| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 36°F | 59°F | 46°F | 3.2" |
| February | 41°F | 64°F | 50°F | 1.9" |
| March | 49°F | 71°F | 59°F | 2.6" |
| April | 56°F | 78°F | 66°F | 3.4" |
| May | 63°F | 84°F | 73°F | 4.4" |
| June | 71°F | 92°F | 81°F | 3.2" |
| July | 74°F | 95°F | 84°F | 1.9" |
| August | 74°F | 97°F | 85°F | 1.9" |
| September | 68°F | 90°F | 78°F | 3.1" |
| October | 57°F | 81°F | 68°F | 4.1" |
| November | 48°F | 70°F | 58°F | 1.9" |
| December | 39°F | 60°F | 48°F | 1.8" |
| Index | Mertens | Texas | National |
| Air quality index | 41 | 52 | 42 |
| Pollution index | 1,309,490 | 2,009,300 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 275 |
| Days with moderate air quality | 80 |
| Days w/ poor A.Q. for sensitive groups | 7 |
| Days with unhealthy air quality | 3 |
| Pollutant | Total |
| Arsenic | 0.430% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.010% |
| Mercury | 0.390% |
| City | Population | Air Quality Index | National Avg. |
| Abbott, TX | 354 | 38 | 24.0% |
| Bynum, TX | 321 | 38 | 24.0% |
| Penelope, TX | 124 | 38 | 24.0% |
| Hillsboro, TX | 8,384 | 38 | 24.0% |
| Frost, TX | 882 | 41 | 18.0% |
| Mertens, TX | 124 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Hyampom, CA | 124 | 27 | 46.0% |
| Nicolaus, CA | 124 | 44 | 12.0% |
| Weott, CA | 124 | 27 | 46.0% |
| Osterdock, IA | 124 | 29 | 42.0% |
| Elk River, ID | 124 | 10 | 80.0% |
| Mertens, TX | 124 | 41 | 18.0% |
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