening during one of those three years, you saw the tail event. That's why 'safe' dates for tender crops should be the 90% or even 95% percentile, and why you should keep frost blankets handy until the 99% date. The calculator is honest about how often the tail hits.

Frost Probability Calculator

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