| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 40°F | 31°F | 7.2" |
| February | 22°F | 38°F | 29°F | 3.1" |
| March | 27°F | 44°F | 35°F | 9.7" |
| April | 36°F | 54°F | 45°F | 8.7" |
| May | 46°F | 62°F | 53°F | 6.7" |
| June | 57°F | 73°F | 64°F | 1.9" |
| July | 64°F | 79°F | 71°F | 5.2" |
| August | 63°F | 81°F | 71°F | 3.1" |
| September | 57°F | 75°F | 66°F | 9.5" |
| October | 48°F | 65°F | 56°F | 7.3" |
| November | 36°F | 54°F | 45°F | 5.9" |
| December | 27°F | 42°F | 34°F | 5.5" |
| Index | South Dennis | Massachusetts | National |
| Air quality index | 36 | 40 | 42 |
| Pollution index | 7,903,830 | 8,399,610 | 2,726,193 |
| Measurement | Total |
| Days measured | 359 |
| Days with good air quality | 348 |
| Days with moderate air quality | 11 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.060% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.080% |
| Mercury | 0.030% |
| City | Population | Air Quality Index | National Avg. |
| Orleans, MA | 1,519 | 36 | 28.0% |
| Brewster, MA | 1,347 | 36 | 28.0% |
| East Harwich, MA | 4,756 | 36 | 28.0% |
| Northwest Harwich, MA | 4,123 | 36 | 28.0% |
| West Dennis, MA | 2,479 | 36 | 28.0% |
| South Dennis, MA | 2,895 | 36 | 28.0% |
| City | Population | Air Quality Index | National Avg. |
| Milaca, MN | 2,894 | 32 | 36.0% |
| Claryville, KY | 2,896 | 54 | 8.0% |
| Lake Wissota, WI | 2,896 | 34 | 32.0% |
| Palo Cedro, CA | 2,893 | 42 | 16.0% |
| Deer Lodge, MT | 2,893 | 20 | 60.0% |
| South Dennis, MA | 2,895 | 36 | 28.0% |
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