Ocean tides are affected by the inability of a single tidal bulge to move rapidly enough to stay beneath the moon. The speed of a bulge is limited by the depth of the water, and the continents get in the way of moving bulges. The oceans respond to these restrictions by generating a number of circular systems of moving bulges called amphidromes. The waters in basins of different shapes respond differently. Tidal range, the height difference between high tide and low tide, also varies widely throughout the world.
Some areas have extremely large ranges.
Tidal patterns and ranges also vary over time. The most outstanding variations are the spring-neap cycle and the tropic cycle. These variations result from changes in the relative positions of the sun, earth, and moon. Spring tides are semidiurnal tides with unusually high ranges twice per month when the sun, earth, and moon are in line. This alignment occurs at times of full moon and new moon.