| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 32°F | 23°F | 1.0" |
| February | 17°F | 35°F | 25°F | 1.1" |
| March | 30°F | 51°F | 39°F | 2.4" |
| April | 41°F | 64°F | 52°F | 4.8" |
| May | 51°F | 74°F | 62°F | 4.6" |
| June | 62°F | 84°F | 73°F | 6.1" |
| July | 66°F | 86°F | 75°F | 4.7" |
| August | 64°F | 84°F | 73°F | 4.5" |
| September | 55°F | 78°F | 66°F | 3.3" |
| October | 43°F | 65°F | 53°F | 3.3" |
| November | 31°F | 51°F | 40°F | 1.9" |
| December | 19°F | 35°F | 27°F | 1.9" |
| Index | Letts | Iowa | National |
| Air quality index | 32 | 39 | 50 |
| Pollution index | 975,807 | 1,643,836 | 5,066,862 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 300 |
| Days with moderate air quality | 65 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.150% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.040% |
| Mercury | 0.150% |
| City, State | Population | Air Quality Index | National Avg. |
| Cotter, IA | 38 | 32 | 36% |
| Winfield, IA | 1,068 | 32 | 36% |
| Fruitland, IA | 1,196 | 32 | 36% |
| Nichols, IA | 360 | 32 | 36% |
| Fredonia, IA | 313 | 32 | 36% |
| Letts, IA | 410 | 32 | 36% |
| City, State | Population | Air Quality Index | National Avg. |
| Shelter Cove, CA | 410 | 27 | 46% |
| Mount Charleston, NV | 410 | 54 | 8% |
| Churdan, IA | 410 | 37 | 26% |
| Rockland, ID | 410 | 27 | 46% |
| Miles, IA | 410 | 42 | 16% |
| Letts, IA | 410 | 32 | 36% |
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