NCEES Principles and Practice of Surveying Practice Exam

How would you describe a semidiurnal tide?

1. One high tide and one low tide each day

2. Two high tides and two low tides each day

3. Periodic high tides of different heights

4. No clear tidal pattern

The correct answer is: Two high tides and two low tides each day

A semidiurnal tide is characterized by the occurrence of two high tides and two low tides each day. This tidal pattern results in relatively equal heights for both high tides and both low tides, though slight variations can sometimes occur due to the influence of the moon's position and other factors. Understanding the terminology is essential. The term "semidiurnal" comes from Latin roots meaning "half" and "day," indicating that tides follow a bi-daily pattern. This phenomenon is most commonly observed in coastal regions around the world, including the east coast of the United States and much of Western Europe. In the context of tides, it's important to recognize how semidiurnal tides differ from diurnal tides (one high and one low tide per day) and mixed tides, which feature a combination of both semidiurnal and diurnal patterns. By identifying the defining characteristics of semidiurnal tides—two cycles occurring each 24-hour period—one can better understand coastal and oceanic behaviors influenced by gravitational forces, primarily from the moon and to a lesser extent, the sun.