Thornton, California has a Warm-Summer Mediterranean Climate climate (Köppen classification: Csb). Average annual temperatures range from 48.1 °F (low) to 73.6 °F (high). Annual precipitation averages 18.2 Inches. 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 | 38.7 °F | 55.8 °F | 3.75 in. |
| February | 40.8 °F | 61.0 °F | 3.32 in. |
| March | 43.9 °F | 66.0 °F | 2.7 in. |
| April | 46.5 °F | 71.4 °F | 1.41 in. |
| May | 51.4 °F | 78.7 °F | 0.73 in. |
| June | 55.6 °F | 85.3 °F | 0.25 in. |
| July | 57.9 °F | 89.4 °F | 0.0 in. |
| August | 57.7 °F | 89.2 °F | 0.02 in. |
| September | 55.3 °F | 86.7 °F | 0.08 in. |
| October | 49.2 °F | 78.0 °F | 0.8 in. |
| November | 41.8 °F | 65.0 °F | 1.72 in. |
| December | 37.9 °F | 56.1 °F | 3.42 in. |
| Annual | 48.1 °F | 73.6 °F | 18.2 in. |
| 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 : 25°F to 20°F |
| Plant Hardiness Zone - 2012 | Zone 9b : 25°F to 20°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Csb - Warm-Summer Mediterranean Climate |