Thornton, California has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 8.9 °C (low) to 23.1 °C (high). Annual precipitation averages 462.3 Millimeters. Thornton is located in USDA Plant Hardiness Zone 9b. The growing season typically runs from the last frost around Jan. 21 - Jan. 31 through the first frost around Dec. 21 - Dec. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 3.7 °C | 13.2 °C | 95.25 mm |
| February | 4.9 °C | 16.1 °C | 84.33 mm |
| March | 6.6 °C | 18.9 °C | 68.58 mm |
| April | 8.1 °C | 21.9 °C | 35.81 mm |
| May | 10.8 °C | 25.9 °C | 18.54 mm |
| June | 13.1 °C | 29.6 °C | 6.35 mm |
| July | 14.4 °C | 31.9 °C | 0 mm |
| August | 14.3 °C | 31.8 °C | 0.51 mm |
| September | 12.9 °C | 30.4 °C | 2.03 mm |
| October | 9.6 °C | 25.6 °C | 20.32 mm |
| November | 5.4 °C | 18.3 °C | 43.69 mm |
| December | 3.3 °C | 13.4 °C | 86.87 mm |
| Annual | 8.9 °C | 23.1 °C | 462.28 mm |
| Average First Frost Date | Dec. 21 - Dec. 31 |
| Average Last Frost Date | Jan. 21 - Jan. 31 |
| Average Days With Frost | 10-20 Days |
| Plant Hardiness Zone | Zone 9b : -3.9°C to -1.1°C |
| Plant Hardiness Zone - 2012 | Zone 9b : -3.9°C to -1.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |