| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 39°F | 58°F | 48°F | 3.3" |
| February | 43°F | 62°F | 52°F | 3.0" |
| March | 45°F | 67°F | 54°F | 1.8" |
| April | 47°F | 71°F | 57°F | 1.9" |
| May | 52°F | 78°F | 63°F | 0.6" |
| June | 56°F | 85°F | 68°F | 0.0" |
| July | 58°F | 90°F | 71°F | 0.0" |
| August | 57°F | 90°F | 70°F | 0.0" |
| September | 56°F | 86°F | 68°F | 0.0" |
| October | 50°F | 77°F | 62°F | 1.4" |
| November | 45°F | 68°F | 55°F | 1.2" |
| December | 40°F | 57°F | 47°F | 2.3" |
| Index | Clayton | California | National |
| Air quality index | 40 | 55 | 42 |
| Pollution index | 13,698,500 | 10,473,190 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 290 |
| Days with moderate air quality | 74 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.040% |
| Benzene | 0.110% |
| Carbon Tetrachloride | 0.330% |
| Lead | 0.070% |
| Mercury | 0.120% |
| City | Population | Air Quality Index | National Avg. |
| Alhambra Valley, CA | 1,051 | 40 | 20.0% |
| Lafayette, CA | 25,949 | 40 | 20.0% |
| Pacheco, CA | 4,775 | 40 | 20.0% |
| San Miguel CDP (San Luis Obispo County), CA | 2,696 | 40 | 20.0% |
| Blackhawk, CA | 10,590 | 40 | 20.0% |
| Clayton, CA | 11,585 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| North Saanich, BC | 11,587 | n/a | |
| Elwood, NY | 11,582 | 43 | 14.0% |
| Accokeek, MD | 11,590 | 47 | 6.0% |
| L'Île-Perrot, QC | 11,591 | n/a | |
| Cedarburg, WI | 11,592 | 44 | 12.0% |
| Clayton, CA | 11,585 | 40 | 20.0% |
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