| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 34°F | 26°F | 4.5" |
| February | 19°F | 35°F | 27°F | 3.6" |
| March | 31°F | 49°F | 39°F | 4.4" |
| April | 40°F | 62°F | 50°F | 5.4" |
| May | 50°F | 72°F | 60°F | 6.5" |
| June | 60°F | 81°F | 70°F | 5.8" |
| July | 64°F | 84°F | 74°F | 8.0" |
| August | 63°F | 82°F | 72°F | 7.5" |
| September | 56°F | 76°F | 65°F | 5.4" |
| October | 45°F | 64°F | 54°F | 6.2" |
| November | 33°F | 51°F | 42°F | 5.4" |
| December | 24°F | 38°F | 31°F | 3.8" |
| Index | Markham | Illinois | National |
| Air quality index | 55 | 41 | 42 |
| Pollution index | 1,558,070 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 121 |
| Days with moderate air quality | 227 |
| Days w/ poor A.Q. for sensitive groups | 16 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.070% |
| Lead | 0.090% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Bedford Park, IL | 669 | 55 | 10.0% |
| Crete, IL | 8,058 | 55 | 10.0% |
| Lynwood, IL | 9,210 | 55 | 10.0% |
| Thornton, IL | 2,308 | 55 | 10.0% |
| Frankfort, IL | 19,166 | 55 | 10.0% |
| Markham, IL | 12,401 | 55 | 10.0% |
| City | Population | Air Quality Index | National Avg. |
| Summerland, BC | 12,400 | n/a | |
| Irmo, SC | 12,397 | 43 | 14.0% |
| Brock, ON | 12,406 | n/a | |
| Patchogue, NY | 12,389 | 43 | 14.0% |
| Sidney, BC | 12,386 | n/a | |
| Markham, IL | 12,401 | 55 | 10.0% |
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