Hartly, Delaware has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 7.5 °C (low) to 18.4 °C (high). Annual precipitation averages 1177.3 Millimeters. Hartly is located in USDA Plant Hardiness Zone 7b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -4 °C | 6 °C | 85.34 mm |
| February | -3.4 °C | 7.1 °C | 75.95 mm |
| March | 0.7 °C | 11.4 °C | 104.65 mm |
| April | 6.3 °C | 17.8 °C | 91.44 mm |
| May | 11.7 °C | 22.8 °C | 99.82 mm |
| June | 16.7 °C | 27.7 °C | 114.55 mm |
| July | 19.5 °C | 30.3 °C | 109.73 mm |
| August | 18.4 °C | 29.3 °C | 110.74 mm |
| September | 14.7 °C | 26.1 °C | 106.43 mm |
| October | 7.9 °C | 19.8 °C | 99.82 mm |
| November | 2.3 °C | 13.7 °C | 80.01 mm |
| December | -1.3 °C | 8.4 °C | 98.81 mm |
| Annual | 7.5 °C | 18.4 °C | 1177.29 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7b : -15°C to -12.2°C |
| Plant Hardiness Zone - 2012 | Zone 7a : -17.8°C to -15°C |
| Current Drought Conditions | Extreme Drought |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |