| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 42°F | 58°F | 50°F | 3.5" |
| February | 46°F | 61°F | 53°F | 3.3" |
| March | 46°F | 64°F | 54°F | 2.6" |
| April | 48°F | 66°F | 56°F | 1.8" |
| May | 52°F | 69°F | 59°F | 0.6" |
| June | 56°F | 73°F | 62°F | 0.1" |
| July | 57°F | 75°F | 64°F | 0.0" |
| August | 58°F | 76°F | 64°F | 0.0" |
| September | 57°F | 76°F | 64°F | 0.2" |
| October | 53°F | 72°F | 61°F | 1.4" |
| November | 48°F | 66°F | 56°F | 1.3" |
| December | 42°F | 58°F | 49°F | 3.2" |
| Index | Castro Valley | California | National |
| Air quality index | 40 | 55 | 42 |
| Pollution index | 6,427,290 | 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 | 0.010% |
| Benzene | 0.110% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.003% |
| City | Population | Air Quality Index | National Avg. |
| Contra Costa Centre, CA | 6,667 | 40 | 20.0% |
| San Miguel, CA | 3,253 | 40 | 20.0% |
| Ashland, CA | 23,566 | 40 | 20.0% |
| Danville, CA | 44,933 | 40 | 20.0% |
| Norris Canyon, CA | 790 | 40 | 20.0% |
| Castro Valley, CA | 64,392 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Janesville, WI | 64,415 | 38 | 24.0% |
| Noblesville, IN | 64,430 | 42 | 16.0% |
| Port Charlotte, FL | 64,335 | 36 | 28.0% |
| Eden Prairie, MN | 64,481 | 46 | 8.0% |
| Tulare, CA | 64,546 | 66 | 32.0% |
| Castro Valley, CA | 64,392 | 40 | 20.0% |
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