What objects can be used for determining visibility?

The use of dark objects against the horizon sky is effective for determining visibility because it allows observers to gauge distance based on how clearly those objects can be seen. When contrasting darker shapes or forms are placed in front of a lighter background, such as the sky, it becomes easier to assess how far away these objects are. This method relies on the observer's ability to discern outlines and silhouettes against the brighter backdrop, which can provide a visual reference point in assessing atmospheric conditions.

In contrast, the other options may not be as reliable for visibility determination. High-intensity focused lights can be seen from great distances but may not provide a clear indication of distance since their brilliance can create glare and obscure the actual visibility conditions of the surrounding environment. Similarly, searchlights, while potentially useful, might draw attention to specific areas without giving a comprehensive view of overall visibility. Lastly, dim lights of unknown intensity are inherently difficult to evaluate, as their indistinct nature does not aid in assessing how far they are or the extent of visibility. Thus, the clarity of dark objects against the horizon sky serves as a practical and effective means of determining visibility in various scenarios.