Under which conditions is visibility generally considered good?

Visibility is considered good when the atmospheric conditions support clear sight lines and minimal obstructions to vision. Unstable air, which is characterized by vertical motion and convective activity, often leads to the dispersal of pollutants, moisture, and particulate matter in the atmosphere. This turbulent mixing helps improve visibility by reducing the concentration of haze and delivering cleaner air to lower altitudes. As a result, when air is unstable, it fosters better visual conditions for pilots, including those operating unmanned aircraft.

In contrast, stable air tends to trap pollutants near the surface, which can lead to reduced visibility. High humidity also contributes to haze, as moisture can scatter light and limit how far one can see. Cold air masses, while they can sometimes have good visibility depending on other factors, often are associated with stable atmospheric conditions that can lead to reduced visibility as well.

Thus, the conditions presented in the question emphasize that unstable air can enhance visibility, making it the correct choice.