| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 39°F | 30°F | 3.0" |
| February | 23°F | 40°F | 31°F | 2.6" |
| March | 30°F | 49°F | 39°F | 3.5" |
| April | 41°F | 62°F | 50°F | 4.2" |
| May | 50°F | 71°F | 60°F | 4.2" |
| June | 60°F | 79°F | 69°F | 5.1" |
| July | 66°F | 84°F | 74°F | 4.4" |
| August | 64°F | 82°F | 72°F | 4.2" |
| September | 57°F | 77°F | 66°F | 4.3" |
| October | 47°F | 65°F | 55°F | 5.1" |
| November | 37°F | 54°F | 45°F | 3.1" |
| December | 29°F | 44°F | 36°F | 4.5" |
| Index | Darien | Connecticut | National |
| Air quality index | 43 | 43 | 42 |
| Pollution index | 16,950,700 | 8,629,883 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 262 |
| Days with moderate air quality | 82 |
| Days w/ poor A.Q. for sensitive groups | 18 |
| Days with unhealthy air quality | 3 |
| Pollutant | Total |
| Arsenic | 0.030% |
| Benzene | 0.140% |
| Carbon Tetrachloride | 0.070% |
| Lead | 0.080% |
| Mercury | 0.030% |
| City | Population | Air Quality Index | National Avg. |
| Byram, CT | 4,855 | 43 | 14.0% |
| Rye Brook, NY | 9,554 | 43 | 14.0% |
| Georgetown, CT | 1,251 | 43 | 14.0% |
| Ridgefield, CT | 7,228 | 43 | 14.0% |
| Trumbull, CT | 36,477 | 43 | 14.0% |
| Darien, CT | 21,519 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Anniston, AL | 21,518 | 38 | 24.0% |
| Klamath Falls, OR | 21,509 | 29 | 42.0% |
| Massapequa, NY | 21,531 | 51 | 2.0% |
| Biddeford, ME | 21,502 | 40 | 20.0% |
| Kapolei, HI | 21,541 | 30 | 40.0% |
| Darien, CT | 21,519 | 43 | 14.0% |
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