| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 40°F | 32°F | 2.5" |
| February | 23°F | 40°F | 32°F | 2.1" |
| March | 30°F | 47°F | 38°F | 3.3" |
| April | 39°F | 58°F | 48°F | 3.1" |
| May | 48°F | 66°F | 57°F | 3.6" |
| June | 58°F | 75°F | 66°F | 3.6" |
| July | 65°F | 81°F | 72°F | 2.8" |
| August | 63°F | 80°F | 71°F | 3.0" |
| September | 57°F | 75°F | 66°F | 3.3" |
| October | 47°F | 65°F | 56°F | 4.4" |
| November | 37°F | 55°F | 46°F | 2.9" |
| December | 29°F | 46°F | 37°F | 3.4" |
| Index | Old Mystic | Connecticut | National |
| Air quality index | 37 | 43 | 42 |
| Pollution index | 4,117,250 | 8,629,883 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 323 |
| Days with moderate air quality | 34 |
| Days w/ poor A.Q. for sensitive groups | 7 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Fishers Island, NY | 61 | 37 | 26.0% |
| Hope Valley, RI | 1,888 | 37 | 26.0% |
| Pawcatuck, CT | 5,024 | 37 | 26.0% |
| Groton, CT | 8,999 | 37 | 26.0% |
| Misquamicut, RI | 378 | 37 | 26.0% |
| Old Mystic, CT | 3,302 | 37 | 26.0% |
| City | Population | Air Quality Index | National Avg. |
| Pistakee Highlands, IL | 3,302 | 39 | 22.0% |
| West Lake Hills, TX | 3,302 | 44 | 12.0% |
| Pagedale, MO | 3,301 | 54 | 8.0% |
| Southampton, NY | 3,301 | 41 | 18.0% |
| Woodbourne, PA | 3,301 | 41 | 18.0% |
| Old Mystic, CT | 3,302 | 37 | 26.0% |
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