| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 21°F | 36°F | 28°F | 3.9" |
| February | 21°F | 37°F | 29°F | 4.4" |
| March | 29°F | 46°F | 37°F | 7.7" |
| April | 39°F | 59°F | 48°F | 9.6" |
| May | 49°F | 68°F | 58°F | 9.0" |
| June | 57°F | 76°F | 66°F | 12.4" |
| July | 63°F | 81°F | 72°F | 9.8" |
| August | 61°F | 79°F | 70°F | 7.9" |
| September | 55°F | 73°F | 63°F | 6.6" |
| October | 44°F | 62°F | 53°F | 10.7" |
| November | 35°F | 51°F | 43°F | 4.9" |
| December | 27°F | 41°F | 34°F | 3.7" |
| Index | Northwest Harwinton | Connecticut | National |
| Air quality index | 37 | 43 | 42 |
| Pollution index | 161,427 | 8,629,883 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 342 |
| Days with moderate air quality | 23 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.002% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| New Hartford Center, CT | 1,210 | 37 | 26.0% |
| Winsted, CT | 6,504 | 37 | 26.0% |
| Bethlehem Village, CT | 1,792 | 37 | 26.0% |
| Thomaston, CT | 1,730 | 37 | 26.0% |
| Litchfield, CT | 1,279 | 37 | 26.0% |
| Northwest Harwinton, CT | 2,897 | 37 | 26.0% |
| City | Population | Air Quality Index | National Avg. |
| Shenandoah, VA | 2,897 | 36 | 28.0% |
| Claryville, KY | 2,896 | 54 | 8.0% |
| Lake Wissota, WI | 2,896 | 34 | 32.0% |
| Onalaska, TX | 2,898 | 52 | 4.0% |
| Stamford, TX | 2,898 | 26 | 48.0% |
| Northwest Harwinton, CT | 2,897 | 37 | 26.0% |
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