| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 42°F | 61°F | 50°F | 2.4" |
| February | 45°F | 63°F | 53°F | 2.4" |
| March | 47°F | 67°F | 56°F | 2.0" |
| April | 49°F | 70°F | 58°F | 1.3" |
| May | 53°F | 73°F | 61°F | 0.3" |
| June | 56°F | 77°F | 65°F | 0.2" |
| July | 59°F | 78°F | 67°F | 0.0" |
| August | 59°F | 78°F | 66°F | 0.0" |
| September | 57°F | 80°F | 67°F | 0.1" |
| October | 53°F | 75°F | 63°F | 0.7" |
| November | 47°F | 67°F | 56°F | 1.3" |
| December | 42°F | 60°F | 51°F | 2.6" |
| Index | Portola Valley | California | National |
| Air quality index | 43 | 55 | 42 |
| Pollution index | 11,477,200 | 10,473,190 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 263 |
| Days with moderate air quality | 98 |
| Days w/ poor A.Q. for sensitive groups | 4 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.070% |
| Benzene | 0.110% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.160% |
| Mercury | 0.090% |
| City | Population | Air Quality Index | National Avg. |
| Pescadero, CA | 418 | 35 | 30.0% |
| Lexington Hills, CA | 2,656 | 35 | 30.0% |
| Loma Mar, CA | 282 | 35 | 30.0% |
| Santa Clara, CA | 126,723 | 43 | 14.0% |
| Belmont, CA | 26,963 | 43 | 14.0% |
| Portola Valley, CA | 4,568 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Humboldt, IA | 4,568 | 37 | 26.0% |
| Longbranch, WA | 4,568 | 30 | 40.0% |
| Rio Communities, NM | 4,566 | 53 | 6.0% |
| Williamstown, MA | 4,565 | 35 | 30.0% |
| Hollister, MO | 4,564 | 18 | 64.0% |
| Portola Valley, CA | 4,568 | 43 | 14.0% |
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