Johnstown, Colorado has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 1.3 °C (low) to 17.8 °C (high). Annual precipitation averages 344.9 Millimeters. Johnstown is located in USDA Plant Hardiness Zone 5b. 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 | -10 °C | 5.4 °C | 7.11 mm |
| February | -8.6 °C | 6.8 °C | 10.92 mm |
| March | -3.8 °C | 12.8 °C | 24.89 mm |
| April | 0.3 °C | 16.5 °C | 37.59 mm |
| May | 5.9 °C | 21.4 °C | 63.75 mm |
| June | 11.2 °C | 28.1 °C | 41.4 mm |
| July | 14.1 °C | 31.4 °C | 42.16 mm |
| August | 12.7 °C | 30.2 °C | 32.77 mm |
| September | 7.8 °C | 26 °C | 33.78 mm |
| October | 1.1 °C | 18.3 °C | 22.86 mm |
| November | -5.1 °C | 10.9 °C | 14.22 mm |
| December | -9.9 °C | 5.3 °C | 13.46 mm |
| Annual | 1.3 °C | 17.8 °C | 344.91 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Severe Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |