What type of objects can effectively be used for determining visibility?

Determining visibility involves using objects that provide clear contrast against their background, which aids in estimating how far one can see. Dark objects positioned against the horizon sky are particularly effective for this purpose because the contrast allows observers to easily distinguish the object from its surroundings. When observing visibility, the clarity of the object against the sky enables a more accurate assessment of distance and atmospheric conditions.

Random patterns against cloud cover may not provide a consistent reference point for visibility assessment, as the inconsistencies can complicate measurements. Bright artificial lights, while visible, can sometimes create glare or distort perception under certain atmospheric conditions, limiting their effectiveness. Cloud formations on the horizon can change shape and density, making it difficult to gauge visibility reliably since they do not remain constant and don't provide a fixed reference point.

Thus, dark objects against the horizon sky stand out distinctly, making them the most reliable indicators for assessing visibility within a specific area.