Hartly, Delaware has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 45.4 °F (low) to 65.1 °F (high). Annual precipitation averages 46.4 Inches. 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 | 24.8 °F | 42.8 °F | 3.36 in. |
| February | 25.9 °F | 44.8 °F | 2.99 in. |
| March | 33.2 °F | 52.6 °F | 4.12 in. |
| April | 43.4 °F | 64.1 °F | 3.6 in. |
| May | 53.1 °F | 73.1 °F | 3.93 in. |
| June | 62.1 °F | 81.9 °F | 4.51 in. |
| July | 67.1 °F | 86.5 °F | 4.32 in. |
| August | 65.1 °F | 84.7 °F | 4.36 in. |
| September | 58.4 °F | 78.9 °F | 4.19 in. |
| October | 46.3 °F | 67.7 °F | 3.93 in. |
| November | 36.2 °F | 56.7 °F | 3.15 in. |
| December | 29.6 °F | 47.2 °F | 3.89 in. |
| Annual | 45.4 °F | 65.1 °F | 46.35 in. |
| 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 : 5°F to 10°F |
| Plant Hardiness Zone - 2012 | Zone 7a : 0°F to 5°F |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |