| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 35°F | 49°F | 41°F | 7.8" |
| February | 35°F | 47°F | 40°F | 8.4" |
| March | 36°F | 49°F | 41°F | 10.1" |
| April | 38°F | 54°F | 45°F | 5.7" |
| May | 46°F | 62°F | 53°F | 3.7" |
| June | 55°F | 71°F | 62°F | 1.4" |
| July | 64°F | 79°F | 71°F | 0.1" |
| August | 63°F | 79°F | 70°F | 0.1" |
| September | 58°F | 74°F | 65°F | 1.4" |
| October | 48°F | 63°F | 54°F | 4.8" |
| November | 40°F | 53°F | 46°F | 7.3" |
| December | 34°F | 46°F | 39°F | 11.7" |
| Index | Downieville | California | National |
| Air quality index | 44 | 55 | 42 |
| Pollution index | 26,667,300 | 10,473,190 | 2,726,193 |
| Measurement | Total |
| Days measured | 361 |
| Days with good air quality | 273 |
| Days with moderate air quality | 83 |
| Days w/ poor A.Q. for sensitive groups | 5 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.360% |
| Benzene | 0.250% |
| Carbon Tetrachloride | 0.100% |
| Lead | 0.250% |
| Mercury | 0.260% |
| City | Population | Air Quality Index | National Avg. |
| Sierra City, CA | 84 | 44 | 12.0% |
| Graeagle, CA | 552 | 44 | 12.0% |
| Whitehawk, CA | 37 | 44 | 12.0% |
| Alleghany, CA | 99 | 44 | 12.0% |
| Blairsden, CA | 0 | 44 | 12.0% |
| Downieville, CA | 99 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Cullomburg, AL | 99 | 36 | 28.0% |
| Faunsdale, AL | 99 | 35 | 30.0% |
| Black Springs, AR | 99 | 38 | 24.0% |
| Tumacacori-Carmen, AZ | 99 | 31 | 38.0% |
| Alleghany, CA | 99 | 44 | 12.0% |
| Downieville, CA | 99 | 44 | 12.0% |
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