| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 28°F | 44°F | 35°F | 6.3" |
| February | 28°F | 46°F | 36°F | 4.5" |
| March | 35°F | 55°F | 44°F | 6.6" |
| April | 45°F | 66°F | 55°F | 6.3" |
| May | 55°F | 75°F | 64°F | 5.8" |
| June | 64°F | 83°F | 73°F | 8.1" |
| July | 69°F | 87°F | 78°F | 10.2" |
| August | 67°F | 85°F | 76°F | 9.2" |
| September | 61°F | 79°F | 69°F | 9.7" |
| October | 50°F | 69°F | 59°F | 9.2" |
| November | 39°F | 58°F | 48°F | 5.9" |
| December | 33°F | 50°F | 41°F | 7.5" |
| Index | Hartly | Delaware | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 3,193,180 | 7,274,307 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 293 |
| Days with moderate air quality | 72 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.010% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Frederica, DE | 948 | 42 | 16.0% |
| Little Creek, DE | 288 | 42 | 16.0% |
| Rodney Village, DE | 1,165 | 42 | 16.0% |
| Camden, DE | 3,529 | 42 | 16.0% |
| Smyrna, DE | 11,694 | 42 | 16.0% |
| Hartly, DE | 167 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Lidderdale, IA | 167 | 37 | 26.0% |
| Sims, IL | 167 | 43 | 14.0% |
| Salvisa, KY | 167 | 42 | 16.0% |
| Burkittsville, MD | 167 | 40 | 20.0% |
| Marietta, MN | 167 | 25 | 50.0% |
| Hartly, DE | 167 | 42 | 16.0% |
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