Frost dates for Boulder, Colorado
Colorado · first & last freeze, from NOAA 1991–2020 climate normals
In a typical year, the last spring freeze around Boulder comes near May 6 and the first fall freeze near October 5. One year in ten, a freeze still arrives after May 18 in spring — and one year in ten, the first fall freeze has already landed by September 21. NCEI's own growing-season statistic (days between freezes, percentiled per year) puts the median at 151 days.
Freeze-date probabilities, three thresholds
| Threshold | Last spring — 9 in 10 years past by | Last spring — typical (median) | Last spring — 1 in 10 years later than | First fall — 1 in 10 years by | First fall — typical (median) | First fall — 9 in 10 years by |
|---|---|---|---|---|---|---|
| 36°F — near-freezing (frost can occur) | May 5 | May 17 | June 2 | September 13 | September 28 | October 10 |
| 32°F — freezing | April 24 | May 6 | May 18 | September 21 | October 5 | October 18 |
| 28°F — hard freeze | April 10 | April 27 | May 10 | September 30 | October 15 | October 30 |
Measured at Boulder, CO (USC00050848), 2.4 mi from the city's census reference point, elev. 5,484 ft. NOAA/NCEI 1991–2020 normals. Read each date column as odds over the 30-year record — e.g. "9 in 10 years past by" means only one spring in ten saw a later freeze than that date.
Frost-free windows (computed)
| Window (32°F ends) | Opens | Closes | Length |
|---|---|---|---|
| Cautious (1-in-10 freeze risk at each end) | May 18 | September 21 | 126 days |
| Typical (median freeze dates) | May 6 | October 5 | 152 days |
| Gambler's (9-in-10 freeze risk at each end) | April 24 | October 18 | 177 days |
Pure arithmetic on the freeze-date percentiles above — the "cautious" window opens on the date only 1 year in 10 still sees a later spring freeze, and closes on the date by which only 1 year in 10 has already frozen.
About this city's numbers
- Census entity: Boulder city, Colorado — official 2024 population estimate 106,803.
- Weather station: Boulder, CO (USC00050848), 2.4 mi from the city's census-gazetteer reference point at elevation 5,484 ft. Frost dates are that station's history — neighborhoods vary.
- Data pulled 2026-09-05; normals period 1991-2020 (replaced by NOAA roughly once a decade).
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