What signal should be given to indicate a fog warning?

To indicate a fog warning, the appropriate signal is to sound a horn. In maritime practice, the use of sound signals, such as a horn, is a recognized method for vessels to communicate their presence when visibility is low, such as in fog. The International Regulations for Preventing Collisions at Sea (COLREGS) specify that vessels must use appropriate sound signals to alert other vessels of their presence, thereby enhancing safety on the water.

A horn is particularly effective because it can be heard over long distances and penetrates through fog better than visual signals. This allows other mariners to understand that there is traffic nearby, which is crucial for avoiding collisions in reduced visibility conditions.

While other options may have specific uses (such as flares for signaling distress or searchlights for illuminating areas), they are not standard signals for fog warnings specifically. A distress flag indicates a vessel in need of assistance, and a searchlight is more suited for visibility and locating objects rather than signaling warning of a vessel's presence in foggy conditions. Hence, sounding a horn aligns directly with the practice of signaling in fog.