| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 41°F | 33°F | 5.6" |
| February | 21°F | 38°F | 29°F | 4.6" |
| March | 29°F | 50°F | 39°F | 6.9" |
| April | 40°F | 63°F | 51°F | 11.6" |
| May | 48°F | 73°F | 61°F | 2.5" |
| June | 61°F | 83°F | 72°F | 4.5" |
| July | 65°F | 86°F | 75°F | 7.2" |
| August | 65°F | 86°F | 75°F | 6.7" |
| September | 56°F | 82°F | 69°F | 1.6" |
| October | 49°F | 70°F | 59°F | 9.8" |
| November | 35°F | 55°F | 44°F | 4.7" |
| December | 26°F | 41°F | 33°F | 5.6" |
| Index | Downingtown | Pennsylvania | National |
| Air quality index | 42 | 45 | 42 |
| Pollution index | 2,169,600 | 4,584,805 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 279 |
| Days with moderate air quality | 80 |
| Days w/ poor A.Q. for sensitive groups | 5 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.010% |
| Mercury | 0.004% |
| City | Population | Air Quality Index | National Avg. |
| Lionville, PA | 6,707 | 42 | 16.0% |
| Caln, PA | 1,335 | 42 | 16.0% |
| Elverson, PA | 1,369 | 42 | 16.0% |
| Royersford, PA | 4,760 | 42 | 16.0% |
| Modena, PA | 655 | 42 | 16.0% |
| Downingtown, PA | 8,012 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Stone Mills, ON | 8,012 | n/a | |
| Jefferson, WI | 8,011 | 37 | 26.0% |
| Whitmore Lake, MI | 8,010 | 40 | 20.0% |
| Franklin, VA | 8,015 | 40 | 20.0% |
| Rockwood, VA | 8,008 | 44 | 12.0% |
| Downingtown, PA | 8,012 | 42 | 16.0% |
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