**Updated 2/2016** Here are some quick examples of how atmosphere can effect sharpness in photos. The clarity of the atmosphere (including dust, pollen, humidity, heat etc) can drastically impact the perceived sharpness of your lens by softening edges and muting colors.
Even a short distance like a soccer field can be effected (especially when it is so dang hot and humid!).
Winter may bring some relief from ozone and humidity
But not necessarily from heat waves.
So even with the sharpest, more clear lenses, you still need to consider the clarity of the atmosphere you are shooting.
This effect is not great for sharpness, but great for separating foreground from background. (This is not a great example for this, but you get the idea.)