Weather in Ontario, California
Fog and sun in daily rhythm. Day 58 of spring. Read this microseason across nine climate regions →
- TodayMay 16Overcast—74°58°
- SundayMay 17Overcast—77°56°
- MondayMay 18Overcast—84°51°
- TuesdayMay 19Overcast—87°61°
- WednesdayMay 20Overcast—84°64°
- ThursdayMay 21Overcast—83°62°
- FridayMay 22Clear—84°63°
- PM 2.5
- 12.2 μg/m³
- PM 10
- 19.3 μg/m³
- NO₂
- 2.0 μg/m³
- Ozone
- 117.0 μg/m³
- UV Index
- 3.3 Moderate
Flowers blooming in late autumn, a sure sign of a bad winter coming.
- Moonrise
- 12:16 PM
- Moonset
- 2:02 AM
- In sign
- ♉︎ Taurus
Fog and sun in daily rhythm
Ontario at a glance
- Today vs. normal: 8°F below the seasonal normal for this latitude
- Last frost: March 11 (climatological average for this latitude)
- Microseason: 28 of 72, May 16–20
- Planting window: Direct sow okra, melons, and southern peas. Thin carrot rows.
Right now in the garden
Peak growing season
As of May 16, 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.