This comes to us from the days of sail and describes the more favorable maneuverability of a fighting sailing vessel. The term is now antiquated in modern Naval warfare, however it still holds true in yacht racing.

It is any position, at sea, upwind of the other vessel. With closer proximity to land, tidal and stream effects, as well as wind variability due to geography also may come into play. An upwind vessel is able to maneuver at will toward any downwind point, since in doing so the relative wind moves aft. A vessel downwind of another, however, in attempting to attack upwind, is constrained to trim sail as the relative wind moves forward and cannot point too far into the wind for fear of being headed.

In sailing warfare, when beating to windward, the vessel heels, rolls, under the sideward pressure of the wind. This restricts gunnery, as cannon on the windward side are now elevated, while the leeward gun ports aim into the sea. In heavy weather the leeward guns may find themselves awash. A ship with the weather gage, turning downwind to attack, may alter course at will in order to bring starboard and port guns to appropriate elevations. Ships seeking to evade capture or attack, however, have the advantage being downwind if they are faster vessels or are close to friendly land.

One of the last times that weather gage was perhaps a factor in a naval engagement was in the Battle of the Denmark Strait in 1941 where the German battleship Bismarck and the heavy cruiser Prinz Eugen held the weather gage over the British battleship HMS Prince of Wales and the HMS Hood. Being upwind the German ships had the advantage that their shells would carry further than those of the British. The British ships also had the disadvantage of spray being blown onto their optical rangefinders.