Frost dates in Ontario, California
NOAA 1991–2020 normals for a 32 °F freeze. Low spots and the edge of town run colder than downtown.
Short answer
The average last spring frost in Ontario is January 4, and the average first fall frost is December 30, from NOAA's Ontario Intl Airport station, 1 mile from the middle of Ontario. 1 year in 10 the last frost comes after February 7, and the first one by December 11. That leaves about 360 frost-free days. Ontario is in USDA planting zone 10a.
Weather stations around Ontario
The first row is the date most people in Ontario go by. The others show how far the dates move across the area.
| Station | Last spring frost | First fall frost | Frost-free |
|---|---|---|---|
| Ontario Intl Airport 1 mi away, 949 ft | January 4 1 in 10 after February 7 | December 30 1 in 10 by December 11 | 360 days |
| Chino Airport 5 mi away, 650 ft | January 17 1 in 10 after February 22 | December 19 1 in 10 by November 27 | 336 days |
| Pomona/Fairplex 10 mi away, 999 ft | January 8 1 in 10 after February 18 | December 28 1 in 10 by December 5 | 354 days |
| Riverside Muni Airport 11 mi away, 805 ft | January 9 1 in 10 after February 21 | December 30 1 in 10 by December 5 | 355 days |
| Riverside Fire Stn 3 14 mi away, 840 ft | January 9 1 in 10 after February 25 | December 25 1 in 10 by December 1 | 350 days |
Distances are from the middle of Ontario as the Census Bureau maps it. The average date is NOAA's 50% date: half of years have a later last frost, or an earlier first one. Plan by the 1-in-10 date if a freeze would cost you plants. The planting zone is the USDA 2023 zone for ZIP code 91761, at the middle of the city.
Your exact ZIP code
Ontario covers a lot of ground. For the station nearest your own street, and its planting zone, type your ZIP code.
How these dates work
Frost dates come from NOAA's 1991 to 2020 Climate Normals, which work out from each weather station's daily lows how likely the last spring freeze and the first fall freeze (32 °F) are by each date. We use the station nearest the middle of your ZIP code and show its height and distance, since a station on a hill or by a lake can differ from your yard.
The average date is the 50% date: half of years have a freeze after the spring date or before the fall one. The 1-in-10 dates are NOAA's 10% dates, the ones to plan by if a late freeze would cost you plants. Within one yard, cold air pools in low spots while walls and paving hold heat, so some corners freeze before others.
More frost dates
All of California, station by station.
Other California cities: Los Angeles · San Diego · San Jose · San Francisco · Fresno · Sacramento · Long Beach · Oakland · Bakersfield · Anaheim · Stockton · Riverside · Irvine · Santa Ana · Chula Vista · Santa Clarita · Fremont · San Bernardino · Fontana · Modesto · Moreno Valley · Oxnard · Huntington Beach · Glendale · Elk Grove · Santa Rosa · Rancho Cucamonga · Oceanside · Garden Grove · Lancaster · Roseville · Palmdale · Corona · Salinas · Hayward · Sunnyvale · Pomona · Visalia · Escondido · Victorville
Where the numbers come from
- USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture
- PRISM Climate Group, Oregon State University (zone data)
- NOAA NCEI, U.S. Climate Normals 1991–2020, annual/seasonal
- NOAA, Normals calculation methodology (frost-freeze probability dates)
- US Census Bureau, 2025 ZCTA gazetteer (ZIP code centers)
- US Census Bureau, 2025 city population estimates and place gazetteer