| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 38°F | 57°F | 47°F | 10.1" |
| February | 41°F | 62°F | 51°F | 8.0" |
| March | 44°F | 67°F | 55°F | 8.4" |
| April | 46°F | 74°F | 59°F | 4.6" |
| May | 53°F | 83°F | 68°F | 2.6" |
| June | 58°F | 91°F | 75°F | 2.6" |
| July | 62°F | 96°F | 79°F | 0.2" |
| August | 60°F | 95°F | 77°F | 0.4" |
| September | 57°F | 90°F | 73°F | 1.3" |
| October | 50°F | 79°F | 63°F | 4.8" |
| November | 43°F | 65°F | 53°F | 7.5" |
| December | 37°F | 56°F | 46°F | 9.8" |
| Index | Penn Valley | California | National |
| Air quality index | 44 | 55 | 42 |
| Pollution index | 4,289,310 | 10,473,190 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 239 |
| Days with moderate air quality | 124 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.050% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.060% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Rough and Ready, CA | 511 | 44 | 12.0% |
| Beale AFB, CA | 955 | 44 | 12.0% |
| Wheatland, CA | 3,631 | 44 | 12.0% |
| Grass Valley, CA | 12,817 | 44 | 12.0% |
| Smartsville, CA | 487 | 44 | 12.0% |
| Penn Valley, CA | 1,073 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Lamar, TX | 1,073 | 42 | 16.0% |
| Lot 50, PE | 1,073 | n/a | |
| Arivaca Junction, AZ | 1,072 | 31 | 38.0% |
| Lyndon, KS | 1,074 | 41 | 18.0% |
| St. George, KS | 1,072 | 32 | 36.0% |
| Penn Valley, CA | 1,073 | 44 | 12.0% |
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