Based on the average depth of the ocean, the average speed at which tide waves can travel across the open ocean is only about 700km (435 miles) per hour. Thus, the idealized bulges that are oriented towards and away from a tide-generating body cannot exist because they cannot keep up with the rotational speed of the earth. Instead, ocean tides break up into distinct large circulation units called cells. In the open ocean, the crest and troughs of the tide wave rotate around an amphidromic point near the center of each cell. There is essentially no tidal range at amphidromic points, but rotating from each point are cotidal lines, which connect all nearby locations where high tide occurs simultaneously. The labels on the cotidal lines indicate the times of high tide in hours as they rotate around the cell. The time indicates that the tide wave rotates counterclockwise in the Northern Hemisphere and clockwise in the Southern hemisphere. The wave must complete one rotation during the tidal period (usually 12 lunar hours), so this limits the size of the cells. The continental affects tides, too, because they interrupt the free movement of the tidal bulges across the ocean surface. Tides are expressed in each ocean basin as freestanding waves that are affected by the position and shape of the continents that ring the ocean basin. In fact, two of the most important factors that influence tidal conditions along the coast are coastline shape and offshore depth.