Checking the Buoy Before the First Lesson
Every morning before the first lesson, surf instructor Kaimana Reyes checks the readings from a weather buoy anchored far offshore. This summer, eighth grader Beatriz Takahara is helping at the surf school, and Kaimana is teaching her to read the buoy data. The waves at this beach usually start as wind blowing over the open ocean far away, and they can travel for days before they reach the shore. The buoy mostly bobs up and down in place as each wave passes under it, while the wave's energy keeps moving toward the beach. Waves near the shore can look quite different from the buoy readings, because waves change as they move into shallow water.
The buoy reports the wave height, the vertical distance from the bottom of a trough to the top of the next crest. Kaimana sketched a wave on a whiteboard and labeled a crest, a trough and the calm water level halfway between them. She pointed out that wave height is not the same as amplitude. The amplitude is the distance from the calm water level up to a crest, so it is half of the wave height. The buoy also reports the period, the time in seconds (s) between one crest and the next crest passing the buoy. The frequency, in hertz (Hz), is the number of waves that pass each second, so it equals 1 divided by the period.
The buoy's computer also estimates the wavelength, the distance from one crest to the next, and the wave energy for each square meter of sea surface, in joules per square meter (J per m²). Kaimana uses the energy number to decide whether beginners can go in the water. Strong waves can knock a new surfer off a board and push the board hard toward the shore. On days when it is too high, she teaches on the sand instead.
Beatriz copied four mornings of readings into a log. The table shows the wave height, period, wavelength and wave energy for each day. The readings changed from day to day as storms far out at sea grew and faded. On Thursday evening, the forecast for Friday called for waves 4.0 m high.