How far can red hand-held flares be visible at sea level on a clear night?

Red hand-held flares are designed to signal distress and can be seen from a considerable distance at sea, particularly under optimal conditions such as a clear night. The visibility range for these flares is typically around ten nautical miles. This distance means that if someone on a boat is in distress and uses a red hand-held flare at night, it can attract attention from other vessels or search and rescue operations that are ten nautical miles away, provided the conditions are excellent.

The specific design and intensity of the flare, as well as environmental factors such as weather, sea state, and light pollution, can influence visibility. However, the standard visibility range is established at ten nautical miles, making it a reliable guideline for safety and emergency signaling practices at sea.