| 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 | 89 | 55 | 42 |
| Pollution index | 858,652 | 10,473,190 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 42 |
| Days with moderate air quality | 178 |
| Days w/ poor A.Q. for sensitive groups | 79 |
| Days with unhealthy air quality | 58 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.020% |
| Mercury | 0.001% |
| City | Population | Air Quality Index | National Avg. |
| West Covina, CA | 105,808 | 72 | 44.0% |
| Cerritos, CA | 49,388 | 72 | 44.0% |
| La Habra Heights, CA | 5,318 | 72 | 44.0% |
| Norwalk, CA | 104,524 | 72 | 44.0% |
| West Whittier-Los Nietos, CA | 26,121 | 72 | 44.0% |
| Walnut, CA | 29,764 | 89 | 78.0% |
| City | Population | Air Quality Index | National Avg. |
| Sanford, NC | 29,758 | 29 | 42.0% |
| Whitehorse, YT | 29,774 | n/a | |
| East Lake-Orient Park, FL | 29,750 | 43 | 14.0% |
| Parkersburg, WV | 29,732 | 35 | 30.0% |
| Leamington, ON | 29,703 | n/a | |
| Walnut, CA | 29,764 | 89 | 78.0% |
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