Monmouth, California has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 49.5 °F (low) to 76.6 °F (high). Annual precipitation averages 10.7 Inches. Monmouth is located in USDA Plant Hardiness Zone 9b. The growing season typically runs from the last frost around Feb. 1 - Feb. 10 through the first frost around Dec. 1 - Dec. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 37.9 °F | 56.2 °F | 2.25 in. |
| February | 40.7 °F | 62.1 °F | 1.94 in. |
| March | 43.5 °F | 68.3 °F | 1.83 in. |
| April | 46.5 °F | 74.1 °F | 0.92 in. |
| May | 53.2 °F | 82.3 °F | 0.44 in. |
| June | 58.7 °F | 90.6 °F | 0.12 in. |
| July | 64.0 °F | 96.5 °F | 0.03 in. |
| August | 62.7 °F | 95.7 °F | 0.0 in. |
| September | 58.1 °F | 90.7 °F | 0.09 in. |
| October | 50.5 °F | 80.1 °F | 0.47 in. |
| November | 41.8 °F | 66.4 °F | 0.87 in. |
| December | 36.6 °F | 56.3 °F | 1.72 in. |
| Annual | 49.5 °F | 76.6 °F | 10.68 in. |
| Average First Frost Date | Dec. 1 - Dec. 10 |
| Average Last Frost Date | Feb. 1 - Feb. 10 |
| 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 | BSk - Cold Semi-Arid Climate |