| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 28°F | 47°F | 37°F | 2.7" |
| February | 29°F | 50°F | 38°F | 2.3" |
| March | 37°F | 60°F | 47°F | 3.2" |
| April | 47°F | 71°F | 58°F | 3.4" |
| May | 56°F | 78°F | 67°F | 4.6" |
| June | 67°F | 87°F | 77°F | 5.3" |
| July | 72°F | 91°F | 81°F | 4.2" |
| August | 70°F | 90°F | 79°F | 3.2" |
| September | 63°F | 83°F | 72°F | 3.5" |
| October | 52°F | 72°F | 61°F | 5.0" |
| November | 40°F | 61°F | 50°F | 2.5" |
| December | 33°F | 52°F | 42°F | 3.8" |
| Index | South West | Washington | District of Columbia |
| Air quality index | 47 | 47 | 49 |
| Pollution index | 1,590,970 | 5,554,010 | 5,554,010 |
| 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 | 0.010% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.004% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Layhill, MD | 5,112 | 47 | 6.0% |
| Travilah, MD | 11,239 | 47 | 6.0% |
| Lake Ridge, VA | 45,111 | 47 | 6.0% |
| Burke, VA | 44,077 | 47 | 6.0% |
| George Mason, VA | 9,948 | 47 | 6.0% |
| Washington, DC | 701,974 | 47 | 6.0% |
| City | Population | Air Quality Index | National Avg. |
| Vancouver, BC | 694,440 | n/a | |
| Boston, MA | 689,326 | 43 | 14.0% |
| Denver, CO | 715,878 | 58 | 16.0% |
| El Paso, TX | 679,879 | 53 | 6.0% |
| Detroit, MI | 672,351 | 56 | 12.0% |
| Washington, DC | 701,974 | 47 | 6.0% |
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