| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 41°F | 26°F | 13.4" |
| February | 19°F | 45°F | 30°F | 13.5" |
| March | 22°F | 49°F | 35°F | 8.0" |
| April | 26°F | 55°F | 39°F | 5.9" |
| May | 30°F | 64°F | 47°F | 4.9" |
| June | 36°F | 74°F | 56°F | 3.2" |
| July | 41°F | 82°F | 63°F | 6.3" |
| August | 39°F | 81°F | 61°F | 3.8" |
| September | 34°F | 75°F | 55°F | 3.6" |
| October | 27°F | 63°F | 43°F | 11.2" |
| November | 22°F | 51°F | 35°F | 6.3" |
| December | 16°F | 41°F | 27°F | 10.8" |
| Index | Portola | 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 Brooks, CA | 441 | 44 | 12.0% |
| Clio, CA | 184 | 44 | 12.0% |
| Gold Mountain, CA | 75 | 44 | 12.0% |
| Lake Davis, CA | 91 | 44 | 12.0% |
| Valley Ranch, CA | 38 | 44 | 12.0% |
| Portola, CA | 1,755 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Kaibito, AZ | 1,755 | 45 | 10.0% |
| Prudenville, MI | 1,755 | 36 | 28.0% |
| Decatur, MS | 1,755 | 36 | 28.0% |
| Corvallis, MT | 1,755 | 40 | 20.0% |
| Mercer, PA | 1,755 | 31 | 38.0% |
| Portola, CA | 1,755 | 44 | 12.0% |
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