Cold fronts generally move from west to east, whereas warm fronts move poleward. Occluded fronts are a hybrid, and stationary fronts are stalled in their motion. Cold fronts and cold occlusions move faster than warm fronts and warm occlusions, because the dense air behind them can lift as well as push the warmer air. Mountains and bodies of water can affect the movement and properties of fronts. When the density contrast between the airmasses is diminished, for instance after flowing out over a uniformly warm ocean, the front can degenerate into a line which separates regions of differing wind velocity known as a shearline. This is most common over the open ocean.