| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 29°F | 47°F | 37°F | 4.0" |
| February | 30°F | 50°F | 39°F | 3.7" |
| March | 39°F | 62°F | 50°F | 4.2" |
| April | 49°F | 73°F | 60°F | 5.8" |
| May | 57°F | 79°F | 68°F | 5.7" |
| June | 66°F | 88°F | 77°F | 4.1" |
| July | 69°F | 90°F | 79°F | 5.7" |
| August | 68°F | 90°F | 78°F | 4.7" |
| September | 60°F | 83°F | 71°F | 4.4" |
| October | 48°F | 72°F | 59°F | 3.9" |
| November | 38°F | 61°F | 49°F | 3.5" |
| December | 32°F | 51°F | 41°F | 4.7" |
| Index | Portland | Tennessee | National |
| Air quality index | 43 | 76 | 42 |
| Pollution index | 1,764,100 | 2,454,916 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 268 |
| Days with moderate air quality | 97 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.030% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Fairfield, TN | 200 | 43 | 14.0% |
| Bransford, TN | 269 | 43 | 14.0% |
| Mitchellville, TN | 160 | 43 | 14.0% |
| Franklin, KY | 8,909 | 43 | 14.0% |
| Bethpage, TN | 481 | 45 | 10.0% |
| Portland, TN | 12,928 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Ridgefield Park, NJ | 12,928 | 51 | 2.0% |
| New Carrollton, MD | 12,925 | 47 | 6.0% |
| Lexington Park, MD | 12,934 | 41 | 18.0% |
| Dawson Creek, BC | 12,921 | n/a | |
| Marshall, MO | 12,920 | 38 | 24.0% |
| Portland, TN | 12,928 | 43 | 14.0% |
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