Thayer, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 43.5 °F (low) to 63.6 °F (high). Annual precipitation averages 38.9 Inches. Thayer is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 19.5 °F | 35.5 °F | 2.22 in. |
| February | 22.8 °F | 40.7 °F | 2.05 in. |
| March | 32.0 °F | 52.1 °F | 2.68 in. |
| April | 42.5 °F | 65.2 °F | 4.29 in. |
| May | 53.9 °F | 75.0 °F | 4.66 in. |
| June | 63.0 °F | 83.2 °F | 4.47 in. |
| July | 66.1 °F | 86.2 °F | 3.52 in. |
| August | 63.9 °F | 85.1 °F | 3.14 in. |
| September | 55.7 °F | 79.9 °F | 3.25 in. |
| October | 44.3 °F | 67.5 °F | 3.24 in. |
| November | 33.8 °F | 52.5 °F | 3.1 in. |
| December | 24.8 °F | 40.6 °F | 2.23 in. |
| Annual | 43.5 °F | 63.6 °F | 38.85 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |