"RIDE THE WAVES"
Crest: The highest point of a wave
Most waves are caused by winds blowing the surface of the water. How long the wind blows, how hard, and over how big of an area all affect the size of the waves it will produce. Most waves are less than 4 meters (12 feet) high, but much larger waves can form during severe storms.
The highest point on a wave is the crest, and the lowest point is called the trough, as shown here. The distance between two crests is the wavelength. The distance from the trough to the crest is the wave height.
When waves reach shallower regions, the wave troughs drag along the shoreline, but the crests keep going. As a result, the waves jam up, shortening their wavelength and increasing in height. At some point, the wave becomes too tall to hold itself up, and it curls over or "breaks" on the shore.
Underwater earthquakes and volcanoes can result in long, high-speed waves called tsunamis. (These waves are sometimes called tidal waves, although they have nothing to do with tides.) Such waves can have a wavelength measuring 160 kilometers (100 miles) and speeds up to 800 kilometers per hour (500 miles per hour). Since tsunami's wavelength is so long, the time between waves hitting the shore can be as long as 15-20 minutes. In the open sea, these waves may be only a half-meter (1.5 feet) tall, but when they build up along a shoreline, they reach heights over 30 meters (100 feet) and have great destructive power. Tsunamis occur most often in Japan, Hawaii, and Alaska, due to earthquakes and volcanic activity in the Pacific Ocean.