A wave is the movement of energy through water that usually begins in the open ocean. Most waves form when wind blows across the water’s surface, transferring energy to the water.

As shown in Figure 1, the crest is the highest point of a wave, while the trough is the lowest point. The wavelength is the distance between two crests or two troughs. The amplitude (or wave height from the still water level) measures the energy of the wave from the still water level up to the crest. Frequency describes how many waves pass a certain point in a given amount of time.

While waves are caused mainly by wind transferring energy across the ocean’s surface, other movements of ocean water are driven by gravity. One important example is tides, which are the regular rise and fall of ocean water along Earth’s coastlines.

Tides are caused by the gravitational interaction between Earth, the Moon, and the Sun. These gravitational forces create two bulges in the ocean’s surface. One bulge forms on the side of Earth facing the Moon, and the other forms on the side facing away from the Moon. These bulges represent high tides. The tidal range is the difference in water level between high tide and low tide. The size of the tidal range depends on the positions of the Moon and the Sun relative to Earth. A spring tide occurs when Earth, the Moon, and the Sun form a straight line, creating the largest tidal range. A neap tide occurs when Earth, the Moon, and the Sun form a right angle, producing the smallest tidal range.

Review:

1. Identify the main parts of a wave.
2. What are tides?
3. Compare a spring tide to a neap tide.