Ontario, California climate: Normal weather by month, frost dates and records
Ontario's normal year runs from 44°F December nights to 95°F August afternoons, with 11.6 inches of precipitation a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 19.3°C, December lows of 6°C, August highs of 35°C, 296 mm of precipitation a year.
- NOAA stationOntario International Airport
- Station IDUSW00003102
- Location34.06°N 117.60°W
- Elevation949 ft (289 m)
- From the center1 miles
- Population185,285
On October 23, the normal high in Ontario is 81°F and the normal low 55°F, and the chance of measurable rain or snow is 8%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Ontario.
- Jan 68° 45°
- Feb 68° 46°
- Mar 72° 49°
- Apr 76° 51°
- May 80° 56°
- Jun 86° 60°
- Jul 94° 65°
- Aug 95° 65°
- Sep 91° 64°
- Oct 83° 57°
- Nov 75° 49°
- Dec 67° 44°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like in Ontario each month?
August is the warmest month, with normal highs of 95°F; December the coldest, with normal lows of 44°F. The most precipitation falls in February (3.07 in), the least in June (0.02 in).
Each month has its own page with the day-by-day normals, the month's records and its recent years:
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 68°F | 45°F | 56.1°F | 2.57 | 5 | <1 | 1 | 10 h 10 min |
| February | 68°F | 46°F | 57.1°F | 3.07 | 6 | <1 | <1 | 11 h |
| March | 72°F | 49°F | 60.2°F | 1.64 | 5 | 1 | <1 | 11 h 50 min |
| April | 76°F | 51°F | 63.4°F | 0.76 | 4 | 3 | <1 | 13 h |
| May | 80°F | 56°F | 67.7°F | 0.30 | 2 | 5 | <1 | 13 h 50 min |
| June | 86°F | 60°F | 73.2°F | 0.02 | <1 | 11 | <1 | 14 h 20 min |
| July | 94°F | 65°F | 79.2°F | 0.05 | 1 | 23 | <1 | 14 h 10 min |
| August | 95°F | 65°F | 80.1°F | 0.03 | <1 | 24 | <1 | 13 h 30 min |
| September | 91°F | 64°F | 77.6°F | 0.10 | 1 | 18 | <1 | 12 h 30 min |
| October | 83°F | 57°F | 69.8°F | 0.41 | 2 | 8 | <1 | 11 h 20 min |
| November | 75°F | 49°F | 61.9°F | 0.80 | 4 | 2 | <1 | 10 h 30 min |
| December | 67°F | 44°F | 55.2°F | 1.89 | 6 | <1 | 1 | 9 h 50 min |
| Year (annual) | 80°F | 54°F | 66.8°F | 11.64 | 36 | 93 | 2 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | 20°C | 7°C | 13.4°C | 65 mm |
| February | 20°C | 8°C | 13.9°C | 78 mm |
| March | 22°C | 9°C | 15.7°C | 42 mm |
| April | 24°C | 11°C | 17.4°C | 19 mm |
| May | 27°C | 13°C | 19.8°C | 8 mm |
| June | 30°C | 16°C | 22.9°C | 1 mm |
| July | 34°C | 18°C | 26.2°C | 1 mm |
| August | 35°C | 18°C | 26.7°C | 1 mm |
| September | 33°C | 18°C | 25.3°C | 3 mm |
| October | 28°C | 14°C | 21.0°C | 10 mm |
| November | 24°C | 9°C | 16.6°C | 20 mm |
| December | 19°C | 6°C | 12.9°C | 48 mm |
Source: NOAA NCEI 1991-2020 monthly normals, station USW00003102; annual normals. Wet days: 0.01 inch or more; 90°F days and freezing nights: NOAA's day counts. Daylight: sunrise to sunset on the 15th, computed from the latitude.
When is the best weather to visit Ontario?
Normal highs sit between 65°F and 85°F in January, February, March, April, May, October, November and December. Of those, May has the fewest days with rain (2).
That is a measurement, not a recommendation: it counts the months whose normal afternoon high sits between 65°F and 85°F, where a light layer is enough by day.
Source: NOAA NCEI 1991-2020 monthly normals, station USW00003102.
When is the last frost in Ontario?
A night at 32°F or colder comes in only 41% of years in Ontario (NOAA). NOAA's median date of the last spring frost is January 4 and of the first autumn frost December 30.
NOAA puts the median frost-free season at 365 days, and the season without a hard freeze (28°F) at 365 days.
The USDA's 2023 Plant Hardiness Zone Map puts Ontario in zone 10a across its 3 ZIP code areas (10a in 91761, the largest). By the map's own method, the average of each year's coldest night over 1991-2020, Ontario's NOAA station is in zone 10a: the coldest night of a typical year reaches 32°F (22 complete years).
Frost dates for every NOAA station in California · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USW00003102; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; the station zone: ACIS daily lows, 1991-2020. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
What hardiness zone is each Ontario ZIP code in?
The USDA's 2023 map gives each ZIP code area one zone. Ontario's 3 ZIP code areas are all in zone 10a. Each ZIP's frost dates are in the frost tool.
Source: USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; ZIP areas in the place: Census Bureau 2020 ZCTA-to-place relationship file. A ZIP area counts when at least a tenth of its land (and 0.5 km²) lies in the place.
How hot and how cold does it get in Ontario?
In a normal year in Ontario, 93 days a year reach 90°F (19 of them 100°F or more), 2 nights fall to 32°F or colder and 36 days bring measurable precipitation.
- Days at 90°F or more
- 93a normal year
- Days at 100°F or more
- 19a normal year
- Nights at 32°F or colder
- 2a normal year
- Days that stay below freezing
- fewer than 1high of 32°F or less
- Days with 0.01 inch of precipitation
- 36a normal year
The highest temperature in this station's record (since 1998) is 118°F, on September 5, 2020; the lowest is 25°F, on January 14, 2007.
The wettest calendar year on that record is 2023, with 21.90 in.
Source: NOAA NCEI 1991-2020 annual normals, station USW00003102; records: ACIS (NOAA Regional Climate Centers), station USW00003102. Records use complete months only; a month missing more than three days is left out.
How sunny is Ontario?
Over a year 92% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Ontario, 5.8 kWh per square metre a day. August is the sunniest month (96%) and December the cloudiest (85%). Among the 459 U.S. places of 50,000 or more, that ranks tied 3rd (with 7 others) in sunshine.
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 85% | 3.3 |
| February | 85% | 4.2 |
| March | 89% | 5.7 |
| April | 90% | 6.9 |
| May | 92% | 7.8 |
| June | 96% | 8.4 |
| July | 94% | 7.9 |
| August | 96% | 7.4 |
| September | 95% | 6.3 |
| October | 93% | 4.9 |
| November | 90% | 3.6 |
| December | 85% | 2.9 |
Source: NASA POWER solar climatology, 2001-2020 (CERES SYN1deg). Satellite-based, for the 1-degree cell (about 100 km across) around the station, so it describes the area rather than the airport.
How much heating and cooling does Ontario need?
A normal year in Ontario brings 1,293 heating degree days, 76% fewer than the median U.S. place (5,373), and 1,970 cooling degree days, 2.2 times the median (908). One month needs 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 6,189 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 280 | 5 |
| February | 228 | 8 |
| March | 172 | 23 |
| April | 105 | 57 |
| May | 37 | 121 |
| June | 4 | 250 |
| July | 0 | 442 |
| August | 0 | 467 |
| September | 1 | 378 |
| October | 26 | 177 |
| November | 135 | 40 |
| December | 304 | 2 |
| Year (annual) | 1,293 | 1,970 |
Source: NOAA NCEI 1991-2020 monthly normals, station USW00003102. A degree day is one degree of a day's mean temperature below 65°F (heating) or above it (cooling); utilities use them to estimate energy use.
Has Ontario been warmer than normal lately?
2025 averaged 67.4°F, 0.6°F above the 1991-2020 normal of 66.8°F. 3 of the 5 complete years since 2021 ran warmer than the normal.
| Year | Mean | Against normal | Precip. |
|---|---|---|---|
| 2021 | 67.8°F | +1.0°F | 12.03 |
| 2022 | 67.5°F | +0.7°F | 5.73 |
| 2023 | 64.8°F | −2.0°F | 21.90 |
| 2024 | 66.5°F | −0.3°F | 16.44 |
| 2025 | 67.4°F | +0.6°F | 13.21 |
Source: observed years: ACIS, station USW00003102. A year counts only with all twelve months complete.
How does Ontario's climate compare?
Ontario is warmer than 205 of the other 218 places with their own NOAA record in California, and wetter than 52. Across the 7,247 U.S. stations with normals, its annual mean of 66.8°F is warmer than 92%.
In the Köppen classification computed from these normals, Ontario is BSh: hot semi-arid steppe. What the classes mean.
Places with the most similar year elsewhere
Measured month by month on the normal high, low and precipitation, among places of 20,000 or more people in other states:
Source: NOAA NCEI 1991-2020 normals for every station PlainClimate covers. Similarity: the root-mean-square difference of the 36 monthly figures, precipitation weighted so an inch counts like 6°F.
Which places near Ontario share this climate record?
4 places have no NOAA station with 1991-2020 normals of their own, and this station is the nearest full record. A place's own climate can differ from these figures, by several degrees where it sits much higher or lower than the station, nearer the coast, or inside a large city's heat; for the larger places the next nearest records are named with their yearly mean, so the spread around them is visible, and places of 50,000 or more have their own page with NOAA's gridded normals.
- Fontana, CAalso near: Riverside (11 mi, 66.2°F a year); Chino (14 mi, 66.1°F a year)218,455 people · 9 mi NE
- Rancho Cucamonga, CAalso near: Chino (11 mi, 66.1°F a year); Pomona (12 mi, 65.0°F a year)176,675 people · 5 mi N
- Upland, CAalso near: Pomona (7 mi, 65.0°F a year); Chino (10 mi, 66.1°F a year)79,684 people · 5 mi NW
- San Antonio Heights, CA3,479 people · 8 mi NW
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
Questions about Ontario's climate
What is the coldest month in Ontario?
December, with a normal high of 67°F and a normal low of 44°F. The warmest is August (95°F by day, 65°F at night).
What is the rainiest month in Ontario?
February, with 3.07 in of precipitation in a normal year; the driest is June with 0.02 in.
What plant hardiness zone is Ontario in?
The USDA's 2023 map puts Ontario in zone 10a across its 3 ZIP code areas (10a in 91761, the largest). By the same method at Ontario's NOAA station, the coldest night of a typical year reaches 32°F (22 years, 1991-2020): zone 10a.
Nearby and next
- Ontario in OctoberThis month day by day, its records and recent years
- California climateThe statewide normals and every place in the state
- Chino, CA6 miles from this station, 66.1°F a year
- Pomona, CA10 miles from this station, 65.0°F a year
- Riverside, CA12 miles from this station, 66.2°F a year
- Anaheim, CA19 miles from this station, 67.7°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made