| 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. |
| Eagleview, PA | 1,920 | 42 | 16.0% |
| South Coatesville, PA | 1,604 | 42 | 16.0% |
| Churchtown, PA | 486 | 42 | 16.0% |
| Morgantown, PA | 1,576 | 42 | 16.0% |
| Exton, PA | 6,297 | 42 | 16.0% |
| Downingtown, PA | 8,331 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Fairwood CDP (Spokane County), WA | 8,331 | 45 | 10.0% |
| Alliance, NE | 8,330 | 19 | 62.0% |
| Hempstead, TX | 8,329 | 52 | 4.0% |
| Whitefish, MT | 8,333 | 37 | 26.0% |
| Monona, WI | 8,334 | 39 | 22.0% |
| Downingtown, PA | 8,331 | 42 | 16.0% |
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