Olmsted, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 47.9 °F (low) to 67.9 °F (high). Annual precipitation averages 49.4 Inches. Olmsted is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Mar. 21 - Mar. 31 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 26.5 °F | 43.2 °F | 3.81 in. |
| February | 29.5 °F | 48.1 °F | 3.61 in. |
| March | 37.4 °F | 57.8 °F | 4.72 in. |
| April | 47.3 °F | 68.6 °F | 5.23 in. |
| May | 57.4 °F | 77.4 °F | 5.2 in. |
| June | 65.9 °F | 85.8 °F | 4.43 in. |
| July | 69.4 °F | 88.8 °F | 3.88 in. |
| August | 67.1 °F | 87.6 °F | 2.68 in. |
| September | 59.2 °F | 82.0 °F | 3.42 in. |
| October | 47.4 °F | 71.2 °F | 3.9 in. |
| November | 37.3 °F | 57.6 °F | 4.2 in. |
| December | 30.2 °F | 46.7 °F | 4.32 in. |
| Annual | 47.9 °F | 67.9 °F | 49.4 in. |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Mar. 21 - Mar. 31 |
| Average Days With Frost | 50-75 Days |
| Plant Hardiness Zone | Zone 7a : 0°F to 5°F |
| Plant Hardiness Zone - 2012 | Zone 7a : 0°F to 5°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |