Olmsted, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 8.8 °C (low) to 19.9 °C (high). Annual precipitation averages 1254.8 Millimeters. 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 | -3.1 °C | 6.2 °C | 96.77 mm |
| February | -1.4 °C | 8.9 °C | 91.69 mm |
| March | 3 °C | 14.3 °C | 119.89 mm |
| April | 8.5 °C | 20.3 °C | 132.84 mm |
| May | 14.1 °C | 25.2 °C | 132.08 mm |
| June | 18.8 °C | 29.9 °C | 112.52 mm |
| July | 20.8 °C | 31.6 °C | 98.55 mm |
| August | 19.5 °C | 30.9 °C | 68.07 mm |
| September | 15.1 °C | 27.8 °C | 86.87 mm |
| October | 8.6 °C | 21.8 °C | 99.06 mm |
| November | 2.9 °C | 14.2 °C | 106.68 mm |
| December | -1 °C | 8.2 °C | 109.73 mm |
| Annual | 8.8 °C | 19.9 °C | 1254.75 mm |
| 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 : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 7a : -17.8°C to -15°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |