Which of the following is not a characteristic of stable air?

The distinguishing characteristic of stable air is that it typically leads to a lack of significant vertical movement in the atmosphere, which directly affects visibility. In stable air conditions, poor visibility can be a common occurrence due to factors like fog, haze, or low clouds, which develop because of minimal vertical mixing and the presence of moisture.

In regard to calm winds and low turbulence, these are also characteristics typically associated with stable air. Calm conditions indicate that there is not enough thermal activity to create strong winds, while low turbulence reflects the smooth layering of air inherent in a stable atmosphere. Therefore, good visibility does not align with the characteristics of stable air, making it the correct answer for what is not a characteristic of stable air.