When is a vertical arrangement of masthead lights typically used?

A vertical arrangement of masthead lights is typically used when towing a long distance. This configuration is designed to signal that a vessel is engaged in towing, which may affect its maneuverability and the navigational awareness of other vessels. The vertical arrangement helps to distinguish the towing vessel from other types, such as fishing or sailing vessels, thus providing clear indications of the vessel's activity to ensure safe navigation and prevent collisions.

Using this arrangement is particularly important during nighttime or in poor visibility conditions, as it allows other vessels to quickly recognize the towing operation. This is critical in maintaining safe distances and ensuring that the tow is properly coordinated. The other contexts mentioned in the options, such as day use or foggy conditions, do not specifically address the primary purpose of the vertical masthead lights in the context of towing activities.