| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 24°F | 41°F | 32°F | 4.0" |
| February | 24°F | 43°F | 33°F | 3.0" |
| March | 31°F | 50°F | 40°F | 4.2" |
| April | 41°F | 61°F | 50°F | 3.5" |
| May | 51°F | 71°F | 60°F | 3.6" |
| June | 60°F | 79°F | 69°F | 4.7" |
| July | 68°F | 85°F | 76°F | 3.1" |
| August | 65°F | 83°F | 74°F | 5.6" |
| September | 60°F | 78°F | 68°F | 3.1" |
| October | 49°F | 67°F | 58°F | 4.1" |
| November | 37°F | 56°F | 46°F | 3.0" |
| December | 31°F | 47°F | 39°F | 4.9" |
| Index | Selden | New York | National |
| Air quality index | 43 | 43 | 42 |
| Pollution index | 39,208,800 | 6,179,275 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 262 |
| Days with moderate air quality | 82 |
| Days w/ poor A.Q. for sensitive groups | 18 |
| Days with unhealthy air quality | 3 |
| Pollutant | Total |
| Arsenic | 0.060% |
| Benzene | 0.320% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.320% |
| Mercury | 0.130% |
| City | Population | Air Quality Index | National Avg. |
| Calverton, NY | 5,280 | 41 | 18.0% |
| Bellport, NY | 1,899 | 43 | 14.0% |
| Central Islip, NY | 33,591 | 43 | 14.0% |
| North Patchogue, NY | 8,202 | 43 | 14.0% |
| Brookhaven, NY | 3,203 | 43 | 14.0% |
| Selden, NY | 21,338 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Cartersville, GA | 21,331 | 44 | 12.0% |
| Montclair, VA | 21,329 | 47 | 6.0% |
| La Porte, IN | 21,328 | 37 | 26.0% |
| Mountlake Terrace, WA | 21,328 | 45 | 10.0% |
| St. Andrews, SC | 21,355 | 43 | 14.0% |
| Selden, NY | 21,338 | 43 | 14.0% |
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