USDA Hardiness Zone 10a
Frost Dates for 91761 — Ontario, California
NOAA 1991–2020 normals from Ontario International Airport, CA (949 ft), 2 miles north.
Updated
Frost is not a yearly event in Ontario: a night at or below 32°F came in 41% of the winters of 1991–2020 at Ontario International Airport, CA, and a 36°F night, cold enough for frost on exposed leaves, in 93%. There is no reliable "last frost date" here; most years have none.
- Winters with a freeze
- 41%
- Winters with 36°F
- 93%
- Coldest month low
- 44°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 Ontario
At Ontario International Airport, CA, 2 miles north of Ontario, 41% of the winters of 1991–2020 had at least one night at or below 32°F, 4% reached 28°F. Nights of 36°F, enough for frost on exposed leaves, are more common (93% of winters) and, when they come, fall between about December 17 and January 27. December is the month most likely to bring one: a 21% 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°F93% of yrs | Dec 24 | Jan 1 | Jan 8 | Jan 17 | Jan 27 | Feb 3 | Feb 11 | Feb 20 | Feb 28 |
| 32°F41% of yrs | too rare for a date | ||||||||
| 28°F4% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F93% of yrs | Nov 27 | Dec 4 | Dec 9 | Dec 13 | Dec 17 | Dec 21 | Dec 25 | Jan 2 | Jan 17 |
| 32°F41% of yrs | too rare for a date | ||||||||
| 28°F4% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Ontario International Airport, 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 91761 →Compare Ontario with another place →
Weather by month in Ontario
August is the warmest month at Ontario International Airport, CA, averaging 95°F by day and 65°F at night; December is the coldest at 67°F and 44°F. From February to November the chance of a 32°F night stays under 10% each month.
Warmth for growing adds up to 6,189 growing degree days a year (base 50°F), more than at 94% of the 283 California stations. 38% of them (2,375) have built up by the end of June, and August adds the most, 932. Precipitation totals 11.7 inches a year, most in February (3.1") and least in June (0.0"); May to September bring 4% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 68 | 45 | 2.6 | 19% | 1 | 202 |
| Feb | 68 | 46 | 3.1 | 6% | <1 | 207 |
| Mar | 72 | 49 | 1.6 | 0% | 0 | 319 |
| Apr | 76 | 51 | 0.8 | 0% | 0 | 403 |
| May | 80 | 56 | 0.3 | 0% | 0 | 549 |
| Jun | 86 | 60 | 0.0 | 0% | 0 | 696 |
| Jul | 94 | 65 | 0.1 | 0% | 0 | 907 |
| Aug | 95 | 65 | 0.0 | 0% | 0 | 932 |
| Sep | 91 | 64 | 0.1 | 0% | 0 | 826 |
| Oct | 83 | 57 | 0.4 | 0% | 0 | 615 |
| Nov | 75 | 49 | 0.8 | 1% | <1 | 357 |
| Dec | 67 | 44 | 1.9 | 21% | 1 | 177 |
Monthly normals 1991–2020 at Ontario International Airport, CA (949 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, Chino Airport, CA, 4 miles south of Ontario and 299 feet lower, has a 32°F night in 85% of winters against 41% here, and a coolest-month low 4°F colder (40°F).
When it snows in Ontario, California
Measurable snow is rare here: only 0 of 19 winters from 1991 to 2020 recorded 0.1 inch or more at Riverside Fire Stn 3, CA, 13.9 miles away, so there is no typical first-snow date to plan around.
Where these figures come from
ZIP 91761 is 2 miles from Ontario International Airport, CA, which stands at 949 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 12 other ZIP codes share the station; its figures are presented in full for 92336 (Fontana). The 2023 USDA Plant Hardiness Zone Map lists ZIP 91761 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.