Mertztown, Pennsylvania has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.7 °C (low) to 17 °C (high). Annual precipitation averages 1260.6 Millimeters. Mertztown is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -6 °C | 3.3 °C | 89.41 mm |
| February | -5.3 °C | 4.8 °C | 72.39 mm |
| March | -1.2 °C | 9.7 °C | 96.77 mm |
| April | 4.5 °C | 16.8 °C | 98.3 mm |
| May | 10.1 °C | 22.3 °C | 104.65 mm |
| June | 15.1 °C | 27.1 °C | 122.94 mm |
| July | 17.6 °C | 29.7 °C | 129.79 mm |
| August | 16.8 °C | 28.7 °C | 119.38 mm |
| September | 12.6 °C | 25 °C | 129.29 mm |
| October | 6.1 °C | 18.3 °C | 111.25 mm |
| November | 0.9 °C | 11.8 °C | 85.34 mm |
| December | -3.1 °C | 6.1 °C | 101.09 mm |
| Annual | 5.7 °C | 17 °C | 1260.6 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |