What are visibility markers used for?

Visibility markers are used to measure visibility based on specific objects. They are strategically placed reference points that observers can identify and assess to determine how far they can see under various weather conditions. For example, if an observer can see a visibility marker that is known to be a specific distance away, they can make an accurate judgment about the visibility in that direction. This is crucial for flight operations, boating, and other activities where clear visibility is necessary for safety.

The other options, while related to various meteorological observations, do not align with the primary function of visibility markers. Measuring cloud thickness involves different instruments and meteorological tools, while indicating wind speed requires anemometers specifically designed for that purpose. Tracking temperature changes involves thermometers and other temperature-measuring devices, not visibility icons. Therefore, the role of visibility markers in determining how far one can see in particular conditions makes the correct answer clearly relevant.