| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 41°F | 31°F | 4.1" |
| February | 26°F | 44°F | 34°F | 5.6" |
| March | 34°F | 55°F | 43°F | 6.4" |
| April | 44°F | 68°F | 55°F | 6.1" |
| May | 55°F | 76°F | 64°F | 9.2" |
| June | 63°F | 84°F | 72°F | 6.0" |
| July | 66°F | 87°F | 76°F | 5.5" |
| August | 64°F | 85°F | 73°F | 7.7" |
| September | 58°F | 79°F | 67°F | 7.6" |
| October | 46°F | 67°F | 55°F | 9.4" |
| November | 36°F | 55°F | 44°F | 5.4" |
| December | 30°F | 46°F | 37°F | 6.8" |
| Index | Darnestown | Maryland | National |
| Air quality index | 47 | 43 | 42 |
| Pollution index | 14,693,900 | 5,342,367 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 209 |
| Days with moderate air quality | 145 |
| Days w/ poor A.Q. for sensitive groups | 10 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.190% |
| Benzene | 0.210% |
| Carbon Tetrachloride | 0.060% |
| Lead | 0.060% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Point of Rocks, MD | 2,067 | 40 | 20.0% |
| Reston, VA | 61,418 | 47 | 6.0% |
| Barnesville, MD | 163 | 47 | 6.0% |
| Brambleton, VA | 20,843 | 47 | 6.0% |
| Dranesville, VA | 11,929 | 47 | 6.0% |
| Darnestown, MD | 6,754 | 47 | 6.0% |
| City | Population | Air Quality Index | National Avg. |
| Lemoore Station, CA | 6,756 | 56 | 12.0% |
| Allendale, NJ | 6,757 | 51 | 2.0% |
| Rumson, NJ | 6,757 | 51 | 2.0% |
| Kingston, MA | 6,750 | 43 | 14.0% |
| Parkwood, WA | 6,758 | 18 | 64.0% |
| Darnestown, MD | 6,754 | 47 | 6.0% |
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