| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 36°F | 60°F | 46°F | 6.0" |
| February | 39°F | 64°F | 50°F | 5.7" |
| March | 41°F | 67°F | 52°F | 4.6" |
| April | 43°F | 71°F | 55°F | 2.4" |
| May | 46°F | 77°F | 60°F | 1.1" |
| June | 50°F | 83°F | 64°F | 0.4" |
| July | 52°F | 85°F | 65°F | 0.0" |
| August | 52°F | 84°F | 64°F | 0.0" |
| September | 49°F | 85°F | 64°F | 0.2" |
| October | 46°F | 78°F | 59°F | 2.0" |
| November | 41°F | 67°F | 52°F | 3.2" |
| December | 36°F | 59°F | 46°F | 6.9" |
| Index | Angwin | California | National |
| Air quality index | 34 | 55 | 42 |
| Pollution index | 360,238 | 10,473,190 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 346 |
| Days with moderate air quality | 18 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.004% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.002% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Middletown, CA | 725 | 31 | 38.0% |
| St. Helena, CA | 6,049 | 34 | 32.0% |
| Calistoga, CA | 5,346 | 34 | 32.0% |
| Santa Rosa, CA | 178,391 | 34 | 32.0% |
| Deer Park, CA | 772 | 34 | 32.0% |
| Angwin, CA | 3,415 | 34 | 32.0% |
| City | Population | Air Quality Index | National Avg. |
| Day Valley, CA | 3,414 | 35 | 30.0% |
| Johnston City, IL | 3,414 | 43 | 14.0% |
| West Liberty, KY | 3,414 | 42 | 16.0% |
| Hanover, MN | 3,414 | 46 | 8.0% |
| Hidden Springs, ID | 3,416 | 36 | 28.0% |
| Angwin, CA | 3,415 | 34 | 32.0% |
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