| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 40°F | 31°F | 3.2" |
| February | 22°F | 41°F | 31°F | 3.2" |
| March | 29°F | 48°F | 38°F | 3.8" |
| April | 40°F | 61°F | 49°F | 3.7" |
| May | 50°F | 69°F | 58°F | 3.8" |
| June | 59°F | 78°F | 67°F | 4.7" |
| July | 66°F | 85°F | 74°F | 4.1" |
| August | 64°F | 83°F | 73°F | 3.5" |
| September | 58°F | 77°F | 66°F | 3.0" |
| October | 48°F | 66°F | 56°F | 5.0" |
| November | 37°F | 55°F | 45°F | 3.3" |
| December | 28°F | 45°F | 36°F | 4.6" |
| Index | Chelsea | Massachusetts | National |
| Air quality index | 43 | 40 | 42 |
| Pollution index | 24,018,000 | 8,399,610 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 291 |
| Days with moderate air quality | 73 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.030% |
| Benzene | 0.220% |
| Carbon Tetrachloride | 0.100% |
| Lead | 0.280% |
| Mercury | 0.130% |
| City | Population | Air Quality Index | National Avg. |
| Dedham, MA | 26,137 | 43 | 14.0% |
| Medfield, MA | 6,867 | 43 | 14.0% |
| North Scituate, MA | 5,501 | 43 | 14.0% |
| Weymouth Town, MA | 59,403 | 43 | 14.0% |
| Swampscott, MA | 15,599 | 43 | 14.0% |
| Chelsea, MA | 41,000 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Montclair, CA | 41,014 | 89 | 78.0% |
| Hagerstown, MD | 40,940 | 40 | 20.0% |
| Hilton Head Island, SC | 41,077 | 39 | 22.0% |
| Marion, IA | 41,096 | 39 | 22.0% |
| Los Banos, CA | 40,884 | 48 | 4.0% |
| Chelsea, MA | 41,000 | 43 | 14.0% |
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