Ontario, California has a Hot-Summer Mediterranean Climate climate (Köppen classification: Csa). Average annual temperatures range from 11.3 °C (low) to 25.7 °C (high). Annual precipitation averages 369.8 Millimeters. Ontario is located in USDA Plant Hardiness Zone 10a.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 6.6 °C | 19.6 °C | 83.82 mm |
| February | 7 °C | 19.8 °C | 98.3 mm |
| March | 8.3 °C | 21.5 °C | 59.18 mm |
| April | 9.7 °C | 23.6 °C | 17.53 mm |
| May | 12 °C | 25.7 °C | 9.65 mm |
| June | 14.2 °C | 29.3 °C | 2.54 mm |
| July | 16.8 °C | 32.9 °C | 1.27 mm |
| August | 17.2 °C | 33.9 °C | 0.51 mm |
| September | 16.2 °C | 32.3 °C | 2.79 mm |
| October | 12.9 °C | 27.7 °C | 15.75 mm |
| November | 8.8 °C | 23.3 °C | 20.57 mm |
| December | 6.2 °C | 19.1 °C | 57.91 mm |
| Annual | 11.3 °C | 25.7 °C | 369.82 mm |
| Average First Frost Date | No Frost |
| Average Last Frost Date | |
| Average Days With Frost | Frost Free |
| Plant Hardiness Zone | Zone 10a : -1.1°C to 1.7°C |
| Plant Hardiness Zone - 2012 | Zone 10a : -1.1°C to 1.7°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csa - Hot-Summer Mediterranean Climate |