| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 34°F | 26°F | 3.3" |
| February | 19°F | 35°F | 26°F | 3.0" |
| March | 32°F | 51°F | 40°F | 4.3" |
| April | 42°F | 64°F | 52°F | 7.2" |
| May | 53°F | 75°F | 63°F | 8.0" |
| June | 62°F | 83°F | 72°F | 7.5" |
| July | 66°F | 85°F | 75°F | 7.1" |
| August | 64°F | 83°F | 73°F | 8.2" |
| September | 56°F | 78°F | 66°F | 6.3" |
| October | 45°F | 65°F | 54°F | 6.6" |
| November | 34°F | 51°F | 42°F | 4.3" |
| December | 23°F | 37°F | 30°F | 3.5" |
| Index | Princeton | Illinois | National |
| Air quality index | 0 | 41 | 42 |
| Pollution index | 2,097,740 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 335 |
| Days with good air quality | 334 |
| Days with moderate air quality | 1 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.300% |
| City | Population | Air Quality Index | National Avg. |
| Dover, IL | 141 | 0 | 100.0% |
| Granville, IL | 1,449 | 0 | 100.0% |
| Standard, IL | 281 | 0 | 100.0% |
| Seatonville, IL | 372 | 0 | 100.0% |
| Malden, IL | 411 | 0 | 100.0% |
| Princeton, IL | 7,589 | 0 | 100.0% |
| City | Population | Air Quality Index | National Avg. |
| Corning, CA | 7,589 | 41 | 18.0% |
| Yreka, CA | 7,589 | 36 | 28.0% |
| Sioux Center, IA | 7,592 | 35 | 30.0% |
| Clear Lake, IA | 7,594 | 15 | 70.0% |
| Glen Ridge, NJ | 7,583 | 51 | 2.0% |
| Princeton, IL | 7,589 | 0 | 100.0% |
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