USDA Hardiness Zone 10a
Frost Dates for 92377 — Rialto, California
NOAA 1991–2020 normals from Riverside Citrus Exp, CA (986 ft), 13 miles south.
Updated
Frost is not a yearly event in Rialto: a night at or below 32°F came in 45% of the winters of 1991–2020 at Riverside Citrus Exp, CA, and a 36°F night, cold enough for frost on exposed leaves, in 98%. There is no reliable "last frost date" here; most years have none.
- Winters with a freeze
- 45%
- Winters with 36°F
- 98%
- Coldest month low
- 43°F
- Hardiness zone
- 10a
32°F or below, 1991–2020
frost possible on leaves
average, December
30 to 35°F (-1.1 to 1.7°C) coldest night
- First snow
- Rare
- Last snow
- Rare
- Winters with snow
- 0%
- Snow per year
- 0"
median, 1991–2020
median, 1991–2020
0 of 19 observed
NOAA normal
Frost risk by date in Rialto
At Riverside Citrus Exp, CA, 13 miles south of Rialto, 45% of the winters of 1991–2020 had at least one night at or below 32°F, 2% reached 28°F. Nights of 36°F, enough for frost on exposed leaves, are more common (98% of winters) and, when they come, fall between about December 13 and February 11. January is the month most likely to bring one: a 25% chance in a given year. 62 of the 283 other California stations behind this site's ZIP pages freeze in fewer than half of winters too.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F98% of yrs | Jan 1 | Jan 14 | Jan 24 | Feb 3 | Feb 11 | Feb 18 | Feb 24 | Mar 3 | Mar 12 |
| 32°F45% of yrs | too rare for a date | ||||||||
| 28°F2% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F98% of yrs | Nov 25 | Dec 1 | Dec 5 | Dec 9 | Dec 13 | Dec 19 | Dec 24 | Dec 29 | Jan 8 |
| 32°F45% of yrs | too rare for a date | ||||||||
| 28°F2% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Riverside Citrus Exp, CA, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 92377 →Compare Rialto with another place →
Weather by month in Rialto
August is the warmest month at Riverside Citrus Exp, CA, averaging 97°F by day and 64°F at night; December is the coldest at 69°F and 43°F. From February to November the chance of a 32°F night stays under 10% each month.
Warmth for growing adds up to 6,087 growing degree days a year (base 50°F), more than at 92% of the 283 California stations. 38% of them (2,336) have built up by the end of June, and August adds the most, 935. Precipitation totals 9.9 inches a year, most in February (2.5") and least in July (0.0"); May to September bring 6% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 70 | 44 | 2.1 | 25% | 1 | 219 |
| Feb | 70 | 45 | 2.5 | 9% | <1 | 209 |
| Mar | 72 | 47 | 1.5 | 1% | 0 | 301 |
| Apr | 75 | 50 | 0.6 | 0% | 0 | 375 |
| May | 80 | 55 | 0.3 | 0% | 0 | 546 |
| Jun | 87 | 58 | 0.1 | 0% | 0 | 687 |
| Jul | 94 | 63 | 0.0 | 0% | 0 | 884 |
| Aug | 97 | 64 | 0.0 | 0% | 0 | 935 |
| Sep | 93 | 61 | 0.2 | 0% | 0 | 813 |
| Oct | 82 | 55 | 0.4 | 0% | 0 | 569 |
| Nov | 76 | 47 | 0.7 | 1% | <1 | 354 |
| Dec | 69 | 43 | 1.5 | 18% | 1 | 198 |
Monthly normals 1991–2020 at Riverside Citrus Exp, CA (986 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Ontario International Airport, CA, 13 miles southwest of Rialto, has a 32°F night in 41% of winters against 45% here, and a coolest-month low 1°F warmer (44°F).
When it snows in Rialto, California
Measurable snow is rare here: only 0 of 19 winters from 1991 to 2020 recorded 0.1 inch or more at Riverside Citrus Exp, CA, 12.9 miles away, so there is no typical first-snow date to plan around.
Where these figures come from
ZIP 92377 is 13 miles from Riverside Citrus Exp, CA, which stands at 986 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 8 other ZIP codes share the station; its figures are presented in full for 92376 (Rialto). The 2023 USDA Plant Hardiness Zone Map lists ZIP 92377 in zone 10a, the band where the coldest night of the year averaged 30 to 35°F (-1.1 to 1.7°C) over 1991–2020; a zone describes winter lows, not frost dates.
Frost dates across California →
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.