Ocean in Motion: Tides - Characteristics

When the Moon, Earth, and Sun fall in a straight line, which we call syzygy (siz-eh-gee), we notice the greatest difference between high and low tide water levels. These spring tides occur twice each month, during the full and new Moon. If the Moon is at perigee, the closest it approaches Earth in its orbit, the tides are especially high and low.

When the Sun and Moon form a right angle, as when we see a half moon, their pulls fight each other and we notice a smaller difference between high and low tides. These are called neap tides.

Factors such as the path the Moon takes around the Earth, our planet's tilt, even the water's depth, and the ocean floor affect tides. Therefore, not all coasts experience two high and two low tides each day.

Semi-diurnal tides occur twice a day. This means a body of water with semi-diurnal tides, like the Atlantic Ocean, will have two high tides and two low tides in one day. Diurnal tides occur once a day. A body of water with diurnal tides, like the Gulf of Mexico, has only one high tide and one low tide in a 25-hour period. Some bodies of water, including parts of the Pacific Basin, have mixed tides, where a single low tide follows two high tides.

The difference in the height of the water surface between the high and low tides is the tidal range. Tidal ranges can be measured in inches, like those in Lake Superior, Michigan, or in feet or yards. In fact, the Bay of Fundy, a V-shaped Canadian inlet in Nova Scotia, has the greatest tidal range known--up to 50 feet! In areas with large tidal ranges, boats anchored at high tide are often left stranded on the dry beach at low tide.

As the sea level rises and falls, it generates a tidal current that flows horizontally. Tidal currents caused by the dropping water level (as the tide "goes out") are called ebb currents. The rising tide generates flood currents. Tidal currents are especially strong where the ocean is connected to an estuary or bay, and boats sometimes have to wait for a current in to enter or leave a harbor.