Frost dates for Rialto, California
California · first & last freeze, from NOAA 1991–2020 climate normals
In a typical year, the last spring freeze around Rialto comes near January 10 and the first fall freeze near January 2. One year in ten, a freeze still arrives after February 17 in spring — and one year in ten, the first fall freeze has already landed by December 11. NCEI's own growing-season statistic (days between freezes, percentiled per year) puts the median at 365 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) | January 1 | February 11 | March 12 | November 25 | December 13 | January 8 |
| 32°F — freezing | December 20 | January 10 | February 17 | December 11 | January 2 | February 7 |
| 28°F — hard freeze | reaches this temperature in only 2% of years — too rare for NCEI to compute dates | |||||
Measured at Riverside Citrus Exp, CA (USC00047473), 10.5 mi from the city's census reference point, elev. 986 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) | February 17 | December 11 | 297 days |
| Typical (median freeze dates) | January 10 | January 2 | 357 days |
| Gambler's (9-in-10 freeze risk at each end) | December 20 | February 7 | 365 days (effectively year-round at this risk level) |
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: Rialto city, California — official 2024 population estimate 104,918.
- Weather station: Riverside Citrus Exp, CA (USC00047473), 10.5 mi from the city's census-gazetteer reference point at elevation 986 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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