As both of two air masses which compose the southern part of the baiu front are air masses which have an ocean as its base (martitime air mass), they absorb a large amount of moisture from the ocean and contain wet air.
梅雨前線の南側を構成する2つの気団はともに海洋を本拠地とする気団（海洋性気団）のため、海洋から大量の水蒸気を吸収して湿潤な空気を持っている。