Lanark, Illinois has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3.1 °C (low) to 14.9 °C (high). Annual precipitation averages 957.3 Millimeters. Lanark is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -11.2 °C | -1.6 °C | 38.61 mm |
| February | -9.2 °C | 0.8 °C | 43.43 mm |
| March | -3.4 °C | 7.7 °C | 56.9 mm |
| April | 2.4 °C | 15.4 °C | 96.27 mm |
| May | 8.9 °C | 21.8 °C | 112.78 mm |
| June | 14.4 °C | 26.9 °C | 132.84 mm |
| July | 16.3 °C | 28.5 °C | 117.35 mm |
| August | 15.1 °C | 27.8 °C | 95.5 mm |
| September | 10.3 °C | 24.4 °C | 89.92 mm |
| October | 4 °C | 17.2 °C | 71.37 mm |
| November | -2.1 °C | 8.4 °C | 54.61 mm |
| December | -7.8 °C | 1.3 °C | 47.75 mm |
| Annual | 3.1 °C | 14.9 °C | 957.33 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |