| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 40°F | 70°F | 54°F | 1.6" |
| February | 41°F | 71°F | 55°F | 1.4" |
| March | 44°F | 75°F | 58°F | 0.8" |
| April | 47°F | 78°F | 61°F | 0.4" |
| May | 52°F | 82°F | 65°F | 0.3" |
| June | 57°F | 89°F | 71°F | 0.0" |
| July | 62°F | 94°F | 76°F | 0.1" |
| August | 61°F | 96°F | 77°F | 0.0" |
| September | 59°F | 94°F | 74°F | 0.1" |
| October | 52°F | 86°F | 67°F | 0.3" |
| November | 44°F | 78°F | 60°F | 0.4" |
| December | 39°F | 69°F | 53°F | 2.0" |
| Index | Walnut | California | National |
| Air quality index | 85 | 55 | 50 |
| Pollution index | 858,652 | 18,106,726 | 5,066,862 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 24 |
| Days with moderate air quality | 211 |
| Days w/ poor A.Q. for sensitive groups | 102 |
| Days with unhealthy air quality | 25 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.020% |
| Mercury | 0.001% |
| City, State | Population | Air Quality Index | National Avg. |
| Arcadia, CA | 57,755 | 74 | 48% |
| Baldwin Park, CA | 76,511 | 74 | 48% |
| Cerritos, CA | 49,797 | 74 | 48% |
| Citrus, CA | 11,780 | 74 | 48% |
| Duarte, CA | 21,792 | 74 | 48% |
| Walnut, CA | 30,026 | 85 | 70% |
| City, State | Population | Air Quality Index | National Avg. |
| North Chicago, IL | 30,023 | 35 | 30% |
| Rochester, NH | 30,052 | 26 | 48% |
| Matthews, NC | 30,054 | 42 | 16% |
| Westchester, FL | 30,058 | 41 | 18% |
| San Pablo, CA | 29,991 | 43 | 14% |
| Walnut, CA | 30,026 | 85 | 70% |
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