Hillsborough, California has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 9.5 °C (low) to 19.5 °C (high). Annual precipitation averages 672.1 Millimeters. Hillsborough is located in USDA Plant Hardiness Zone 10a. The growing season typically runs from the last frost around Frost Rare through the first frost around Jan. 21 - Jan. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 6.7 °C | 14.7 °C | 133.1 mm |
| February | 7.3 °C | 15.9 °C | 138.43 mm |
| March | 7.8 °C | 17.2 °C | 91.19 mm |
| April | 8.3 °C | 18.2 °C | 50.04 mm |
| May | 10 °C | 19.8 °C | 23.11 mm |
| June | 11.2 °C | 22 °C | 4.83 mm |
| July | 12.3 °C | 23.4 °C | 2.79 mm |
| August | 12.6 °C | 23.8 °C | 2.29 mm |
| September | 12.4 °C | 24.4 °C | 2.03 mm |
| October | 10.9 °C | 22.4 °C | 26.92 mm |
| November | 8.5 °C | 17.8 °C | 61.47 mm |
| December | 6.4 °C | 14.7 °C | 135.89 mm |
| Annual | 9.5 °C | 19.5 °C | 672.09 mm |
| Average First Frost Date | Jan. 21 - Jan. 31 |
| Average Last Frost Date | Frost Rare |
| Average Days With Frost | Frost Rare |
| 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 | Csb - Warm-Summer Mediterranean Climate |