In oceanography, a front is a boundary between two distinct water masses. The water masses are defined by moving in different directions, i.e. on one side of the front the water is generally moving in one way, and on the other side of the front, the water is moving in another. Depending on the directions of the water masses, a front may be defined as or . The water masses on either side of a front may also have different temperatures, salinities, or densities, along with differences in other oceanographic markers. While most fronts form and dissipate relatively quickly, some, such as the fronts caused by the antarctic circumpolar current, persist for long periods of time.