Ontario, California Weather
Coastal fog thickens at dawn. Day 77 of spring. Read this microseason across nine climate regions →
Ontario weather forecast — hour by hour, 7-day outlook, NOAA radar
- TodayJun 4Overcast——88°58°—
- FridayJun 5Foggy——87°59°-1°
- SaturdayJun 6Overcast——82°58°-5°
- SundayJun 7Clear——81°61°-1°
- MondayJun 8Partly Cloudy——82°62°+1°
- TuesdayJun 9Overcast——74°61°-8°
- WednesdayJun 10Clear——79°58°+5°
Ozone at AQI 106 now. AQI flat over the last 6 hours (within ±3 points). With UV 10 peaking around 1 PM under clear skies, surface ozone likely climbs to AQI 148 by mid-afternoon.
CAUTION Sensitive groups (children, elderly, respiratory conditions) should limit prolonged outdoor exertion.
What's driving it
Ozone × UV × Sky
Ozone at AQI 106 now. With UV 10 peaking around 1 PM under clear skies, surface ozone likely climbs to AQI 148 by mid-afternoon.
- Present
- AQI 106
- UV peak
- 10.5 at 1 PM
- Sky at peak
- clear
- Projected peak
- AQI 148
PM × Wind × Precip
PM2.5/PM10 ratio 0.70 with 10 mph wind — characteristic of long-range haze transport rather than a local source.
- PM2.5/PM10
- 0.70
- Wind
- breezy
- Recent rain
- 0h in last 6h
- Pattern
- transport
Trends
Seven days of AQI and PM2.5.
Hourly air-quality data from the Copernicus Atmosphere Monitoring Service, charted across the past and next several days. Dashed lines mark the AQI breakpoints at 50 (Good → Moderate) and 100 (Moderate → Unhealthy for Sensitive Groups).
A summer fog for fair, a winter fog for rain.
- Moonrise
- 6:04 AM
- Moonset
- 4:13 PM
- In sign
- ♒︎ Aquarius
Coastal fog thickens at dawn
Ontario at a glance
- Today vs. normal: 83°F — typical for the season
- Last frost: March 11 (climatological average for this latitude)
- Microseason: Jun 1–5
- Planting window: Harvest spring lettuce before it bolts. Sow heat-tolerant greens.
Right now in the garden
Peak growing season
As of June 4, the growing season is at its peak — frost is months away. Continue succession-planting beans and summer squash. Start fall brassicas (broccoli, cabbage, kale) from seed indoors for transplanting in late summer.
Planting calendar
| Month | Plant | Harvest |
|---|---|---|
| January | — | — |
| February | — | — |
| March | lettuce, peas, spinach, radishes | — |
| April | lettuce, peas, spinach, radishes, tomatoes, peppers, beans, squash | — |
| May | tomatoes, peppers, beans, squash | lettuce, peas, radishes |
| June | tomatoes, peppers, beans, squash | lettuce, peas, radishes |
| July | — | tomatoes, beans, summer squash |
| August | — | tomatoes, beans, summer squash |
| September | — | tomatoes, beans, summer squash |
| October | fall brassicas, garlic (overwinter), carrots | winter squash, tomatoes (last) |
| November | fall brassicas, garlic (overwinter), carrots | — |
| December | — | — |
A year in weather
August brings Ontario's warmest temperatures (~81°F mean) while December delivers the coldest (~56°F). February sees peak rainfall at 2.5 inches, dropping to just 0.1 inches in June.
| Month | Mean temp | Precip | Rainy days |
|---|---|---|---|
| January | 56° | 2.2″ | 4 |
| February | 57° | 2.5″ | 5 |
| March | 60° | 1.6″ | 3 |
| April | 64° | 0.6″ | 1 |
| May | 68° | 0.2″ | 1 |
| June | 74° | 0.1″ | 0 |
| July | 80° | 0.1″ | 0 |
| August | 81° | 0.1″ | 0 |
| September | 77° | 0.1″ | 0 |
| October | 69° | 0.4″ | 1 |
| November | 61° | 0.7″ | 2 |
| December | 56° | 1.4″ | 3 |
Regional context
Ontario belongs to a hot-summer Mediterranean climate region — sharing similar temperature ranges, seasonal patterns, and growing conditions with nearby cities.
Similar climates: Los Angeles, CA, San Francisco, CA, San Diego, CA, Riverside, CA, Sacramento, CA.
Naturalist notes
Late May brings the peak migration of Yellow Warblers through Ontario's parks and green spaces.
Coastal live oaks begin dropping their acorns in September as temperatures start their gradual decline.
Frequently asked
- When does it freeze in Ontario?
- Ontario's last spring frost typically falls around mid-March, and the first fall frost arrives around mid-December.
- What is the rainy season in Ontario?
- February is the wettest month with about 2.5 inches of rain on average; the city receives roughly 10 inches annually.
- What is the warmest month in Ontario?
- August is typically warmest, averaging about 81°F.
- What is the coldest month in Ontario?
- December is typically coldest, averaging about 56°F.
- When can I start a vegetable garden in Ontario?
- Cool-season crops (peas, lettuce) can be sown around the last spring frost (mid-March); warm-season crops (tomatoes, peppers) wait until 1–2 weeks after.
- How many rainy days does Ontario get?
- Ontario averages about 19 rainy days per year.
- What hardiness zone is Ontario?
- Ontario's USDA hardiness zone is determined by its lowest average winter temperature; check the USDA's online lookup with the city ZIP for the current zone designation.
Climate
Ontario, California experiences a hot-summer Mediterranean climate zone. January temperatures hover near 56°F while July averages 80°F — a 23°F seasonal swing.
Throughout the year, Ontario receives about 10 inches of precipitation spread over roughly 19 rainy days.
Geography shapes Ontario's climate through its latitude (34.0°N), proximity to large water bodies, and elevation — factors that determine what grows here, when frost occurs, and daily weather patterns.