Tidal Characteristics

The rise and fall of the ocean water level is called the tide. The tide is caused primarily by the pull of the moon and the sun. Each month, the moon, Earth, and sun are in alignment during a full or new moon. Twice a month, the moon, Earth, and sun are at right angles to one another during a first-quarter and third-quarter moon phase. This has an effect on the tides.

When the moon, Earth, and sun are in alignment, the effect of gravity is at its strongest.. This is called a spring tide. This happens twice a month and will cause areas to have one really high tide and one really low tide. The tidal range is the greatest for that day. Tidal range is the difference in water levels between a high and low tide.

When the moon, Earth, and sun are at right angles to one another, the effect of gravity is its weakest. This means that the tidal range is very low, with the difference between high tide and low tide being minimal. This is called a neap tide.

Tidal Patterns

Did you know there are different tidal patterns? Some areas receive only one high tide and one low tide per day. These areas are called diurnal tides. Some places, like Hampton Roads in Virginia, receive two high tides and two low tides per day. These are called semidiurnal tides. Some places receive two high tides and two low tides per day, but the water levels are all mixed up; thus they are called mixed tides. Mostly, the shape and depth of the ocean basins cause these tidal patterns. Look at the tidal patterns around the world as depicted in the image above.

The difference between tides is so great in the Bay of Fundy, Nova Scotia, that boats will rest on the sandy bottom during low tide! People actually kayak, raft, or surf the flood currents here! When the flood current is so great it creates a wave, oceanographers actually call it a tidal bore. This is the true tidal wave.

When tides leave a rocky area, they also leave behind pools of water called tidal pools. These pools serve as shelter for some organisms that live close to the shoreline. Some organisms will survive out of water until the high tide returns. Tide pools harbor organisms adapted to the rise and fall of the tides!

A tidal bore comes in China's Qiantang River (left). A tidal pool on Matinicus Island, Maine, harbors algae
and other organisms (right).

 

Tides Review

Now that you have learned all about tides, check your knowledge. In this non-graded interactivity, follow the instructions on each question slide. Click SUBMIT to check your responses. Click the player button to begin.