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

The characteristic that is not associated with stable air is good visibility. In stable air conditions, atmospheric stability often leads to limited vertical mixing of air. This can result in the development of low clouds and fog, which can significantly reduce visibility. On the other hand, the other characteristics mentioned are typical of stable air:

Smooth air is commonly found in stable conditions as there is less turbulence due to the lack of significant vertical air movements. Steady precipitation usually occurs in stable air when light rain can continue for extended periods as moisture is gradually released from stratiform clouds. Lastly, stratiform clouds are indicative of stable air, forming in layers and typically producing steady, gentle precipitation without the turbulence associated with more unstable conditions.

Hence, while stable air features smooth conditions, steady precipitation, and stratiform clouds, it does not generally promote good visibility, making this the correct response.